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Title: Non-technical chats on iron and steel and their application to modern industry
Author: Spring, la Verne W.
Language: English
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NON-TECHNICAL CHATS ON IRON AND STEEL
AND THEIR APPLICATION TO MODERN INDUSTRY
BY
LA VERNE W. SPRING, A.B.
CHIEF CHEMIST AND METALLURGIST, CRANE CO., CHICAGO
_WITH TWO HUNDRED AND NINETY-FOUR ILLUSTRATIONS AND DIAGRAMS_
[Illustration: [Logo]]
NEW YORK
FREDERICK A. STOKES COMPANY
PUBLISHERS
_Copyright, 1917, by_
FREDERICK A. STOKES COMPANY
_All rights reserved, including that of translation
into foreign languages_
TO
MY FELLOW WORKERS
IN THE VAST IRON AND STEEL INDUSTRY
THIS BOOK IS
AFFECTIONATELY DEDICATED
FOREWORD
It has long been a desire of the author to put into non-technical form
the interesting data here given. During several years spent in the
service of one of the great steel companies of this country, first in
the laboratories and afterward in the rolling mills, he acquired a love
for the industry that is now of fairly long standing. Spending as much
of his spare time then and since in visiting various parts of that
mammoth plant and as many others as he was able, he has always felt that
this extremely interesting subject could not fail to prove fascinating
even to those who had previously known little of the manufacture of
steel and steel products. Later work with gray and malleable cast irons
and with cast steel enlarged the outlook and further urged his sharing
these interesting things with others not so fortunately situated.
Such an inspiration, if it may so be called, is the reason for the
appearance of these articles. Practically as here reprinted, the first
thirteen of them were published during 1915 and 1916 in serial form in
the “Valve World,” the house organ of Crane Company of Chicago, with
which the writer for some time has been connected. The enthusiasm with
which they were received has been very gratifying, while the scores of
letters bearing favorable comment testify to the correctness of the
judgment that the metallurgy of our most useful metal, iron, is of very
general interest.
It may be remarked by some that certain of the statements made in the
book are not strictly accurate in that few details have been stated or
exceptions made. This is true, but it seemed necessary if main facts
were to be made to stand forth with the boldness required to accomplish
the purpose which the author had in mind. The chapters are in no way
intended to be an encyclopedia of the subject. The idea throughout has
been to present only the main points and to show the derivation of the
products from the raw materials and their relationships to each other.
In other words the book is intended only as a sort of outline.
References are given which will aid in the selection of works to be
consulted by any who are sufficiently interested to go farther.
Without the encouragement and coöperation of Crane Company and the kind
assistance of friends, some within and some without the iron and steel
industry, this little book would not have been possible. Special mention
must be made of the aid given by Messrs. I. M. Bregowsky, J. A.
Matthews, C. D. Carpenter, and others whose reading of and suggestions
concerning parts of the manuscript were of much help. Thanks are due
also to many individuals and firms for their very hearty coöperation in
furnishing information and the photographs which appear in the pages of
the book.
L. W. S.
_Illustrations are from the following sources:_
A. M. Byers Co., Pittsburg.—National Tube Co., Pittsburg.—U. S. Steel
Corporation, New York.—Tennessee Coal, Iron & Railroad Co., Birmingham,
Ala.—Shenango Furnace Co., Pittsburg.—Pickands, Mather & Co.,
Pittsburg.—United States Geological Survey.—Wellman-Seaver-Morgan Co.,
Cleveland, O.—Lackawanna Steel Co., Buffalo.—Cleveland-Cliffs Iron Co.,
Ishpeming, Mich.—J. H. Hillman & Sons Co., Pittsburg.—Harbison-Walker
Refractories Co., Pittsburg.—By-Products Coke Corporation, Chicago.—H.
Koppers Co., Pittsburg.—Federal Furnace Co., Chicago.—Crucible Steel
Company of America, Pittsburg.—Crane Co., Chicago.—Interstate Iron &
Steel Co., Chicago.—LaBelle Iron Works, Steubenville, O.—Morgan
Construction Company, Worcester, Mass.—J. A. Matthews. Syracuse, N.
Y.—McLain’s System, Milwaukee, Wis.—Allis-Chalmers Co., Milwaukee,
Wis.—J. H. Williams & Co., New York.—Griffin Wheel Co., Chicago.—U. S.
Molding Machine Co., Cleveland.—Bradley Manufacturing Co., Bradley, Ill.
(Sears, Roebuck & Co., Chicago).—Snyder Electric Furnace Co.,
Chicago.—Commonwealth Steel Co., St. Louis.—John A. Crowley & Co.,
Detroit.—Illinois Steel Co., Chicago.—Pickands, Brown & Co.,
Chicago.—“Sketches of Creation,” by Alexander Winchell. Harper & Bros.,
New York.—“Descriptive Metallurgy of Iron,” by S. Groves.—“Materials of
Engineering,” by R. H. Thurston. John Wiley & Sons, New York.—“Chambers’
Encyclopedia.” J. B. Lippincott, Philadelphia.—“Cast Iron in the Light
of Recent Research,” by W. H. Hatfield. Charles Griffin & Co., Ltd.,
London.—“Handbook of Chemical Technology,” Wagner-Crookes. D. Appleton &
Co., New York.—“The Ore Deposits of the United States and Canada,” by J.
F. Kemp. McGraw-Hill Book Co., New York.—“The Valve World.” Crane Co.,
Chicago.—“The Romance of Steel,” by H. Casson. A. S. Barnes & Co., New
York.—“The Metallurgy of Steel,” by Harbord & Hall. Chas. Griffin & Co.,
Ltd., London.—“Metallurgy of Steel,” by H. M. Howe. McGraw-Hill Book
Co., New York.—Tomlinson’s “Encyclopedia of Useful Arts” (1854). G.
Virtue & Co., New York.—“Liquid Steel,” by E. G. Carnegie. Longmans,
Green & Co., New York.—“Iron and Steel in All Ages,” by James Swank.
American Iron & Steel Association, Philadelphia.—“The A.B.C. of Iron and
Steel.” Penton Publishing Co., Cleveland, O.—“The Iron Age.” David
Williams, New York.—“The Iron Trade Review.” Penton Publishing Co.,
Cleveland, O.—“High Speed Steel,” by O. Becker. McGraw-Hill Book Co.,
New York.
[Illustration: CONTENTS]
CHAPTER PAGE
I THE EARLY HISTORY OF IRON 1
II THE RAW MATERIALS 17
III RAW MATERIALS (_Continued_) 37
IV THE BLAST FURNACE 52
V A GENERAL GLIMPSE AHEAD 69
VI WROUGHT IRON 91
VII CEMENTATION AND CRUCIBLE STEELS 106
VIII BESSEMER STEEL 123
IX THE OPEN-HEARTH PROCESS 142
X CAST IRON 160
XI CAST IRON (_Continued_) 178
XII MALLEABLE CAST IRON 195
XIII CAST STEEL 214
XIV THE ALLOY STEELS 233
XV THE HIGH-SPEED STEELS 240
XVI THE MECHANICAL TREATMENT OF STEEL 245
XVII THE ROLLING PROCESS 259
XVIII THE ROLLING OF RODS 277
XIX WIRE AND WIRE DRAWING 284
XX THE MANUFACTURE OF PIPE AND TUBES 292
XXI THE MANUFACTURE OF SEAMLESS STEEL TUBES 302
XXII TRANSFORMATIONS AND STRUCTURES OF THE STEELS 310
XXIII THE EQUILIBRIUM DIAGRAM OF THE IRON-CARBON ALLOYS 335
REFERENCES 350
INDEX 355
NON-TECHNICAL CHATS ON IRON AND STEEL
CHAPTER I
THE EARLY HISTORY OF IRON
[Illustration:
PREHISTORIC MAN
]
When in imagination we see the iron maker of early days sitting
cross-legged on his platform between two crude bellows formed from goat
skins with slits for air intakes and nozzles of bamboo, working them
alternately to deliver their pitifully small streams of air into the
hole in the side of a bank of clay which served as a furnace, we wonder
at his patience; and after long hours of such effort his reward was only
a few pounds of iron!
Contrast with this, if you please, the modern blast furnace with its
towering height of 100 feet, its four huge heating stoves, the big
blowing engines which each minute deliver to the furnace 50,000 cubic
feet of blast, and the whole array of dust arresters, gas washers, and
automatic ore and coke handling machinery which are essentials of this
king of modern metallurgical devices. How insignificant seems the output
of the ancient furnace when compared with the daily yield of 500 tons
from this giant of to-day!
How has this come about?
[Illustration:
THE FIRST RAZOR
]
Looking back over the centuries we see a period many thousands of years
ago when primitive man lived in caves or other rude habitations and was
entirely without the implements which we now consider indispensable. The
weapons with which he defended his wife and babes from the wild beasts
and from his warlike neighbors were clubs, wooden spears with perhaps a
bone or shell tip, and hatchets of chipped stone tied with thongs of
hide into a split stick. He managed by ingenious snares and his crude
weapons to provide game and fish for the support of his family.
He did not shave often, for his wife was not as particular in regard to
his appearance as are modern women, but when such a thing happened, a
piece of shell was his razor. The good wife had no steel needles with
which to sew together skins for their crude clothing. If she darned her
husband’s socks it is not recorded, nor did she use steel crochet hooks
in making the “doilies” for their parlor table.
When grain began to supplement the wild game, fruit, and berry diet, it
was broken between flat stones or ground in stone mortars. Fires were
kindled after long and laborious twirling or rubbing together of two dry
pieces of wood. With his stone hatchets and by liberal burning away of
parts by fire he formed his canoes from trunks of fallen trees.
This was the “Stone Age,” and iron and steel were unknown and not to be
heard of for many thousands of years.
In various parts of the world copper always has occurred “native”; i.e.,
in the metallic form and not in combination with other elements as an
earth or ore. As the centuries rolled on, man eventually learned that
this soft red metal could be pounded into thin-edged implements and that
it made more useful tools than those of stone, which his ancestors had
taught him to form. Some of these metal implements were hard and had
fairly good cutting edges, made so by accidental or intentional presence
of tin, and little did he dream that the twentieth century upon finding
his buried bronze implements would think his crude alloy so wonderful
and talk reverently of a “lost art of tempering copper.”
[Illustration:
IMPLEMENTS OF THE STONE AGE
]
Gold, too, became known to him because it also occurs “native.” Its
melting point was low enough that he could fashion it into ornaments,
idols, and other articles for religious purposes. But during the
hundreds of centuries of the “Stone Age” and during much of this—the
“Bronze Age”—copper, bronze, and gold were the only metals used. Though
the smiths became very dextrous in casting and modeling these metals,
they yet knew not iron or steel.
[Illustration:
IMPLEMENTS OF THE BRONZE AGE
]
Round about them during these many centuries, as multi-colored earths or
rocks, were the ores of various metals. They little dreamed that when
rightly treated certain of the heavy red, yellow, or black earths which
lay right at their doors could give up that most useful metal, iron. No
one even had knowledge of such a substance, for, unlike copper and gold,
iron never occurs “free,” having too great a tendency to chemically
combine with other elements, for example, oxygen of the air, with which,
in moist climates, it so readily forms “iron rust.” Besides, its melting
point is high and so much heat and carbon are needed for its “reduction”
from the ore that, during the thousands of years that had gone before,
it had never been produced.
[Illustration:
PRIMITIVE FURNACE FOR SMELTING IRON
]
But one day by accident and under fortunate coincidence of rich ore,
high heat, and plenty of carbon in the form of charcoal from the wood, a
lump of metallic iron was formed underneath a pile of logs which had got
afire and burned fiercely because of a high wind. When pounded between
two stones this new heavy metal, too, was malleable and could be formed
into a spearhead superior to anything yet known. Every one was
interested and an observant one soon “doped out” that certain earths
could be made to yield this new metal, iron.
The art of extracting it spread slowly, each artisan learning from his
neighbor, and, as rich ores were plentiful in many districts, iron
became more and more generally produced. Not only in one country was
this so but evidence shows that in many others—in Egypt, Chaldea,
Borneo, India, China, etc.,—roughly similar processes and crude furnaces
came to be used.
Tubal-Cain, supposedly about 4000 years B.C., is mentioned in the Bible
as an “artificer in iron and brass,” and a wedge of wrought iron was
buried in the great pyramid of Cheops probably as early as 3500 B.C.
This wedge was recently found and is now the property of the British
Museum. The Chinese made use of iron many centuries before the Christian
era, but the Assyrians are supposed to have been the first to use the
metal on a really extensive scale.
[Illustration:
THE PILLAR AT DELHI, INDIA
]
The much discussed pillar at Delhi, India, which is still standing in a
remarkable state of preservation, is twenty-two feet high. It is made up
of several wrought iron sections cleverly welded together. As the
natives regard it with religious awe, metallurgists have been unable to
make thorough investigation and chemical analysis. While the date of its
erection is somewhat in doubt, it is supposed to have been about the 4th
or 5th century A.D.
But from our modern viewpoint those early iron furnaces were queer
things. The first were little more than piles of ore and wood or
charcoal on the tops of hills where a brisk wind would make a hot fire.
Later, with the invention of the crudest of bellows, the smelting was
done in small holes in the side of banks of clay, charcoal made from the
forest trees being used as fuel. Indeed, some of these types of furnaces
still exist and are so operated to-day in neglected districts in Western
India and elsewhere, producing their little five to 100 pound balls of
iron after several hours of tedious work.
When the Romans invaded Britain (now England), they found the Britons
making iron in crude furnaces called bloomaries; and not a great deal of
improvement, except in size, was made up to Queen Elizabeth’s time, when
strict laws had to be enacted to prevent destruction of the forests
which were being denuded for production of charcoal, coke, which we know
so well, not yet having been produced for fuel.
[Illustration:
MILADY’S NEEDLE
]
[Illustration:
THE CATALAN FORGE
]
But the real forerunner of our modern blast furnace was the Catalan
forge, developed in and named from Catalonia, north Spain, where it
originated. The Catalan, however, and all of such crude early furnaces,
including those thus far described, produced a variable kind of what we
now know as “wrought iron,” and our modern “cast” iron did not appear
until about 1350 when, with larger furnaces, an excess of charcoal, with
greater heat and other favorable conditions, the Germans found that the
pasty, difficultly melting metal could be made to absorb carbon enough
to make it easily fusible. _This_ was the secret.
To state the matter in a simple way, iron ore, which is essentially a
natural “iron rust,” is the metal, iron, held in the strong chemical
grip of the gas, oxygen, which normally forms one-fifth of the air we
breathe. As you note, the combination forms a substance entirely unlike
either the iron or the oxygen, but both of these can be regenerated from
it (the ore) by chemical methods. Under influence of high heat (this is
one of the chemical methods, by the way), this stranglehold can be
broken by carbon, of which lampblack, graphite, charcoal, and coke, are
our most familiar examples. The result, in the small, crude, and
inefficient furnaces of long ago was a disappointingly small ball of
crude iron, pasty and scarcely meltable, even at highest heats, but soft
and malleable when cold. As mentioned, it was a variety of what is now
commonly called “wrought iron.”
[Illustration:
A CATALAN FORGE WITH ITALIAN TROMPE, OR WATER BLOWER
]
The ancients got this far.
But this was not “cast iron.” When, however, much more charcoal was
present in the highly heated furnace than was necessary simply to
combine with the oxygen of the ore, the liberated iron greedily absorbed
enough of the extra carbon to change its own nature. The metal then
became very fluid, whereas before it had been pasty and stiff even at
much higher temperatures, or, indeed, at white heat. This liquid iron
could be “cast,” that is, poured into molds and in that way made into
various useful shapes. It therefore became known as “cast iron” because
of this property.
So the brittle metal (cast iron) in our kitchen ranges, for instance, is
only the early malleable form of the metal _surcharged with_ or having a
large amount of carbon (3½ per cent to 5 per cent) in its make-up, and
it is this supercarbon content which confers the fluid quality while hot
and the extreme brittleness when cold. True, there are other important
constituents in our modern cast iron, but for our present purpose they
need not be dealt with.
It has been stated that the ancients got only as far as balls of wrought
iron. They really got further as their very fine sword steels show—the
“Wootz” of India, the “Damascus” of Syria, and later the “Toledo” of
Spain. These they produced by heating rich ore in very small, closed
crucibles with just enough carbon (pieces of wood or green leaves) to
make what we now call “carbon tool steel.” As carbon steel is simply
iron which has absorbed _not over_ 2 per cent of carbon (cast iron
described above has a supersaturation with its 3½ per cent to 5 per cent
of carbon and therefore is entirely different) they were able to make it
in small quantities. When hardened by cooling quickly in water, a
forged-out blade of this product would cleave without dulling its edge a
piece of iron, it is said, or cut cleanly a tuft of silk floss tossed
into the air. These steels attained well deserved renown.
While no one can desire to cast the slightest disparagement on the
product of that period, much of which was excellent, astonishingly so
considering the period, a moment’s consideration will convince one that
modern products not only do not suffer in comparison but in reality are
immensely superior. The ancients had little or no knowledge of the
reason for the proper qualities of their tools and they made the metal
from variable materials in a crude way in such small quantities that
little uniformity was possible. While some of the product was
undoubtedly excellent, much must have been less desirable.
Modern discoveries and inventions, with the great mechanical progress of
the last three centuries and the scarcely half-century-old application
of chemical control, have given during recent years products of great
uniformity and marvelous quality. What can compare with thirty thousand
pound lots of steel turned out from one Bessemer converter each
seventeen minutes during the 24 hours in the day, that is, a total of
1300 tons or 2,600,000 pounds, in which not only the main controlling
element, carbon, but also four lesser ones, silicon, manganese, sulphur,
and phosphorus, are held within extremely narrow limits; or the modern
blast furnace which produces a million pounds each 24 hours, run with
the same certainty of control? Modern high-speed steels which are every
day being made have such high quality that tools formed from them will
stand up for hours working red-hot under a lathe speed of two or three
hundred linear feet per minute taking a deep cut and “plowing out” chips
faster than a laborer can carry them away.
[Illustration:
THE GERMAN STUCKOFEN
]
Modern war armament which has recently been so well advertised is
sufficient answer as to whether modern metallurgy is in advance of that
of centuries ago.
The only necessity for such comparisons is that it seems to be a failing
of many to think that our forefathers were more wise and better in other
ways than we. It was but a few years ago that the fallacious
announcement was made that during archeological excavations in Egypt
there had been found a fully equipped telephone system. The inference
intended to be conveyed, of course, was that Bell’s invention of the
telephone had been antedated many hundreds of years.
[Illustration:
A GERMAN BLAST FURNACE OF FIFTY YEARS AGO
]
The forerunner of the modern steels was crucible steel, first made by
Huntsman about the middle of the eighteenth century. Previous to his
time steel had been made by the “cementation” process by which method
hammered-out bars of wrought iron were given a hard steel crust by
heating to a red heat in charcoal or bone dust. Huntsman’s product began
to come so uniform and of such quality that his competitors were quite
outdistanced. It is related that one of them took advantage of a very
severe storm to gain admittance to the forest forge of Huntsman, who, he
knew, could not refuse shelter at such a time. What he beheld was a very
simple thing—the melting in a clay pot of pieces of cementation steel.
Even to-day the crucible process is holding its own where quality is the
main consideration. It is the method by which practically all of the
tool, automobile, and other special steels of to-day are manufactured
and can hardly be given too high a rating. The newly devised electric
furnace process is the only possible competitor in sight. Of course for
quantity and for lower cost the Bessemer and the open-hearth processes
are the only available ones, but crucible steel has been the mighty
factor in the commercial development of the world—at least until the
latter half of the last century when the two other processes last
mentioned began to acquire honor of their own without, however,
detracting much from the importance of crucible steel as the steel of
“quality.”
[Illustration:
THE FIRST IRON CASTING MADE IN AMERICA
]
Though more interesting than any of the “six best sellers” much of the
subsequent history of iron will have to be passed over at this time. We
can now only mention those very great and revolutionary discoveries and
inventions which led to and absolutely are the basis of the quantity and
excellence of modern irons and steels; namely, the trial for a time of
coke made from pit coal by Dud Dudley of England and its failure which
was turned into a great success a century later (about 1713) by Abraham
Darby; Watt’s invention of the steam engine in 1770 which made possible
application of a strong continuous blast; invention of the process of
“puddling” of iron and of the rolling mill by Cort about 1784; the
introduction by Neilson about 1830 of the hot instead of the cold blast
which increased blast furnace production fourfold; the regenerative
system of furnace heating invented by Frederick and William Siemens; and
the invention of the Bessemer and Siemens-Martin or open-hearth
processes which provided methods for steel making on such an immense
scale that this invaluable material was made available for general
purposes.
It should be repeated that the inventions just mentioned have been of
the utmost importance to the iron industry, and through them only has it
acquired its consequence of to-day. Without them we would not have the
wonderful steel bridges, the skyscrapers, the gigantic steel ships, the
all-steel railway trains, etc., and the hundreds of iron products that
are to-day so plentiful and so constantly about us that we disregard
their presence. It is difficult thus to pass them by, but as most of
them will be referred to in later chapters we must do so.
Early iron making in America is of interest to us and must be briefly
stated.
The colonists were aware of some of the iron ore deposits about them and
sent samples to England where these yielded very fine iron. In 1619 a
company known as the “London Colony” was sent out from England to engage
in the manufacture of iron at Falling Creek, near Jamestown, Virginia,
but three years later all were massacred by Indians. It was many years
before attempt was again made to manufacture iron in Virginia.
About 1637 the General Court of Massachusetts granted to Abraham Shaw
one-half of the benefit of “any coles or yron stone w^{ch} shall bee
found in any comon ground w^{ch} is in the countryes disposing.”
Apparently little resulted from this high-sounding grant.
Real iron making in America began six years later with John Winthrop,
Jr., and his “Company of Undertakers for the Iron Works” which for many
years operated in several localities in the New England States. Heaps of
cinders left from their furnaces may still be seen and testify to their
very extensive operation. One of Winthrop’s men was Joseph Jenks, who
became known as the “Tubal-Cain” of New England. What is claimed to have
been the first casting made on the Western Continent was made by him. It
is a small pot, which was acquired and is said to be still owned by the
family of Thomas Hudson, a descendant of Hendryk Hudson.
Sand molding as used at present was introduced by an ingenious
Englishman, Jeremy Floris, and is vastly superior to the previously used
system of molding in clay. Hollowware began to be extensively produced
about this time.
As the country developed, iron works sprung up here and there and
various kinds of articles came to be regularly manufactured. Of the
early plants we can only mention the Stirling Iron Works, at Warwick,
New York, which made the great 186–ton chain with links weighing 140
pounds each, which spanned the Hudson River near West Point, and where
in 1816 was cast the first cannon made in America; the foundry of Sharp
& Curtenius, in New York, where was cast the first steam cylinder; and
the Trenton Rolling Mills, which first rolled iron as fireproof
structural material.
Before the Revolutionary War the colonies exported considerable bar and
pig iron to Europe, and as early as 1791 England began to foresee that
this country would eventually be a serious rival.
Pittsburg’s great advantage as an iron and steel center has been due to
its proximity to an extensive seam of bituminous coal and ore in
adjacent counties, and to its location so near the Great Lakes, which
provided cheap water transportation for the Lake Superior ores. The
first iron works there was that of Turnbull & Company, which was
established in 1790.
Though Reameur, a Frenchman, is the accredited discoverer of the process
of malleableizing cast iron, Seth Boyden, in a little shop in Newark,
New Jersey, made malleable iron castings a commercial success.
[Illustration:
TWO MODERN BLAST FURNACES, SHOWING SKIP-HOISTS, CAST HOUSES, STOVES,
AND ORE PILE
]
The utilization of the great beds of high grade coking coal of eastern
Pennsylvania, well known as the Connellsville district, and the
discovery and development of the Lake Superior ore deposits have made
the United States the leading producer of iron and steel of the world.
The development of the Birmingham, Alabama, district, also has been a
chapter of great importance but lack of space forbids description at
this time.
We can have only a very slight appreciation of the debt which
civilization owes to iron, for practically everything we see or with
which we daily come in contact contains or has resulted from application
of iron in some way or other. Our cooking utensils and implements (even
the enameled and tinned ones), the kitchen range, the water and drain
pipes, and the furnace and heating plants of our houses, are they not
largely of iron? Our main building materials—the steel frames of
skyscrapers and bridges, and are not even wood, brick, stone, and cement
either shaped, molded, or of necessity made by aid of iron machinery?
The conveyances by which we travel—wagons, automobiles, street cars,
steam railways and steamships—how would they be possible without iron or
steel? Consider the power plants of our factories, of gas and electric
lighting plants, the pumping machinery and distribution systems of water
works, mines, etc. Would the electric current which supplies so much of
our power and light be known to-day or even be possible but for the
magnetic properties of iron? And how many of the materials and articles
which we wear, use, and have about us constantly would be in any way
possible without the wealth of steel machinery and tools which are
available and absolutely necessary for their production?
The iron industry is often spoken of as the barometer of a people’s
civilization. If all iron and iron products and their influence upon the
world should be obliterated, it seems impossible that we could be even
started on the road to civilization.
No matter how we try, probably none of us ever realizes the immensity
and importance of the iron and steel industry with approximately 460
huge blast furnaces here, 5000 cast and malleable iron foundries, about
1000 Bessemer and open-hearth steel and some 3000 puddling furnaces, and
the many thousands of factories which each day are turning the products
of these into rails, plate, wire, pipe, and the infinitude of finished
articles which enter into and are mighty factors of our civilization.
Yet with these furnaces, forges and factories at our very doors, 99.9
per cent of us are entirely oblivious to their wonders and to their
presence except to be annoyed by their noise and smoke. Even the
blacksmiths and their service we scarcely note, though they are daily
fashioning for us a material which is vastly more important and more
wonderful than any of the “Seven Wonders of the World.”
CHAPTER II
THE RAW MATERIALS
A story has it that a minister once visited a friend who was a
zoölogist. Upon realizing for the first time how highly organized a
creature was the humble earth-worm with its three-layer skin covering,
alimentary canal, nephridia or excretory system, reproductory organs,
rude nervous system, and setæ for purposes of locomotion, he exclaimed:
“Wonderful! I had always supposed that worms were only skin and squash.”
With millions of tons of heavy reddish-brown earth from northern
Michigan and Minnesota going by our doors continuously during the
shipping season, the position of most of us is very similar to that of
the minister relative to the earth-worm. We know that something is going
on but we are not aware of its importance or the immensity of it.
Iron Ore
Almost every one knows that there are extensive copper deposits along
the Lake Superior shore of what is now northern Michigan. In the 17th
century word of these was several times taken to Europe where in old
publications was mentioned a huge ingot of copper from which the Indians
chopped pieces with their hatchets. At that early date maps of the
region were drawn which are wonderfully accurate, and, from time to time
over a period of a century and a half, adventurers attempted to gain
wealth in this favored region.
[Illustration:
MAP SHOWING DISTRIBUTION OF IRON ORES OF AMERICA
]
However, despite the definite knowledge of considerable mineral wealth
there and rumored claims of much more, Michigan at the time of her
admission to the Union in 1836 bitterly opposed having what is now the
northern peninsula included within her territory in lieu of a ten mile
wide strip of northern Indiana and Ohio, and it has been said that she
nearly went to war to resist it. Even after the discovery of the iron
ore deposits, no one realized the full importance of the minerals of
this region.
[Illustration:
SHOWING TYPICAL MESABA ORE BED WHICH, AFTER EARTH COVERING IS REMOVED,
BECOMES AN OPEN PIT MINE
]
During the many years of campaigning for Federal help in the building of
a canal at Sault Sainte Marie, the fisheries, valued at $1,000,000 a
year, were given as the leading reason why a canal should be built, and
it was no less a personage than Henry Clay, who, in opposing
appropriation for this purpose, referred to the project and the district
as “beyond the remotest settlement, if not in the moon.”
Now, within eighty years of that time, the annual tonnage of shipping
passing through the Sault Sainte Marie canal is as great as the combined
tonnage from the ports of New York, London, Liverpool, Antwerp, and
Hamburg, and, as against the $1,000,000 value of the fisheries, the
value at the mines of the ore alone shipped from this region amounts to
about $100,000,000 yearly.
[Illustration:
SHENANGO OPEN PIT MINE, CHISHOLM, MINN.
]
[Illustration:
INTERIOR OF A HARD OR LUMP ORE MINE
]
As the copper deposits are mostly along the shore of the lake and the
great iron ore beds occur seven or more miles inland, the latter were
not discovered until Sept. 19, 1844, when William A. Burt, a Deputy
United States Surveyor, noticed that the needle of a solar compass of
which he was the inventor became unreliable. In looking about to
discover the magnetic source which must be the cause of the variation,
members of his party discovered ore just beneath the sod near what is
now Negaunee, Michigan. Inventor-like, Burt’s only concern was to devise
some preventive for future interference by stray magnetic currents. He
simply noted in his book that there was here a deposit of iron ore and
neither he nor any of his party profited or apparently attempted to
profit from the discovery.
[Illustration:
SHOWING ORE BODY AND SHAFT METHOD OF MINING
]
The Indians seem to have had no previous knowledge of these ore
deposits.
The first shipments of ore naturally were samples taken from what
afterward came to be known as the Jackson mine and trials of them in
blacksmiths’ forges were made at Jackson, Michigan (from which town went
the first seriously-minded pioneer, Philo M. Everett), and at Cucush
Prairie. They were soon afterward tried in a blast furnace at Sharon,
Pa.
The first plan was to build forges and manufacture iron near the mines.
So there was established on Carp River a forge to which the ore was
hauled in winter when the ground was frozen. It turned out, however,
that while very good bar iron was manufactured here, it could not be
delivered in Pittsburg at a cost less than $200 a ton. As the market
rate for iron then was but $80 a ton the plan was not financially
successful.
[Illustration:
LIGHTING THE FUSES IN A SHAFT MINE
]
Attention was turned to the shipping of ore to furnaces better located
as regards coke supply and market. It was possible to make this a
profitable undertaking only through cheap ore handling and
transportation. So, to-day, the ore which is better adapted than the
other raw materials for handling by labor-saving devices and
transporting without deterioration, is taken to the coke and limestone,
and to the market for the product. While the weight of coke used is but
half that of the ore smelted, its greater bulk, loss by breakage when
handled in quantities, and deterioration upon exposure preclude its
manipulation in the way which would be necessary to get it to the ore.
[Illustration:
VIEW OF HULL-RUST MINE, NEAR HIBBING, MINN.
]
The marvelous development of this territory into the greatest ore
producer of the world, including the hauling of the first small shipment
on mule back, the building of the plank and then the “strap” railroad
with grades so steep that the small trucks often ran over and killed the
mules, the building of the steam railroad, the successive building and
enlarging of canal locks at Sault Sainte Marie connecting for use of
ever larger and larger ore boats the waters of Lake Superior and Lake
Michigan, and the growth of ore boat fleets to such size that during the
shipping season scarcely ever is one boat out of sight of another over
the entire 800 mile journey from Duluth to the furnaces along Lake
Michigan and Lake Erie, is within the memory of men still living.
Marquette was the shipping point during the earlier days and the history
of this and adjacent regions during the latter half of last century vies
in pioneering flavor with the tales of our early western frontiers, and
with the more recent Yukon mining camps.
[Illustration:
THE ROUTES BY WHICH LAKE SUPERIOR ORES GO TO THE FURNACES
]
The first two mines, the Jackson and the Marquette, have come to be
particularly well known historically. The development of these and other
ranges in northern Michigan and Minnesota, particularly the Menominee,
Gogebic, Vermillion, etc., and, since 1890, the Mesaba and Cuyuna, have
brought about revolutions in ore digging, handling and transportation,
which followed each other with extreme rapidity. The ore carrying boats,
for instance, may almost be said to have jumped from a length of three
hundred to six hundred feet, and the Sault Sainte Marie canal locks were
several times almost immediately outgrown, though rebuilt again and
again, each time so much larger than before that they were deemed
impossible to be outgrown.
[Illustration:
LOADING ORE AT AN OPEN PIT MINE
]
[Illustration:
SHOWING OTHER TYPICAL ORE BODIES, SHAFT MINED
]
Practically all of the ore beds, with the exception of the Mesaba, yield
hard or lump ores and most of them are shaft mines in which mining has
to be done underground and the ore blasted down. Blast furnaces had
never used any but lump ores when along came discovery of the immense
soft ore deposits lying just beneath the surface of the ground over a
region one-hundred miles long in what is known as the Mesaba district of
Minnesota. These soft ores were so accessible and so rich that they drew
the attention of the iron makers of the whole country. But, alas! While
perfectly good in every other respect, they were merely dry powder and
not adapted to blast furnace methods. There were great discussion,
excitement, and ridicule among or at those who invested in these soft
ore mines. Eventually, of course, blast furnace men worked out feasible
methods of converting soft or what are termed “Mesaba Range” ores into
iron in the blast furnace. Those brave spirits, who in the face of
ridicule dared to invest in and develop Mesaba properties, have long
been reaping their financial reward which still shows no sign of
diminishing, as “Mesaba Range” mines are “the” mines of to-day.
[Illustration:
A CLOSE VIEW OF THE HOOVER & MASON UNLOADERS
]
While it may be done, it has not been found desirable to make up the
entire “burden” or furnace charge of soft ores alone as long as lump or
hard ores are obtainable to mix with them, but often more than half is
soft ore.
[Illustration:
STARTING AN OPEN PIT MINE. THE EARTH COVERING IS BEING REMOVED
]
While ores from the shaft mines, called “Old Range” ores, are won by
going down into the earth sometimes as far as 3,000 feet, drilling holes
in the rock, blasting down the ore, and loading it into buggies which
are hoisted to the surface, “Mesaba Range” ores are made available by
simply “stripping” off the thin earth covering, then caving or loading
with steam shovels the soft ore into railway cars. Some of the
illustrations presented show the ore trains and shovels, and the manner
in which the open pits are worked in terraces.
One naturally wonders how it is possible to mine and carry these ores to
the shipping ports of Duluth, Superior, Two Harbors, Marquette, Ashland,
Escanaba, etc., put them aboard ore carrying boats, transport them by
steam power to Milwaukee, Chicago, Gary, Detroit, Cleveland, Pittsburg,
Buffalo, and the many other iron centers and convert them into iron and
steel at a profit. This, of course, is only possible because of the
inventive genius of man. For every operation ingenious machinery has
been constructed which has brought the cost of such operations to its
lowest terms.
[Illustration:
UNLOADING CARS OF COAL OR ORE IS A SIMPLE MATTER WITH THE MODERN CAR
DUMPER
]
[Illustration:
ORE CARS ARE UNLOADED BY GRAVITY AT DOCKS. THE CHUTES THEN CONVEY THE
ORE FROM THE ORE POCKETS INTO THE BOAT’S HOLD
]
By modern methods the cars carrying the ore from the mines are run up
trestle-work into positions above the ore bins high over the docks. The
mammoth ore carrying boats are merely steel shells with quarters for
crew and machinery at bow and stern, and hatches built with exact twelve
foot centers between. They are tied alongside the dock, long steel
chutes, also spaced twelve feet apart, are lowered along most of the
length of the boat and the ore slides into the vessel’s hold evenly all
along as it has to do, else the buoyancy of lighter parts of the boat
might break the frail shell. The entire load of 10,000 tons of ore is
ordinarily taken aboard in less than one hour. Pulling out immediately
the vessel traverses Lake Superior, the Sault Sainte Marie canal, Lake
Michigan or Lakes Huron and Erie as the case may be, and ties up at the
dock at destination. Years ago it would have been unloaded by men with
buckets or wheel-barrows, requiring some days at best. Now, however, the
hatches are uncovered and several ore unloaders with huge clam shell
buckets taking as high as fifteen tons of ore at a “bite” descend like
vultures upon it. Within four or five hours the boat is again empty with
no manual labor having been done upon the ore from the mine to the
furnace pile with the exception of a little heaping up of the ore in the
corners of the boat’s hold, which the ore unloaders could not reach.
[Illustration:
HOOVER & MASON UNLOADERS AT THE ILLINOIS STEEL CO., SOUTH CHICAGO,
ILL.
]
A young man named Alexander E. Brown could not bear, in the old days, to
see the ore so awkwardly unloaded, and in 1880 started the procession of
ore unloading devices. There are now several successful ore unloaders of
which the Brown hoist, the Hoover & Mason, and the Hulett are probably
the best known. With the Hulett unloader the operator has to be an
aviator, as his position is directly above the grab bucket. He descends
into the hold with the bucket, comes up with it and is with it in its
entire journey from the boat’s hold to the dump and back again. It must
be dizzy business.
[Illustration:
THE HULETT UNLOADER. NOTE THE OPERATOR’S HEAD IN WHITE SPOT JUST ABOVE
THE GRAB
]
Time is too precious to hold the boat at the dock long enough that each
bucketful, large as it is, can go directly to its final bin. It is
dropped just back of the unloading machine from which it is again picked
up by other buckets which carry it back toward the furnaces and deposit
it in cement ore troughs awaiting further journey to the ore house, from
which it goes to the furnaces. The empty ore boat immediately coals with
whole car loads of the fuel dumped into chutes leading to her bunkers by
the car dumper and proceeds on her way back to the mines for another
cargo of ore. The round trip, including loading and unloading, requires
but seven days.
[Illustration:
A GOOD VIEW OF THE HATCH SYSTEM OF MODERN ORE BOATS AND FOUR HULETT
UNLOADERS AT WORK
]
As in many other lines of commercial endeavor of to-day, speed and large
tonnage have been the aim and it would seem that in ore handling and
conveying devices the limit has about been reached. The big steam
shovels, gravity docks, ore tanks or boats, and unloading and coaling
devices, with the low cost of water transportation have made our modern
iron and steel preëminence possible. To show the importance to us of
this water transportation, we might mention that the rate for carrying
ore from Lake Superior ore ports to the Lake Erie furnaces has been as
low as $.0007 per ton per mile while the transportation cost by way of a
well operated railway at that particular time was more than $.005 per
ton per mile—more than seven times as much.
[Illustration:
THE REHANDLING BRIDGE WITH STOCK ORE PILE AND BLAST FURNACE AT REAR
]
Though for some years past more than three-quarters of all of the iron
ore used in the United States has come from seven or eight mines in the
northern peninsula of Michigan and the adjacent part of Minnesota, it
must not be understood that the Lake Superior mines are the only ore
deposits in this country. Figures show that such an inference is far
from the truth. It is true, however, that they have made the United
States what it is, the leading iron producer of the world. There are
still immense quantities to be mined on the Lake Superior ranges. Their
heavy production of cheaply handled high grade ore has, of course, held
back development of other districts, which also have great natural
resources. The Birmingham, Ala., region for instance, is a great ore and
iron producer, right now producing the third largest tonnage of any
district in the country. Some time in the not far off future, Alabama
with her great deposits of iron ore, coal, and other natural resources
is going to announce herself in no small voice. New York, Pennsylvania,
Tennessee, and Virginia rank next after Minnesota, Michigan, and Alabama
as ore producers, and several other states of the Union are not paupers
in resources of iron ore.
[Illustration:
HULETT GRAB BUCKETS IN THE HOLD OF AN ORE BOAT
]
We should not get so enthusiastic over our ore supply and iron
production as to think that other countries are devoid of such material.
Almost every civilized country has ore enough that it does pretty well.
With many the trouble is that the ore has objectionable constituents or
that supply of cheap fuel is not available. Germany has large deposits
of iron ore, but until the invention of the basic Bessemer process about
1870 she was handicapped because of the high phosphorus content of her
ore. The basic processes, both Bessemer and open-hearth, allow of the
removal of this phosphorus during the conversion into steel, and they
therefore brought Germany to the front as an iron producer.
The excellence of Sweden’s iron and steel has long been known the world
over. Sweden produces approximately one per cent of the world’s total
production of iron and steel, but her ore has been of such high grade
that iron made from it has maintained its position as a standard for use
in the manufacture of highest grade crucible steels. The very finest
steels for cutlery and tools, and even the softer grades of steel of
northwestern Europe, have been made from Swedish iron as a base.
Iron ore, of course, is classified by geologists and chemists into
varieties with such names as hematite, magnetite, siderite, etc., which
here little concern us.
To be worked at a profit, the iron content of the ore must be high with
the smallest possible amounts of undesirable impurities, particularly
phosphorus, sulphur, and silica. There are, however, certain impurities
which are not undesirable, for instance, lime, which will act as a flux
and neutralize the effect of some of the undesirable impurities. For
these reasons the prices for iron ore are based on the iron content and
modified by the relative amounts of undesirable and desirable
impurities. Phosphorus is almost a domineering factor and at present
approximately fifty cents a ton more is paid for Bessemer ore (that
containing less than .050 per cent phosphorus) than for non-Bessemer
ore. As might be expected the best ores have been the first used and the
grade is constantly falling. Instead of the 66 per cent iron ores of
some years ago those coming nowadays contain not much more than 59 per
cent of iron and the Bessemer ores described above are getting scarcer,
so that for some years practically all of the furnaces have been mixing
with them as much higher phosphorus ore as could be used without pushing
the phosphorus content of the mixture over the allowable limit.
We often hear people surmising what is to become of us when all of the
iron ore of this planet has been used. There is no harm in taking stock
of resources and in this case it does us much good. It happens that each
time the count is taken of iron ore available and that which under
future and better methods of working can be utilized, we find ourselves
immensely better off than the previous report had made out and we have
less cause to worry about the future. The last inventory was taken by
the extremely ambitious International Geological Congress held at
Stockholm, Sweden, in 1910. It shows that the world yet has enough rich
ore to make 10,192,000,000 tons of iron, and, a further supply of ore
for 53,136,000,000 tons of iron, which could be used if necessary.
So we will get along for a while yet.
CHAPTER III
THE OTHER RAW MATERIALS
[Illustration:
OLD CHARCOAL KILNS, NEAR NEGAUNEE, MICH.
]
Since the beginnings of iron manufacture, charcoal has been a favorite
fuel. Though during the past two centuries coke has grown to be the
standard, with anthracite and some few bituminous coals finding use in
certain favored localities, charcoal may be considered the fuel which
developed the iron industry, at least until recent years.
Charcoal
As most of us know, charcoal is completely charred wood, usually hard
wood, though sometimes resinous or other soft woods are used. Of
well-dried timber more than 50 per cent by weight is moisture. This and
certain other constituents are driven off by heat in the absence of air,
which process is usually called “destructive distillation.”
By primitive methods a considerable part of the wood was completely
burned and wasted during the production of charcoal. Stacked in piles or
long rows the cut wood was well covered with earth, except for a small
opening at the top through which the fire was lighted down a center
cavity left to the bottom of the pile. The air coming in through the
opening at the top was sufficient to keep the wood smoldering. After a
period, which had been shown by experience to give the best results, the
opening was closed and the fire smothered.
[Illustration:
BEEHIVE COKE OVENS
]
Brick ovens of the beehive shape were built at a later date where
considerable charcoal was to be made. These were operated on much the
same general principle as the _meillers_ or earth-covered piles,
described above. The fire was lighted at the bottom of the central
cavity of the corded wood, the only air at first coming from the top,
though later in the process a little was admitted through holes in the
walls. After about ten days, when gas ceased to come off, the kiln was
tightly closed for a period of twenty days more for the fire to die out
and the charcoal to cool.
By both of these processes valuable constituents were burned or driven
off by the heat and lost. These were mainly methyl alcohol, acetic acid,
and wood tar.
Modern industry so emphatically disapproves of any waste of materials
that apparatus has been devised to produce charcoal which allows of
recovery of the by-products at the same time. In northern Michigan,
which is practically the only district in the United States in which the
charcoal industry as an industry still survives, long steel tubes or
retorts are built with brick fire-boxes under each end, much as a
stationary boiler is set. Into these retorts are run steel cars loaded
with the wood. The retorts being closed, the heat drives or distills off
the moisture and gaseous compounds through pipes connecting them with
condensing apparatus. After about twenty hours the wood has been
charred, the doors of the kilns are suddenly opened and the cars are
rushed into other and similar retorts for cooling, while fresh loads of
wood replace them in the first.
[Illustration:
STANDARD BEEHIVE COKE OVEN
]
[Illustration:
BEEHIVE OVENS
]
As may be surmised vast quantities of wood and of wood-producing land
are required for extensive charcoal manufacture, and this is the most
serious problem for the manufacturer of charcoal. Several square miles
of timber land must be cut over each year and the wood efficiently
transported in order to operate a large plant profitably.
Pig iron as a _by-product_ is a rather novel idea, but that is
practically what the charcoal pig iron produced in our Lake Superior
region is. Several companies operate wood distillation plants for the
production of methyl alcohol, acetic acid, acetate of lime, etc., and
use their charcoal in the manufacture of charcoal pig iron from the ores
so close at hand.
The very low sulphur content and the small amount of ash have been the
great advantages possessed by charcoal over other solid fuels. Resulting
characteristics made charcoal pig iron a former favorite for manufacture
of certain articles such as chilled car wheels, etc., and it, therefore,
brought a higher price than coke pig iron. During recent years, however,
by careful selection of coal and improvements in the coking process the
sulphur and ash of coke have been so reduced that charcoal has not so
great an advantage as formerly. Charcoal iron to-day brings only about
$1.50 per ton more than coke iron; whereas, the differential a few years
ago was as great as $5.00 or $6.00 per ton.
Charcoal is quite fragile and structurally weak, so much so that blast
furnaces for its use cannot be built higher than sixty feet; whereas,
the great strength of coke allows them to be built to exceed one hundred
feet in height with correspondingly increased output. What this means
may be realized by every one conversant with the demands of modern
industry.
Coke
[Illustration:
CHARGING COAL INTO THE OVENS
]
As charcoal is completely charred wood, so coke for analogy’s sake may
be said to be completely charred coal, practically always of the
bituminous type. By “baking” bituminous coal at a cherry-red heat, its
volatile constituents are driven off as the well-known “coal-gas” of
almost every small town, and a strong, brittle and porous material or
coke residue is left. If the baking is done without any admission of air
to the retort, practically none of the coal burns and the “cake” or coke
which is left contains the ash of the original coal and what is known as
the “fixed carbon,” i.e., carbon which cannot be distilled or driven off
by heat alone, though it would burn were air admitted.
The gases or volatile constituents which are given off consist mainly of
moisture and a mixture of gaseous chemical compounds, which are known as
“hydro-carbons.” These contain that part of the carbon of the original
coal which does not remain as “fixed carbon” in the coke.
[Illustration:
QUENCHING AFTER COAL HAS BEEN COKED
]
Just why some coals will coke while others of apparently the same
composition as shown by the chemist’s analyses, will not, but instead of
the hard brittle mass will leave a heap of brown or black powder, is not
as yet definitely known. It is easy enough for chemists to determine
with accuracy the amounts of hydrogen, nitrogen, oxygen, carbon,
sulphur, and other elements; but it is a difficult and perhaps an
impossible matter to determine just how these elements are “hitched up”
in the very complex mineral, coal,—one of the most complex substances
which we know.
Various theories have been advanced in the attempt to explain the coking
quality. A bulletin of the United States Geological Survey claims that
the relative percentages of hydrogen and oxygen in the coal determines
it; others have held that it depends upon the compounds of a tarry or
asphaltic nature present. The fact remains that some coals coke without
trouble, while others do not coke at all. As yet the only real way to
tell whether a new variety of coal will or will not coke is to try it.
Since 1713, when Abraham Darby in England succeeded in introducing it as
a substitute for the fast disappearing charcoal for use in blast
furnaces, coke has become the standard fuel. It is very strong and will
bear up under the great weight of iron ore and limestone with which the
furnace is charged. So furnaces for use with coke may be built much
larger than those in which charcoal is to be the fuel. The porous nature
of coke allows it to burn rapidly with intense heat, so that the output
of an iron works is greatly increased through its use—a very desirable
thing in these days of big things. It has its disadvantages, of course,
mainly high sulphur, a deleterious substance for which molten iron,
unfortunately, has a voracious appetite, and a rather high percentage of
ash which must be fluxed out. But all in all, it is a very desirable
fuel for blast furnace and other metallurgical purposes, as is shown by
the fact that it is used in the production of about ninety-nine per cent
of all iron and steel now made.
[Illustration:
DRAWING THE COKE
]
What is known as the Appalachian coal region produces coal for more than
seventy-five per cent of the coke made in the United States. This region
includes the strip of territory extending from Western Pennsylvania and
Ohio down to Tennessee, Georgia, and Alabama. The famous Connellsville
district is a part of this region.
Illinois and Indiana have a great deal of coal, which, however, has
rather indifferent coking qualities. Almost constant experimentation has
been carried on in the attempt to induce these semi-coking coals to
coke. The best that has so far developed is the use of a considerable
percentage of them in admixture with coals of good coking qualities.
Such mixtures yield quite satisfactory coke.
The Beehive Oven Process
[Illustration:
LARGE PIECES OF COKE
]
In the old days there was no desire or incentive to avoid waste of coal
resources. If during the coking process some air got into the oven and
part of the coal was burned, or if all of the gas given off was wasted,
it did not matter. There was plenty more of coal and the thing desired
was to get the requisite coke in the quickest and cheapest way.
[Illustration:
WHERE COALS ARE PULVERIZED AND MIXED FOR COKING
]
In Western Pennsylvania, Ohio, and Virginia, were great beds of high
grade coking coal. In this region and particularly around Pittsburg,
numerous blast furnaces and steel mills grew up. The coke for these was
made in the most convenient way—in the wasteful beehive ovens.
[Illustration:
BATTERY OF BY-PRODUCT COKE OVENS, SHOWING GAS-COLLECTING MAIN
]
As the name signifies, these ovens or retorts are brick chambers shaped
like beehives. In the larger plants they are built either in single rows
against long hills or in double rows back to back. Over the tops of the
ovens in each row runs a car called a charging “lorry.” Coal is poured
from the bottom of this through a hole in the top of each oven while it
is still hot from the preceding charge. No air gets in except that
admitted through the hole in the oven top and a small slit left over the
one side door, through which the coke is drawn when the coking process
is finished. The heat of the oven starts the distillation of the
moisture and the volatile compounds which escape through the hole in the
oven top. The small amount of air admitted burns a little of the coal
and gas and raises the temperature of the oven to that required for
coking.
After 48 or 72 hours a spray of water is thrown in over the glowing coal
to quench the fire. The partially cooled coke is drawn through the open
door, sorted and loaded into cars for shipment.
Though this method of coking is a very wasteful one, it yet produces the
larger quantity of the coke made in the United States. However,
conditions are rapidly changing and it will not be many years before the
much less wasteful “by-product” process gains the ascendency. By 1914 it
had already come to produce about twenty-five per cent of the total coke
made here, and since that date the percentage has been rapidly
increasing.
The By-product Process
[Illustration:
TOP OF OVENS WITH CHARGING BIN AND LORRY AT FAR END
]
By this system of coking a greater yield of coke is obtained and most of
the by-products are saved. The value of the latter depends largely, of
course, upon local conditions, such as transportation, costs of the
material, cost of labor, and available market for the coke oven gas.
They are usually figured as having a value of $1.50 per ton of coal
coked, equivalent to a total of $71,000,000 per year for the coal coked
in the United States.
[Illustration:
LORRY FOR CHARGING COAL INTO OVENS
]
The ovens and apparatus required are considerably more expensive, but,
since this industry has developed in this country during the last
twenty-two years to a point where one-quarter of all of the coke
manufactured is made by the by-product process, there can be no doubt
that it is a profitable proposition and that eventually the wasteful
beehive ovens will be a thing of the past.
Practically all of the types of by-product coke ovens in use have been
developed in Germany or Belgium, where circumstances forced earlier
conservation of resources than in this country. The three best known
types are the Semet-Solvay, the Otto Hoffman, and the Koppers—the latter
a recent arrival. They differ mainly in details of construction and
operation.
[Illustration:
MACHINE FOR PUSHING COKE FROM OVENS
]
In a general way a “battery” of coke ovens consists of from 40 to 80
long narrow brick-walled chambers placed closely side by side with
heating flues or “checker-work” between them. The fire for the baking
process is in these flues, which are interconnected, and the heat
developed is sufficient to drive off the moisture and volatile
substances of the coal in the narrow chambers just on the other side of
the brick walls. Charging is done by a “lorry” as in the beehive
process. After from seventeen to twenty-four hours at a red heat, the
coke is “pushed” from the ovens, one after another, by an electric ram
which enters at one end. The 30 × 7 × 1½ foot block of glowing coke
emerges from the other end, where, breaking under its own weight into
good-sized pieces, it falls into a steel car on a track just beneath. A
spray of water quenches it and it is taken to the storage bins to be
sorted.
Rich coal-gas is the main by-product. That which comes off during the
first seven hours is the richest and has the greatest illuminating or
“candle” power. After washing free from dust, tar, ammonia, etc., the
gas is usually run into holders or tanks from which it is distributed
for use for illuminating or for heating purposes. That which comes off
during the latter part of the coking period has much less of those
constituents which give illuminating value. It has good heat value,
however, and as fuel is required for keeping the ovens up to the coking
temperature, this poorer gas from the coking chambers is switched into
and burns in the flues between the coking chambers as mentioned.
Thus the larger part of the gas is sold to customers, usually in the
city near which the ovens have been located, and the poorer part is
utilized in heating the ovens and the steam boilers which run the plant.
[Illustration:
QUENCHING CAR AWAITING ITS LOAD
]
The coal tar, which the German chemists have made so famous through its
manufacture into the almost endless variety of beautiful dyes, is
another of the by-products which is recovered by this, but burned or
lost in the beehive oven process. From a long main over the tops of the
ovens which connects the gas pipes, the tar flows along with the gas to
the scrubbing and gas cleaning plant, where by rather intricate
operations it is freed from other substances.
In this country much of the tar is used for building purposes, etc., and
some as fuel, but not much has been made into the chemical products for
which Germany is so famous. For a long time a few dyes and other
chemical compounds have been made here from coal tar. Since the early
days of the war in Europe and the cessation of imports of such materials
on this account, there has come about considerable expansion in their
manufacture here; but it is doubtful if the time is yet ripe for a
wholesale entry into the manufacture of these coal tar “derivatives,”
especially the very extensive variety of dyestuffs.
Naphthalene and benzol from which many other chemical compounds as well
as munitions of war can be made, are among the by-products.
[Illustration:
QUENCHING THE COKE
]
Most of the ammonia which the corner drug store sells, comes from the
by-product manufacture of coke. The largest part of the ammonia which is
produced in the process, however, is manufactured into sulphate of
ammonia, a well-known fertilizer.
Coal
Anthracite or hard coal has been used in certain districts in the United
States, especially in New Jersey and eastern Pennsylvania. It is not an
ideal fuel as it is too solid to burn rapidly, spalls or cracks under
heat and interferes with the blast. Since 1860 when coke became
available here much less coal has been used, though some is yet used in
admixture with coke. Some bituminous coals which contained little tarry
matter also have been used in this way.
Fluxes
Limestone, the rock which is ordinarily used for fluxing purposes, needs
no introduction to any of us. As the marble of statuary, the material of
which oyster and other sea shells and the white tombstones of our
cemeteries are composed, it is well known. Any of these varieties of the
material may be used for fluxing purposes, but usually it is limestone
which is quarried for the purpose or obtained as chippings or spalls
from building blocks.
[Illustration:
COKE GOING FROM QUENCHING CAR TO BINS
]
[Illustration:
LOADING COKE IN BOX CAR
]
The active agent, which produces the chemical or fluxing action in the
blast furnaces, is carbonate of calcium (lime) of which limestone
contains about 98 per cent. Dolomite is a mixture of carbonates of lime
and magnesium, about 53 per cent of the former and 45 per cent of the
latter, and is sometimes used in place of limestone. Fluor spar, a rock
composed of calcium and fluorine, is used in small quantities in some of
the metallurgical processes. It is a very powerful flux.
CHAPTER IV
THE BLAST FURNACE
Up the dark tower shoots the elevator with its “buggy” of coke. Its
speed is not conditioned to the comfort of man, who is not supposed to
be a passenger, except the occasional laborer whose duty as buggy-pusher
requires his presence on twelve-hour shifts at the top. So we, whose
exploratory proclivities have led us at the office to sign away our
lives for grant of a pass to the blast furnace, find our breath about
taken from us with the first mad dash into the darkness of the climb.
That stone tower had looked much more innocent from below.
But now the rickety elevator has as suddenly emerged into the light
again and stopped abruptly at the charging floor which extends across
the chasm to the top of the furnace.
As the smoke-begrimed buggy-pushers rush the buggy of coke across to the
furnace bell, we have opportunity to notice that we are a full hundred
feet above ground. Just here, seemingly so close that we can put our
hands on them, in a row, are the round steel tops of the four stoves
which are for the purpose of preheating the blast. The huge pipes, dust
arresters, tanks, and buildings, all so necessary to the plant, look
almost like a tangled mass from our high station, while the charging
floor upon which we stand, the shoulder-high steel fence around it, the
furnace top, the adjacent stoves and in fact everything for a half mile
around us, is colored yellow-red with iron dust. We understand the
reason for this when the buggies of ore which have succeeded the coke
are dumped into the funnel-shaped depression around the conical bell at
the center. As the huge bell is lowered and the charge slides in there
is considerable blowing out of the fine ore dust, which, in fact,
continually “oozes” out of all crevices under the heavy pressure of the
blast inside.
[Illustration:
HAND-FED BLAST FURNACE
]
Day and night, month in and month out, during the life of the fire-brick
lining of the furnace, this routine of charging, first coke, next the
theoretically correct charge of analyzed iron ores, then limestone, in
rotation goes on. From 6 A.M. to 6 P.M. and from 6 P.M. to 6 A.M. on
twelve-hour shifts, alternating gangs of laborers push the buggies
across to the furnace top, dump and return them to the elevator already
up with another load.
The incessant quiver of the iron plates beneath our feet with the
rumbling and groaning from the inside of this monster are disquieting
and the thought constantly recurs: “What if this powerful creature
should just now rebel, as quite occasionally occurred in the old days
when all of its moods had not been so well understood?” For this king of
metallurgical devices, though gentle and obedient as a lamb under proper
treatment, is a domineering fury when it has dyspepsia as occurs
whenever its attendants are remiss in their attentions to its diet.
“Those explosion doors just below the furnace top—are they in working
order and would they be adequate?” But whether, as in recorded
instances, the whole furnace top is torn off as evidence of its wrath,
or its displeasure is exhibited in a milder way, we much prefer to be
absent. The thought is disquieting and we are glad to leave.
[Illustration:
CINDER OR SLAG FLOWING INTO LADLES
]
[Illustration:
“FIREWORKS” AT THE CINDER-NOTCH
]
Unwilling to test again the elevator for the downward trip, we take to
the narrow iron stairway which leads from the top of the furnace to the
ground. But this is worse than the elevator, for the stair treads are
very narrow and made only of three slender iron rods. To our palpitating
hearts they seem to give very insecure foothold and the gaps show that
there is nothing but earth beneath us, and that a hundred feet below. To
make matters worse, before we creepingly get half way down some visitors
below have stopped to watch our slow and trembling steps and our nervous
clutch on the low “stingy” hand rail. We hear them innocently inquire of
one another why we move so slowly. We wish that we could appear brave,
especially before the women in the party, but we could not move with
greater alacrity if our lives depended upon it.
Once below again with our breath regained, things are more interesting.
The red-hot molten slag which has just been tapped out is running from
the furnace along a long trough into a ladle six feet high resting upon
a car on the railway track alongside the “cast house,” as the huge
structure which houses the lower part of the furnace is called. This
smoking, molten slag stream gives off a powerful sulphur smell and
throws a lurid glare over everything round about.
[Illustration:
THE SLAG DUMP
]
The furnace superintendent is just explaining to the other party that
No. 2 furnace has been “hanging” for a couple of days and is still
dangerous. “If you realize,” said he, “the great weight of coke, ore,
and limestone in that furnace, you can see what a splash it would make
if the clogged, bridged part with all above it should fall suddenly into
the molten pool in the ‘hearth’ of the furnace. Two years ago No. 1
broke out and the molten metal caught and burned to death one of our men
and injured several others. ‘Hanging’ does not occur when the furnace is
working right, but failure of the charge to come down evenly is very
serious sometimes. A blast furnace is like a coquette; she has to be
handled just so, and even then you cannot always be sure what she is
going to do next.
“Oh yes,—where does the iron come from? Well, we often read nowadays of
a man and his ‘affinity.’ Now the chemist has used that word ‘affinity’
for years to describe the liking or attraction of one chemical ‘element’
for another; in fact, that is where this recent colloquial use of the
word originated. Iron ore is nothing more nor less than metallic iron,
the element, chemically combined with oxygen, another element which
constitutes one-fifth of the air we breathe. Under conditions produced
in the blast furnace, though for centuries wedded to iron, oxygen
deserts him for her ‘affinity’ carbon, which is best known to you as
coke, coal, or charcoal. The iron, now free, becomes molten at the high
temperature encountered (about 2800° to 3000° F.) and descends into the
‘hearth’ or bottom of the furnace, while oxygen and her new partner
escape out of the top of the furnace in gaseous form. When molten iron
has accumulated in the hearth to the extent desired it is tapped out as
you will soon see.
[Illustration:
DIAGRAMMATIC SKETCH OF BLAST FURNACE
]
“The limestone charged has ‘affinity’ for dirt and certain other
impurities of the ore which it removes in molten form as the ‘cinder’ or
‘slag’ which is there running into the ladle. When cold, this cinder is
a dark greenish-black, glassy substance, which of recent years has come
to be used to a certain extent in the manufacture of Portland cement but
is mainly used for filling-in purposes. Some is crushed and utilized in
concrete mixtures and for road building. In Chicago considerable land
has been ‘made’ by dumping slag into the lake, and South Chicago is
reported as standing on a swamp which has been filled in with slag from
the steel works there.
[Illustration:
OLD-FASHIONED PIG BED
]
“This immense furnace is simply a strong steel shell lined two or three
feet thick with fire bricks. At the ‘bosh,’ which is the region where
the greatest heat is produced, hollow bronze plates are inserted among
the bricks of the lining through which circulating cold water keeps the
bricks from being fused.
“The hot gases which are led from the upper part of the furnace through
that brick-lined ‘downcomer’ are burning in three of those ‘stoves’ to
heat them, while cold, clean air from the blowing engines is coming
through the other stove, which, ten minutes ago, when it was put on
blast, was the hottest of the four. It is now giving up part of its
accumulated heat to the blast on its way to the furnace. Switching the
cold incoming air every little while from a partially cooled stove
through a hotter one while allowing the former to reheat, provides
continuous blast of a temperature of 800° to 1400° F. The ‘hot blast’
idea originated about 1830 with James Neilson, a gas engineer of
England, and its introduction revolutionized the blast furnace industry
and made the highly efficient modern practice possible.
[Illustration:
FURNACE AND SAND BED READY FOR IRON
]
“This big pipe above our heads which encircles the furnace is the
‘bustle pipe.’ It also has to be lined with fire bricks. The hot blast
is distributed by this ‘bustle pipe’ to the tuyères here—these L-shaped
pipes—which shoot it directly into the furnace. Through the peep-holes
in the tuyères you can get a glimpse of the dazzling interior of the
furnace. The blast of heated air is causing the coke to burn fiercely
there so that it melts the iron, which farther up in the furnace has
been forced to part from the oxygen, as I explained to you.”
[Illustration:
LAYOUT OF BLAST FURNACE PLANT
]
But now six men with a long steel bar are starting to break through the
two or three feet of clay with which the “tap hole” of the monster
furnace is plugged, and our informant hurries away.
[Illustration:
SKIP HOIST AT WORK. SKIP DUMPING INTO HOPPER
]
For ten minutes with strong sledge blows the tappers struggle to break
through the plug of burned clay. Meanwhile the monotonous whistle of the
heavy blast into and through the bustle pipe and tuyères goes on and the
discharge pipes of the water-cooling plates empty into the gutter around
the furnace bottom the water which has been circulating to keep the
inner bricks from fusing.
[Illustration:
SECTION OF BLAST FURNACE SHOWING FILLING ARRANGEMENT, BINS AND ORE
BRIDGE
]
And now a shout and the strong red glow throughout the cast house tell
us that the tap hole is open and the iron is running down the main
channel of the sand bed. Past the plugged entrances to the lateral
branches runs the molten iron stream to the end of the cast house nearly
one hundred feet distant where it divides, filling the laterals on each
side and from them running into many open molds arranged like the teeth
of a comb. Each of these molds is about five feet long. They form the
“pigs,” and the laterals what are known as “sows.” As each lateral and
its molds fill, the lateral ahead of it is opened and the process
repeated, laterals and pigs filling up simultaneously on opposite sides
of the main channel till the whole cast house floor is filled nearly up
to the furnace with the red smoking metal.
[Illustration:
SAND CAST PIG IRON
]
There are few sights more glorious than the cast house with bed just
filling with metal, and especially is it so at night. The strong
yellow-red light of the flaming metal issuing from the furnace and the
intense glow of that already in the bed illuminates everything in and
about the building. But already, before the furnace is empty, the
workmen are spraying with water the earliest cast pigs. Covering them
with a light layer of sand they venture upon them with thick-soled shoes
and break the “pigs” from the “sows” with sledge hammers.
This is the old-fashioned “sand cast” pig iron. After remelting in the
cupola furnaces of neighboring towns and casting into stove parts or
other forms it is known as “cast iron”; through “puddling” in
reverberatory or special furnaces it becomes “wrought iron”; after
decarbonizing treatment in steel furnaces of various design it is
changed into the wonderful material called “steel.”
Pig iron is thus the _intermediate_ or _semi-raw_ material from which
practically all of our various iron and steel products are made and the
transition product through which they pass.
[Illustration:
MACHINE CAST PIG IRON
]
But the romantic period of the hand-fed furnace and the gloriously
beautiful pig beds at casting time are rapidly passing; in fact, are
almost past. Modern “skip-hoists” carrying automatically dumped buckets
or cars charge the furnace more economically than even low-waged
laborers can do it. The two charging cars alternate, one filling at the
bottom in the stock house while the other is dumping through the double
bell at the top of the furnace. Furnaces are now tapped by power driven
drills which make quick work of a formerly difficult operation. Instead
of running it into the sand bed, the molten iron from the furnace is
nowadays run into ladles alongside the cast house as is the cinder which
was described above. If the metal is to be made into pigs it goes to the
pig casting machine, where the traveling iron molds very quickly convert
the entire cast into “chilled cast” pigs. At the top of the incline
these pigs which have been cooling under sprays of water fall from the
traveling molds into railroad cars below which deliver them to the
consumer.
[Illustration:
HOW PIG IRON IS NOW CAST INTO TRAVELING MOLDS
]
[Illustration:
UPPER END OF CASTING MACHINE WHERE PIGS ARE DUMPED FROM TRAVELING
MOLDS INTO RAILWAY CARS
]
In large steel works the greater part of the molten iron is not cast
into pigs at all but while yet molten is directly charged into the
open-hearth or Bessemer furnaces which convert it at once into steel of
which the greater part is made into plate, rails, or other shapes before
being allowed to cool. Even the gas is recovered nowadays. Its journey
through the “dust arrester” rids it of most of its dust, after which
filters and washers clean it thoroughly. That not required for the
heating of the stoves is used for firing the steam boilers about the
plant and as fuel for batteries of huge gas engines which in large
plants have been installed to generate low-priced electric current.
It should be noted that in modern practice iron is mined, loaded,
transported to the furnaces, unloaded, charged and made into pigs or
converted into steel and even into the finished products with
practically no hand labor, all operations being performed by machinery.
Though a “direct” process for converting the ore into wrought iron or
steel has been long sought, a method has never been found, except that
used in the very small way followed by the old iron-workers with their
crude furnaces. It has always proved commercially advantageous to make
pig iron in the blast furnace as an intermediate step and then by a
second step convert it into wrought iron, steel, etc. So the ore is
brought from the mines to the furnace, the coke and limestone arrive
from another region, and batteries of huge blast furnaces through the
country make from them the pig iron.
BLAST FURNACE DATA AND ANNUAL PIG IRON PRODUCTION
═════╤══════════╤══════════╤══════════╤════════════════════════════════
Year │ Average │ Average │ Average │ Tons Pig Iron Produced During
│Height[1] │ Cu. Ft. │ Daily │ Year
│ of Blast │ Capacity │Output in │
│ Furnace │ of Blast │Tons Each │
│ │ Furnace │ │
─────┼──────────┼──────────┼──────────┼──────────┬──────────┬──────────
„ │ „ │ „ │ „ │ United │ Great │ Germany
│ │ │ │ States │ Britain │ and
│ │ │ │ │ │Luxemberg
─────┼──────────┼──────────┼──────────┼──────────┼──────────┼──────────
1850 │ 30′│ 2000│ 29│ 565,000│ │
1860 │ │ │ │ 920,000│ │
1870 │ │ │ │ 1,865,000│ 5,963,000│ 1,391,000
1880 │ 70′│ 8200│ 117│ 3,835,000│ 7,749,000│ 2,729,000
1890 │ 90′│ 18200│ 360│ 9,000,000│ 7,904,000│ 4,658,000
1900 │ 100′│ 24000│ 600│13,790,000│ 9,003,000│ 7,550,000
1905 │90 to 100′│ 24000│ 600│23,000,000│ 9,746,000│10,988,000
1910 │90 to 100′│ 24000│ 600│27,300,000│10,380,000│14,495,000
1912 │90 to 100′│ 24000│ 600│30,000,000│ 9,037,000│17,869,000
1913 │90 to 100′│ 24000│ 600│31,000,000│10,654,000│19,292,000
─────┴──────────┴──────────┴──────────┴──────────┴──────────┴──────────
Footnote 1:
No advantage has been found in furnaces having a height of over 110
feet.
The chemical laboratory plays a very important rôle in iron making.
Analyses made of each car or boat load of ore by the furnace chemists
representing the buyer must check very closely the analyses made by the
mine chemists for the seller, as the price of every ton of ore is based
on its iron and its phosphorus contents and the percentages of certain
other constituents present. In calculating the “burden” or charges for
the furnace, each of these constituents is estimated in pounds actually
present per charge, losses or gains during the journey through the
furnace are allowed for, and by combining the various ores the charges
are so made up that the resulting iron will be of certain desired
composition. The closeness of actual composition obtained to calculated
results is startling.
Truly this is an age of efficiency.
Though technical information and statistics are not to be inflicted upon
our readers to any extent in this series of articles, it may not be
amiss to give one table of interesting figures which show the increase
in height and capacity of furnaces from 1850 to the present time. During
this period the annual production of pig iron in the United States has
risen from 565,000 to 31,000,000 long tons (2240 pounds). It will be
noted that since 1887, when we passed Great Britain, the United States
has been the champion pig iron producer of the world.
The record production of a single blast furnace to date was that of one
of the United States Steel Corporation’s furnaces at Duquesne, Penn.,
which produced 900 tons of pig iron in one day.
CHAPTER V
A GENERAL GLIMPSE AHEAD
We have arrived at the parting of the ways. From the vast beds of iron
ore, the coal fields and coke ovens, and from the quarries of limestone,
all roads have led to the blast furnace. This we have visited and we now
know how pig iron is made.
From this point the several paths diverge. One by one we are to follow
them to get acquainted with the interesting country which they traverse
and the regions to which they lead. However, before choosing any one of
these paths for our first trip, it will be to our advantage to pause, to
study for a moment our position and get a general view of the country
ahead of us. We should know the relative locations and importance of the
places we are going to visit, for only by getting a comprehensive idea
of the general plan of this ferrous (meaning iron) world, can we
understand to the best advantage the position of each of the main
products, wrought iron, steel, cast iron, malleable iron, etc., and
acquire a satisfactory knowledge of them. In the last chapter pig iron
was called the intermediate stage between the ore and the finished iron
product, and such the sketch given shows it to be. It is only as an
intermediate product that pig iron has value, for nowhere in the
commercial world has it a purpose except as a material to be chemically
and structurally transformed into other materials which may themselves
be used without further transformation. It is desirable that we fully
realize the position of this very important semi-raw material, pig iron,
before passing on to the study, one at a time, of the refined ferrous
products which _are_ susceptible of _direct use_ in our commercial life.
[Illustration:
ELECTRIC ARC, MICROSCOPE, AND CAMERA, PART OF METALLOGRAPHIC APPARATUS
]
From Table A (page 71) it will be seen that from blast furnace metal
several well-known products are formed by the various processes of
refinement. Each of these methods of purification may be said to result
in a certain general composition and structure which give to the
material formed its character and properties.
The Metallographic Method of Classification
By aid of the microscope it is possible actually to look into the
structure of these materials.
TABLE A
IRON AND STEEL PRODUCTION OF THE UNITED STATES FOR 1912
┌──────────┬─┬──────────┬─┬────────────────────┬─┬─────────────────────┐
│ │ │ │ │ 3000 Furnaces. │ │ Bar Iron, Sheets. │
│ │ │ │ │ WROUGHT IRON. │ │ Pipe, Boiler Tubes, │
│ │ │ │↗│ 1,637,582 tons │→│Stay Bolts. Stock for│
│ │ │ │ │ (750,000 tons Pig │ │ Cementation and │
│ │ │ │ │ Iron used). │ │ Crucible Steel. │
└──────────┴─┴──────────┴─┴────────────────────┴─┴─────────────────────┘
┌──────────┬─┬──────────┬─┬────────────────────┬─┬─────────────────────┐
│ │ │ │ │ 102 Plants. │ │ Rails, Axles, Rods, │
│ │ │ │ │ BESSEMER STEEL. │ │ Wire, Nails. │
│ │ │ │↗│ 1,327,900 tons │→│ Structural Shapes. │
│ │ │ │ │(11,000,000 tons Pig│ │ Pipe, Bar Steel, │
│ │ │ │ │ used). │ │ Steel Castings. │
└──────────┴─┴──────────┴─┴────────────────────┴─┴─────────────────────┘
┌──────────┬─┬──────────┬─┬────────────────────┬─┬─────────────────────┐
│ │ │ │ │ 78 Plants. │ │ │
│ │ │ │ │ ACID OPEN-HEARTH │ │Rails, Plate, Axles. │
│ │ │ │↗│ STEEL. │→│ Steel Castings. │
│ │ │ │ │ 1,139,221 tons │ │ Miscellaneous. │
│ │ │ │ │ (500,000 tons Pig │ │ │
│ │ │ │ │ used). │ │ │
└──────────┴─┴──────────┴─┴────────────────────┴─┴─────────────────────┘
┌──────────┬─┬──────────┬─┬────────────────────┬─┬─────────────────────┐
│ │ │ │ │ 100 Plants. BASIC │ │Rails, Plate, Axles, │
│ IRON ORE │ │ │ │ OPEN-HEARTH STEEL. │ │ Rods. Structural │
│59,975,000│↖│ │↗│ 19,641,502 tons │→│Shapes. Pipe, Boiler │
│ TONS │ │ │ │(11,500,000 tons Pig│ │ Tubes, Bar Steel, │
│ │ │ │ │ used). │ │ Steel Castings. │
└──────────┴─┴──────────┴─┴────────────────────┴─┴─────────────────────┘
┌──────────┬─┬──────────┬─┬────────────────────┬─┬─────────────────────┐
│ │ │ │ │ │ │ Stove, Radiator, │
│ │ │466 BLAST │ │ │ │ Machinery Parts, │
│COKE USED │ │ FURNACES │ │ CAST IRON. │ │ Valves, Pipe │
│35,491,292│←│ PIG IRON │→│ 6,750,000 tons │→│Fittings, etc., Bath │
│ TONS │ │ MADE │ │(5,000,000 tons Pig │ │ Tubs, Sinks, │
│ │ │29,726,937│ │ used). │ │ Carwheels, │
│ │ │ TONS │ │ │ │ Brakeshoes, Rolls, │
│ │ │ │ │ │ │ Misc. Castings. │
└──────────┴─┴──────────┴─┴────────────────────┴─┴─────────────────────┘
┌──────────┬─┬──────────┬─┬────────────────────┬─┬─────────────────────┐
│ │ │ │ │ │ │ Railway Car and │
│ CHARCOAL │ │ │ │ MALLEABLE IRON. │ │ Agricultural │
│35,436,017│↙│ │↘│ 1,000,000 tons │→│ Machinery Castings, │
│ BUSHELS │ │ │ │ (750,000 tons Pig │ │ Pipe Fittings, │
│ │ │ │ │ used). │ │ Miscellaneous │
│ │ │ │ │ │ │ Castings. │
└──────────┴─┴──────────┴─┴────────────────────┴─┴─────────────────────┘
┌──────────┬─┬──────────┬─┬────────────────────┬─┬─────────────────────┐
│LIMESTONE │ │ │ │ 108 Plants. │ │Tool Steels, Ordnance│
│15,092,166│↙│ │↘│ CRUCIBLE STEEL. │→│Materials, Bar Steel,│
│ TONS │ │ │ │ 121,517 tons. │ │ Castings. │
└──────────┴─┴──────────┴─┴────────────────────┴─┴─────────────────────┘
┌──────────┬─┬──────────┬─┬────────────────────┬─┬─────────────────────┐
│ │ │ │ │ 14 Plants. │ │ Rails, Bar Steel, │
│ │ │ │↘│ ELECTRIC STEEL. │→│ Castings. │
│ │ │ │ │ 18,309 tons. │ │ │
└──────────┴─┴──────────┴─┴────────────────────┴─┴─────────────────────┘
┌──────────┬─┬──────────┬─┬────────────────────┬─┬─────────────────────┐
│ │ │ │ │ 4 Plants. │ │ │
│ │ │ │↘│ OTHER STEEL │→│ Tool Steels, Bar │
│ │ │ │ │ PROCESSES. │ │Steel, Miscellaneous.│
│ │ │ │ │ 2853 tons. │ │ │
└──────────┴─┴──────────┴─┴────────────────────┴─┴─────────────────────┘
* Figures for Pig Iron Used are Estimates, no Conclusive Statistics
being available.
[Illustration:
PART OF A CHEMICAL LABORATORY WHERE 6,000 SAMPLES A YEAR ARE ANALYZED
]
By cutting in any direction through a piece of metal or alloy, polishing
the surface exposed very clean and smooth and then etching (corroding)
slightly with acid, the exposed grains of the metal may be seen when
sufficiently magnified. Not only may the grains of metal be seen, but
certain of the other constituents which are present are visible.
Photographs, also, can be taken by attaching a camera to the microscope.
This method of analysis, which is known as “metallography,” has proved
as valuable an aid to the metallurgist as it proved to the geologist
when applied to the study of rocks and geological specimens.
[Illustration:
MACHINE FOR DETERMINING STRENGTH OF IRON AND STEEL BARS
]
The metallography of wrought iron, cast iron, malleable iron and steel
differentiates them to us with considerable accuracy, as is shown by a
glance at the accompanying photomicrographs, as photographs taken
through the microscope are called.
For our immediate purpose of gaining a general knowledge of the relative
positions of these products, this method of analysis probably cannot be
excelled.
After the processes of manufacture of these materials have been taken up
one by one in later chapters, the photomicrographs will be even better
understood than now, as will the differences of chemical composition and
physical properties of the alloys, such as strength, hardness,
brittleness, forging quality, etc. The photomicrographs are given at
this time to show that the materials are structurally very different and
to aid in the general classification.
To make them comparative, all have been taken at the same magnification
of seventy diameters; i.e., the microscope has made everything shown
just seventy times as large as it actually was in the alloy.
[Illustration:
No. 198. PHOTOMICROGRAPH OF SAND CAST PIG IRON. THE THICK BLACK LINES
ARE GRAPHITE FLAKES
]
As stated before, the alloy _pig iron_ normally contains from 3 to 5½
per cent of carbon. This was absorbed during the journey through the
blast furnace. As long as the iron was molten all of this carbon was in
the “combined” form; i.e., in chemical combination with the iron itself.
Cold iron, however, cannot retain in the chemically combined form as
much carbon as does molten iron, so, during the solidification and
cooling of the alloy, more or less of its carbon was precipitated, i.e.,
thrown out of solution and from chemical combination with the iron, the
amount depending mainly upon the speed of the cooling. It appeared then
as the “free” carbon (crystalline graphite) which remained distributed
throughout the alloy and may be seen as the jet black flakes in
photomicrograph No. 198.
Every pure metal is supposed to be composed of crystals or grains which
would have been of the true cubic form if the severe internal pressure
during solidification and cooling had not distorted them.
Photomicrograph No. 99b represents quite well a pure metal. It is that
of an extremely mild steel made by special methods in the open-hearth
steel furnace. It is so highly refined that it can hardly be called
steel at all but is often called “_open-hearth iron_” or “_ingot iron_.”
It is probably the purest iron on the market in commercial quantities
to-day. While in the chemical laboratory iron of considerably greater
purity can be made, for a commercial product this is remarkably pure,
seldom containing more than ¼ per cent of elements other than the metal,
iron.
[Illustration:
No. 99b. OPEN-HEARTH IRON. PROBABLY THE PUREST COMMERCIAL IRON PRODUCT
]
The boundary lines of the crystals or grains may be plainly seen. Each
grain should show practically white. The dark parallel lines, the dots,
and the grayish portions result from inequalities in the polishing and
etching.
[Illustration:
No. 1d. SECTION OF WROUGHT IRON CUT LENGTHWISE OF THE BAR. BLACK
PATCHES AND FILAMENTS ARE “SLAG” OR “CINDER”
]
After noting the appearance of photomicrograph No. 99b, which is of a
nearly pure iron, one need have no difficulty in realizing that pig iron
and the steels are alloys and not simple metals. The truth is that of
all alloys some of the well-known iron products which we are studying
are by far the most complicated, much more so than are the nonferrous
alloys, which include the brasses, bronzes, babbitts, German silver,
etc.
[Illustration:
PHYSICAL TEST ROOM WHERE 15,000 SPECIMENS ARE TESTED EACH YEAR
]
This should not worry us, however, for we shall not attempt to follow
them into their complications.
[Illustration:
No. 3b. STEEL CONTAINING .10 PER CENT OF CARBON. THE CARBON IS IN THE
BLACK PATCHES
]
The important point just now is to observe the crooked black flakes of
crystalline graphite in this photomicrograph No. 198. It is largely
because of these flakes of brittle, soft graphite that pig iron and the
cast irons are so fragile. One has no difficulty in realizing that these
flakes, which cut in every direction through the metal, make it
structurally weak, especially toward a sudden blow.
No. 1d shows a typical section of _wrought iron_ cut lengthwise of the
rolled bar. It will be noted that, as in photomicrograph No. 99b, there
are no graphite flakes.
In the process of wrought iron manufacture practically all of the carbon
of the original pig iron is burned out, leaving little besides the iron
itself and some viscous cinder or slag. In the rolling or hammering out
of the resulting white-hot “bloom,” the slag enclosures which remain
after the squeezing process are extended lengthwise through the bar.
Upon observing with the microscope any prepared section of wrought iron
which has been cut lengthwise of the bar the filaments of slag may be
plainly seen, all parallel or practically so. When such filaments of
slag can be discerned in a longitudinal section it is practically an
absolute indication that the material in question is wrought iron.
[Illustration:
No. 22c. STEEL WITH .50 PER CENT OF CARBON
]
[Illustration:
No. 36a. STEEL WITH 1.98 PER CENT OF CARBON
]
Photomicrograph No. 3b is that of _mild steel_ containing .10 per cent
(⅒ per cent) of carbon. Here we have neither the graphite flakes of No.
198 nor the slag filaments of No. 1d. We can plainly see the boundary
lines of the grains. The irregular dark patches which are evenly
distributed throughout are the defining features of steel. In what might
be called a chemical-mechanical combination, these dark patches contain
all of the small percentage of carbon which gives to carbon steel its
definite properties.
During the manufacture of this alloy all but a small amount of carbon is
eliminated by burning it out, as happens with wrought iron. But the
steel is _molten_ or _fluid_ when finished and the slag which has been
formed floats on top and is also eliminated, which does not occur with
wrought iron, which is thick and pasty at its finishing temperature.
[Illustration:
No. 74. GRAY CAST IRON. THE CROOKED BLACK LINES ARE GRAPHITE FLAKES
]
Therefore, steel contains no graphite and no slag but has only the small
percentage of carbon which was purposely put back to give to it its
valuable properties.
[Illustration:
No. 92d. SEMI-STEEL, A STRONGER GRAY CAST IRON
]
Photomicrograph No. 22c is typical for steel which contains .50 per cent
(½ per cent) of carbon. The irregular patches containing the carbon are
much larger and more frequent. It will be seen, therefore, that through
metallography the various iron alloys, to a considerable extent, may be
analyzed as well as classified.
Photomicrographs Nos. 74 and 92d represent soft and stronger grades of
ordinary _gray cast iron_ respectively. It will be noted at once that
both much resemble pig iron in structure, as of course they should, for
simple remelting in the cupola does not effect much modification in
composition or structure.
Occasionally castings are made from molten iron direct from the blast
furnace, but such practice is not very satisfactory and is little done.
It forms probably the only exception to the statement made above that
pig iron has no useful purpose in the commercial world except as
something to be transformed by some refining process into another
material.
[Illustration:
No. 132. GRAY CAST IRON 200 YEARS OLD
]
[Illustration:
No. 109. MALLEABLE CAST IRON BEFORE ANNEALING
]
The remelting for the well-known cast iron is usually of such selected
brands of pig iron and cast scrap as will produce cast iron best adapted
to the purposes intended. The resulting alloy still contains 3 per cent
or 3½ per cent of carbon.
No. 132 is interesting. It is a photomicrograph of a piece of cast iron
which is approximately 200 years old. From the standpoint of the
metallurgist it is the same as other photomicrographs of cast iron, the
difference in appearance resulting probably from casting conditions,
etc.
_Malleable cast iron_ is made much as is gray cast iron except that it
is brought to such a composition that sections of castings made from the
melt show a white fracture when broken. Castings of gray iron of course
show a gray fracture. The former are extremely hard and are as brittle
as glass until they have gone through a careful long anneal or heat
softening process. By maintaining them at cherry-red heat away from air
for sixty hours or more and cooling slowly, they become “malleable”;
i.e., not brittle at all but capable of considerable distortion without
cracking.
[Illustration:
No. 35. MALLEABLE CAST IRON ANNEALED
]
From the viewpoint of malleability, malleable iron may be considered to
occupy a position between gray cast iron and steel.
Photomicrographs Nos. 109 and 35 are those of malleable iron before and
after the annealing treatment. The former shows the typical structure of
_white cast iron_, while the latter plainly shows the minute lumps of
pure carbon and the surrounding grains of pure iron metal, the two
having become divorced through the annealing process. Note that the
large amount of carbon in this, the “temper carbon” form, does not make
the alloy brittle through cutting of the grains as does the crystalline
graphite form of carbon.
The above gives very briefly the most essential points in the
classification of the irons and steels from the structural standpoint.
True it has not entered into the vast complications of the higher carbon
or tool steels which are those which will take a “temper”; i.e., the
tool steel’s quality of hardening by quenching in water from a cherry
red heat. The simpler points of these will be taken up later. But enough
has been given that we now understand something of the relative
positions of the great main products, some reasons therefor, and their
general structures.
[Illustration:
NO. 8. BAR ROLLED FROM SCRAP; CONTAINS BOTH WROUGHT IRON AND STEEL
]
It has been seen that their micrographs quite definitely differentiate
them and probably no one will have difficulty in recognizing and naming
such specimens. As testing this it may be interesting to note
photomicrograph No. 8. Two different iron alloys made up the material
from which this photomicrograph was taken. Apparently the bar was one
made by heating and rolling together scrap metals. Such material is on
the market. The photomicrograph shows that some of the scrap used was
wrought iron and some of it mild steel of about .08% carbon.
Classification by Chemical Analysis and by Physical Tests
So far not much has been said about composition except that upon it to a
great extent depends the structure and physical properties of the alloy.
Composition and physical characteristics as well as structures are
necessary to a fair
TABLE B—CHEMICAL COMPOSITION AND PHYSICAL PROPERTIES
═══════════════╤═══════╤═════════╤════╤══════╤═══════════╤═══════════
│ Per │Per Cent │Per │ Per │ Beginning │ Tensile
│ Cent │Graphitic│Cent│ Cent │ of │Strength[3]
│Silicon│ Carbon │ │Total │Freezing[2]│
│ │ │ │Carbon│ │
───────────────┼───────┼─────────┼────┼──────┼───────────┼───────────
1. Sand Cast │ 2.50│ 3.75│ .25│ 4.00│ 2100│
Pig Iron │ │ │ │ │ │
2. Machine │ 2.50│ 3.00│1.00│ 4.00│ 2100│
Cast Pig │ │ │ │ │ │
Iron │ │ │ │ │ │
───────────────┼───────┼─────────┼────┼──────┼───────────┼───────────
3. Open-Hearth│ .03│ .00│ .02│ .02│ 2740│ 50,000
Iron │ │ │ │ │ │
4. Wrought │ Slag│ .00│ .05│ .05│ 2740│ 52,000
Iron │ │ │ │ │ │
│ │ │ │ │ │
5. Mild Steel │ .05│ .00│ .10│ .10│ 2740│ 55,000
│ │ │ │ │ │
│ │ │ │ │ │
│ │ │ │ │ │
6. Medium │ .05│ .00│ .25│ .25│ 2720│ 65,000
Steel │ │ │ │ │ │
7. Rail Steel │ .05│ .00│ .60│ .60│ 2680│ 90,000
8. Low Carbon │ .15│ .00│ .75│ .75│ 2660│ 100,000
Tool │ │ │ │ │ │
Steel │ │ │ │ │ │
9. Medium │ .15│ .00│1.00│ 1.00│ 2630│ 120,000
Carbon │ │ │ │ │ │
Tool │ │ │ │ │ │
Steel │ │ │ │ │ │
10. High Carbon│ .15│ .00│1.25│ 1.25│ 2600│ 135,000
Tool │ │ │ │ │ │
Steel │ │ │ │ │ │
11. Very High │ .15│ .00│1.50│ 1.50│ 2560│ 124,000
Carbon │ │ │ │ │ │
Tool │ │ │ │ │ │
Steel │ │ │ │ │ │
12. High Steel │ │ .00│1.75│ 1.75│ 2530│
│ │ │ │ │ │
13. White Iron │ │ .00│2.50│ 2.50│ 2460│
│ │ │ │ │ │
14. White Cast │ .70│ .10│2.65│ 2.75│ 2400│ 41,000
Iron │ │ │ │ │ │
│ │ │ │ │ │
15. Annealed │ .70│ 2.70│ .05│ 2.75│ │ 40,000
Malleable│ │ │ │ │ │
Cast Iron│ │ │ │ │ │
│ │ │ │ │ │
16. Cast Iron │ 1.00│ 1.00│2.00│ 3.00│ 2330│ 35,000
for │ │ │ │ │ │
Chilled │ │ │ │ │ │
Castings │ │ │ │ │ │
17. Semi-Steel │ 1.75│ 2.80│ .40│ 3.20│ 2300│ 35,000
│ │ │ │ │ │
│ │ │ │ │ │
18. Gray Cast │ 2.00│ 3.10│ .30│ 3.40│ 2200│ 25,000
Iron │ │ │ │ │ │
│ │ │ │ │ │
│ │ │ │ │ │
│ │ │ │ │ │
19. Soft Gray │ 2.50│ 3.30│ .15│ 3.45│ 2200│ 23,000
Cast Iron│ │ │ │ │ │
│ │ │ │ │ │
│ │ │ │ │ │
───────────────┴───────┴─────────┴────┴──────┴───────────┴───────────
═══════════════╤═════════════╤══════════╤═════════════════
│Elongation[4]│ Welding │ Principal Uses
│ │Properties│
│ │ │
│ │ │
───────────────┼─────────────┼──────────┼─────────────────
1. Sand Cast │ │ │Further Refining.
Pig Iron │ │ │
2. Machine │ │ │Further Refining.
Cast Pig │ │ │
Iron │ │ │
───────────────┼─────────────┼──────────┼─────────────────
3. Open-Hearth│2½″ │ Good │Bars, pipe,
Iron │ │ │ sheet.
4. Wrought │2¼″ │ Fine │Bars, pipe,
Iron │ │ │ boiler tubes,
│ │ │ stay bolts.
5. Mild Steel │2½″ │ Good │Bars, pipe, wire,
│ │ │ sheet,
│ │ │ shafting,
│ │ │ tubes.
6. Medium │2¼″ │ Good │Bars, plate,
Steel │ │ │ structural.
7. Rail Steel │1¾″ │ Fair │Rails, gears.
8. Low Carbon │1¼″ │ Fair │Hammers, cold
Tool │ │ │ chisels, saws,
Steel │ │ │ springs.
9. Medium │1″ │ Poor │Lathe tools,
Carbon │ │ │ chisels,
Tool │ │ │ drills, dies,
Steel │ │ │ springs.
10. High Carbon│1⅛″ │ Slight │Lathe tools,
Tool │ │ │ chisels, files,
Steel │ │ │ saws.
11. Very High │ ¼″ │ Slight │Razors, lancets,
Carbon │ │ │ graving tools,
Tool │ │ │ saws for steel.
Steel │ │ │
12. High Steel │0 │ None │Dies for wire
│ │ │ drawing.
13. White Iron │0 │ None │Dies for wire
│ │ │ drawing.
14. White Cast │0 │ None │For
Iron │ │ │ malleableizing,
│ │ │ carwheels.
15. Annealed │ ½″ │ None │Railway and
Malleable│ │ │ agricultural
Cast Iron│ │ │ castings,
│ │ │ fittings.
16. Cast Iron │0 │ None │Rolls, gears,
for │ │ │ brakeshoes.
Chilled │ │ │
Castings │ │ │
17. Semi-Steel │0 │ None │Gears, steam
│ │ │ cylinders,
│ │ │ valves.
18. Gray Cast │0 │ None │Machine parts,
Iron │ │ │ grates,
│ │ │ radiators,
│ │ │ valves, soil
│ │ │ pipe.
19. Soft Gray │0 │ None │Stoves,
Cast Iron│ │ │ hollowware,
│ │ │ pipe fittings,
│ │ │ misc.
───────────────┴─────────────┴──────────┴─────────────────
Footnote 2:
In degrees Fahrenheit. For our purposes, the same as the _melting
points_ of the alloys.
Footnote 3:
Pounds required to pull apart lengthwise a bar one inch square.
Footnote 4:
Number of inches that a bar eight inches long will stretch before it
breaks.
[Illustration:
IRON AND STEEL TEST PIECES AND INSTRUMENTS USED IN MEASURING THEIR
SIZE, ELASTIC LIMIT AND ELONGATION
]
understanding of the subject. There is given therefore, a table showing
approximate comparative values of chemical compositions and physical
properties of the alloys under discussion.
It should be distinctly understood that the figures given in Table B are
_approximate_ only and are intended to be average, or rather, perhaps,
typical.
There are all sorts of conditions which in practice modify the figures
given in the table, and criticism may be maintained justly against some
of the too specific statements which here it was necessary to make. The
classification is given with considerable hesitation and only because,
arranged in this way, it brings out existing relationships which
otherwise would escape notice and which we cannot afford to overlook.
But please do not gain the impression that these alloys are divided into
distinct classes. There are no dividing lines at all. One group merges
into the next so gradually that it is impossible to tell where the one
ends and the other begins.
It is to be hoped that no one will make himself miserable by trying to
digest these rather formidable figures of Table B all at one sitting.
They are given mainly for comparison and for reference. It is suggested
that after noting carefully the similarities and differences to which
attention is called, they be reserved until the processes of manufacture
of the various alloys are taken up one by one. Reference to these
figures on those occasions should be profitable.
The main points to be noted at this time are:
1. Open-hearth iron is practically pure iron, having no constituents or
slag inclusions which materially affect its properties.
2. Wrought iron, for all practical purposes, is pure iron except for its
content of slag. It is the only one of the iron family which does
normally contain slag.
3. Neither open-hearth iron nor wrought iron contains carbon in
appreciable quantities.
4. The distinguishing and active element of the steel family is carbon.
With increase of carbon the hardness of the alloy increases as does its
tensile strength, but the ductility (elongation or stretch) decreases.
5. Other conditions being equal, the more carbon the alloy contains the
more easily it melts; i.e., at lower temperature. So the purer irons
such as open-hearth iron, wrought iron, and mild steel (i.e., steel with
low carbon, usually under 0.15 per cent) have relatively high and the
cast irons lower melting points.
TABLE C—PERCENTAGES OF THE SO-CALLED “METALLOIDS” BY WEIGHT AND BY
VOLUME
═══════════════════╤═══════════╤══════════╤═════════╤══════════
│ Silicon │Manganese │ Sulphur │Phosphorus
│ │ │ │
───────────────────┼────┬──────┼────┬─────┼───┬─────┼────┬─────
│Wt. │ Vol. │Wt. │Vol. │Wt.│Vol. │Wt. │Vol.
───────────────────┼────┼──────┼────┼─────┼───┼─────┼────┼─────
Gray Cast Iron │2.50│=10.0=│ .70│ =.7=│.09│=.36=│ .75│=3.2=
Semi-Steel │1.75│ =7.0=│1.00│=1.0=│.10│=.40=│ .35│=1.5=
Malleable Cast Iron│ .70│ =2.8=│ .50│ =.5=│.15│=.60=│ .17│ =.7=
Cast Steel │ .30│ =1.2=│ .50│ =.5=│.05│=.20=│ .04│=.17=
Mild Steel │ .05│ =.2=│ .40│ =.4=│.04│=.16=│ .03│=.13=
Open-hearth Iron │ .03│ =.1=│ .02│=.02=│.01│=.06=│ .01│=.06=
Wrought Iron │1.20│ =5.0=│ .08│=.08=│.01│=.04=│ .15│=.65=
───────────────────┴────┴──────┴────┴─────┴───┴─────┴────┴─────
═══════════════════╤═══════════╤═══════╤══════════
│ Total │Iron[5]│ Total
│ Carbon │ │Metalloids
───────────────────┼────┬──────┼───────┼───┬──────
│Wt. │ Vol. │ Wt. │Wt.│ Vol.
───────────────────┼────┼──────┼───────┼───┼──────
Gray Cast Iron │3.45│=12.1=│ 92.5 │7.5│=26.4=
Semi-Steel │3.20│=11.2=│ 93.6 │6.4│=21.1=
Malleable Cast Iron│2.75│ =9.6=│ 95.7 │4.3│=14.3=
Cast Steel │ .35│ =1.2=│ 98.8 │1.2│ =3.3=
Mild Steel │ .10│ =.35=│ 99.4 │ .6│ =1.2=
Open-hearth Iron │ .02│ =.07=│ 99.9 │ .1│ =.3=
Wrought Iron │ .05│ =.17=│ 97.7 │2.3│ =5.9=
───────────────────┴────┴──────┴───────┴───┴──────
Footnote 5:
These alloys contain small amounts of other elements so these
percentages of iron are a little high, though approximately correct.
Note the purity (high percentage of iron) of the open-hearth iron.
6. In the cast irons, the carbon occurs not in one only but in two
different forms; i.e., as graphitic carbon, commonly called graphite
(Gr. C.) and the combined form (C. C.). The sum of these is usually
between 3.00 per cent and 3.50 per cent. It is not so much the total
amount of the carbon that causes the differences in structure and
physical properties which have been noted in 1 and 2 and in 16 and 17
above, as it is the _relative proportions_ in which these two varieties
occur.
[Illustration:
EXHIBITION CASE SHOWING FOUR WELL-KNOWN IRON ALLOYS WITH THEIR
METALLOIDS
]
7. No one knows just when, with increase of carbon, steel ceases to be
steel and becomes white cast iron. There is no definite dividing line
either in chemical or physical properties. The changes are extremely
gradual throughout the scale. Aided by the microscope, modern physical
chemistry has disclosed the fact that alloys of iron with carbon
“freeze” from molten to solid condition according to two different laws.
The change from one to the other occurs somewhere between 1.7 per cent
and 2.2 per cent of carbon as is described in Chapter XXII. This is our
only basis for calling alloys with less than 2 per cent of carbon,
steels, and those with greater amounts, cast irons.
8. For our immediate purposes the other “metalloids” or constituents are
of secondary importance and will not be taken up now. From this it must
not be understood that they can be slighted by the metallurgist and
furnace man in his work. They cannot. Every one of them is of importance
and must be accounted for in the final product or trouble results.
Volumetric Analysis of the Iron Alloys
There is a way in which we may visually get a very intimate idea of the
relative composition of these alloys.
The cabinet of which a photograph is given is partitioned into four
sections. Each one of these contains a bar and six specimen jars. As you
may or may not be able to read from the labels, the bars, all of exactly
the same size, are soft cast iron, semi-steel (a stronger cast iron),
annealed malleable iron and cast steel. The six jars above each bar
contain the exact amounts of the various constituents other than iron
which are in the bar beneath.
As none of the constituents except manganese are as heavy as iron, their
volumes per unit of weight are correspondingly greater. Putting it into
approximate figures we have the percentages by weight and by volume
shown in Table C.
This means, of course, that _of the cast iron plates of your cook stove
or steam or water radiators fully one-quarter (26 per cent by volume) is
not iron at all_ but brittle substances of little or no strength. These
elements, silicon, sulphur, phosphorus, and carbon, are commonly called
“metalloids.” While the first three named are not in “free” form in the
alloy and therefore allow of some doubt as to just the space they
require, we have good reason to suppose that the figures given are not
far from correct.
With such a volume of weakening constituents and particularly with the
graphite flakes cutting through and separating the iron grains as the
photomicrographs show, can one wonder that cast iron is fragile—more so
than steel or wrought iron?
To sum up, naming only the most familiar alloys and the two or three
qualifying features of each which stand forth with particular boldness,
we have:
_Pig Iron_—Very High Carbon. Brittle.
_Gray Cast Iron_—High Carbon. Brittle.
_Malleable Cast Iron_—High Carbon. Made Malleable by Annealing.
_Wrought Iron_—Slag. Little or no Carbon. Very Malleable without
Annealing.
_Mild Steel_—Very Low Carbon. No Slag. Very Malleable without Annealing.
_Carbon Tool Steel_—Medium Carbon. No Slag. Of Medium Malleability.
So steel which is not called “iron” at all is a very pure metal in
comparison with gray and malleable cast iron and usually has a larger
percentage of the chemical element, iron from which all are derived,
than has the well-known wrought iron itself.
CHAPTER VI
WROUGHT IRON
We of America, and especially of the West, never have been particularly
devoted to the study of our genealogies. However, it is likely that most
of us at one time or other in our mind’s eye have seen that little
country town back in Massachusetts or New York State, whence came our
forebears. Since those days of long ago, when, with you upon his knee,
grandfather waxed reminiscent and related tales of his boyhood, haven’t
you many times wished that Father Time could carry you back with him a
hundred and fifty years and allow you for a few hours to walk among
those good people with their quaint dress and customs? How interested
you would be in the relatives and friends whose queerly transcribed
verses, bearing date of a century and a half ago, adorn the yellow pages
of your great aunt’s autograph album, which is one of your treasures!
To the present rather unsentimental world we admit of much the same sort
of reverent feeling as we traverse the paths, centuries old, of the
wrought iron region. Here is something which links us with the
antiquities and we “take off our hats and tread lightly” over the lands
and the centuries in and through which this primal material, wrought
iron, has been produced.
But we come back to a plain statement of the case. Let not the fact be
overlooked that it is wrought iron which in some form or other has been
made during the forty or more centuries that iron has been known. It
undoubtedly was the pioneer of the iron family and through all of the
centuries it has maintained its importance. From it as a base during the
earlier centuries were made the steels for which the ancients were so
famous, and similar practice has prevailed from those early times up to
the present day.
[Illustration:
WROUGHT IRON BAR Steel has no “fiber” and can not be split in this
way.
]
We say that the iron made by the ancients was in reality a variety of
what we now term wrought iron, for it fulfilled three of the principal
requirements for wrought iron, viz., it was not necessarily melted
during production, it was a malleable metal, and it contained cinder or
slag, but practically no carbon.
Early iron and steel, of course, were comparatively rare, so much so as
to be available only for implements of war and for particular purposes.
Moreover, all of the steel made was of the high carbon, hard variety,
none of it being of the low carbon sort which we know as the soft or
mild steels. The latter have all come since the invention of the
Bessemer process in 1855 and the Siemens-Martin (the open-hearth)
process of a few years later.
As you will remember the next development in the iron industry was the
use of the larger blast furnace and much more fuel, with higher
temperatures and the production of cast iron which, by reason of the
absorption of considerable carbon from the fuel, was fluid enough to run
out of the furnace. And, as you also know, this blast furnace metal has
come to be the great intermediate product in what has come to be known
as the _Indirect Process_ of making iron and steel.
[Illustration:
PICKLING IN WEAK ACID EXPOSES THE “FIBERS” OF WROUGHT IRON
]
As pig iron contains 3½% or more of carbon, while wrought iron has none
or comparatively little of this embrittling element, any process for
converting the pig iron into wrought iron must eliminate the carbon.
Following the advent of pig iron as a commercial product several
furnaces and processes were developed to turn it into wrought iron. All
of these were based upon the elimination of the carbon by _oxidation_,
which means the burning of the carbon in presence of oxygen of the air.
The best known ones were the Walloon or Swedish Lancashire process, the
South Wales process, the Finery Fire, the Running-Out Fire and the
Charcoal Finery or Knobbling process. The Walloon furnace, largely used
in Sweden, is shown in the illustration on page 94.
While by the process generally used to-day wrought iron is more easily
and cheaply produced, it must be said that by the processes above named
has been made as fine iron as has ever been produced. The Sheffield
manufacturers, whose high grade steel made by the crucible process, is
so widely and favorably known for cutlery, tools, etc., in many cases
yet demand the Walloon process iron made from high grade Swedish ores as
the starting point for their product.
[Illustration:
BAR IRON WILL STAND SEVERE BENDING EVEN AFTER IT HAS BEEN NICKED
]
[Illustration:
SWEDISH WALLOON FURNACE
One of several types of furnaces that preceded the Reverberatory. Some
of them are still used.
]
In these furnaces which consisted mainly of a shallow hearth filled with
glowing charcoal, with tuyères for blast supply, the pigs of iron were
heated until molten. At high temperatures the metalloids of pig iron
will readily burn in a blast of air. After the air blown over and upon
the molten metal has burned out most of the carbon, the main metalloid,
the melting point of the purified metal becomes so much higher that the
heat of the furnace is not sufficient to keep it molten. It becomes more
and more pasty and stiff therefore. This is what the iron maker calls
“coming to nature.” It is the signal that the carbon is about gone and
that he must be careful or the iron will suffer in quality. At this
stage he sees to it that the mass of pasty iron is well protected by
glowing charcoal until it is removed to be hammered or worked into a
bar.
This in general is the process which was used from about the fourteenth
century up to 1783 when Cort invented the reverberatory furnace which
has since been the type generally used.
In the processes just referred to, all very similar, it should be noted
that the iron was _in contact_ with the fuel, which, therefore, _had to
be charcoal_, the fuel with little or no sulphur.
[Illustration:
CORT’S REVERBERATORY FURNACE
This is the type of furnace generally used to-day.
]
Charcoal was an expensive fuel, and, moreover, there was an insufficient
supply because of the great destruction of forests necessary for its
production. Naturally the thing to do was to substitute coal, which was
plentiful. But the sulphur of the coal spoiled the iron.
This proved to be a great barrier until 1783 when Henry Cort of England
succeeded in making wrought iron in a new type of furnace wherein the
iron was refined in one compartment while the fuel was made to furnish
its heat from another; i. e., the fuel and the metal _were not in
contact_ and the refined metal did not suffer from the sulphur content
of the coal.
The figure illustrating Cort’s furnace plainly shows how this was
accomplished and it represents almost as well the furnace used to-day.
In the one-hundred and thirty-three years that have elapsed since it was
designed, his furnace has been changed only in certain details, and but
two important changes have been made in his process—the use of an iron
bottom instead of the sand bed by S. B. Rogers in 1804 and the
introduction of “pig boiling” by Joseph Hall in 1830.
Cort’s process is known as “dry puddling” and his trouble was the
excessive loss of iron, due to his use of the sand bottom and the
absence of a proper cinder. The loss was said to have been from 50% to
70% of the iron charged; i. e., it took about 2 tons of pig iron to make
one ton of wrought iron. Because of the great demand for iron and the
fact that he was using such a cheap fuel, coal, his process at that time
was a success financially.
[Illustration:
EARLY IRON ROLLS
While Cort did not originate the idea, it was he who first made the
rolling process successful, and, therefore, he is given credit for
the invention.
]
But Cort did not stop here. He saw the desirability of a quicker and
more economical method of reducing the “blooms” or balls of iron into
bars or other finished shapes than the hammering process up to that time
used. He accomplished this by the use of power driven rolls, such as
those shown in the cut.
These two inventions of Cort’s were epoch making, though of the two the
more important one was the invention of the rolls from which has
developed the modern rolling mill.
There remains to be described in a little more detail the making of
wrought iron as practiced to-day. The puddling furnace used has a grate
upon which is built the coal fire. The long flame passes over the fire
wall and is deflected by the roof down upon the charge of pig iron piled
upon the “fettling” or lining of iron oxide (iron ore or mill scale)
over the air or water cooled iron plates which form the bottom and the
sides of the hearth.
With a long iron bar the “puddler” (the expert attendant) turns the pigs
until they have melted down into the bath of slag or cinder charged and
continually being formed by the chemical union of iron from the ore or
pig and some of the sand and other impurities present. This cinder,
which is largely a silicate of iron, is a protection for the bath of
molten iron and its use prevents excessive oxidation and loss of metal.
During the melting of the charge the heat is kept as high as possible
and the molten iron is “puddled,” i. e., stirred, by the “puddler” or
his helper with a “rabbler” or iron bar. In order to take out the
phosphorus and sulphur, which for best results should be removed before
the carbon is eliminated, the heat of the furnace is lowered somewhat as
soon as all of the pig has melted, and some iron oxide is mixed into the
“bath” of molten iron and cinder. Most of the phosphorus and sulphur are
chemically acted upon and pass into the cinder which covers the iron.
Soon the mass begins to boil or seethe and small blue flames break
through the cinder covering. This indicates that the carbon is being
oxidized by the oxygen of the iron ore which was added, the oxygen, as
in the blast furnace, having a greater “affinity” for carbon than it has
for the iron.
This “pig boiling” goes on for twenty or thirty minutes, the “puddler”
meanwhile “rabbling” the charge in order to hurry the reactions and to
make sure that all parts of the bath of molten metal are uniformly
exposed to the oxidizing conditions.
[Illustration:
THE MAKING OF WROUGHT IRON TO-DAY. CHARGING THE PIG IRON
]
Soon the metal begins to “come to nature” and little lumps of pure iron
here and there through the bath stiffen up into little pasty balls. Some
may be seen sticking out through the cinder covering of the bath and
others must be torn loose from the bottom with the rabbler. This
“balling” period lasts from fifteen to twenty minutes, during which time
all of the iron has “come to nature.”
[Illustration:
THE COAL PILE AND FIRE BOX OF A REVERBERATORY FURNACE
]
[Illustration:
TURNING THE PIGS TO INSURE EVEN MELTING
]
The puddler quickly separates into two or three white-hot balls the 400
pounds or so of spongy iron. These balls, full of and dripping with
cinder, are seized, one at a time, with long tongs, removed from the
furnace and rushed to the rotary squeezer through which they are twisted
and turned, ever becoming longer and smaller in diameter until they
emerge from the other and narrower side much compacted and with but
little of the cinder which they originally carried. Without being given
time to cool, the blooms are seized and shoved into the “muck rolls,”
whence after a few passes they emerge as long flat bars, very rough and
imperfect in appearance.
After shearing into short lengths, many pieces of this “muck bar,” as it
is called, are made into “box piles” and tied together with wires. These
are charged into the furnace of the finishing mill. After coming to a
white heat the box piles go to the finishing rolls where they are rolled
into bars, rods, plates, or other shapes desired. By repeated cutting,
piling, heating, and rolling, double refined and other high grades of
wrought iron are made, each repiling and rerolling, of course, producing
a more compact and better product.
[Illustration:
PUDDLING THE MELTED CHARGE
]
[Illustration:
CONSIDERABLE CINDER OVERFLOWS DURING THE “BOILING” STAGE
]
Wrought iron has a “fiber,” as may be seen from two of the illustrations
presented. In “coming to nature” small particles of iron crystallize out
as they become purer. These measure perhaps ¹⁄₃₂″ or ¹⁄₁₆″ in diameter.
As it goes to the squeezer the ball of “sponge” is made up of such
grains of iron loosely welded together with the interstices filled with
cinder. The squeezer, and later the rolls, elongate the particles of
iron into threads, which, welded together, make the bar.
It is thought by some that films of cinder, too thin to be seen under
the microscope, surround each fiber of iron and that these afford
protection from rusting and give to wrought iron the excellent welding
quality that it possesses.
[Illustration:
DRAWING ONE OF THE BALLS FROM THE FURNACE
]
[Illustration:
ON THE WAY TO THE “SQUEEZER”
]
Steel has no fiber and for this reason it cannot be split as was the
wrought iron bar shown in the illustration of page 92.
It will have been noticed that only small amounts of iron can be refined
at one time. This, indeed, has been the unfortunate part of wrought iron
manufacture, for it may readily be seen that production of any such
material in lots as small as a quarter ton results in labor costs which
are almost prohibitive in these days of machine-made goods. Not only is
the output of a furnace small but much skill and judgment are necessary
for the production of a high grade product. Very sturdy and strong men,
too, are required as puddlers, for the work is heavy and the extremes of
heat and cold to which they are exposed necessitate men of rugged
health.
[Illustration:
CROSS-SECTION OF A “SQUEEZER”
]
This material in bar iron, engine stay bolts, butt and lap-welded pipe
and certain other products has long held a high place. Though not as
strong as steel, its very excellent welding properties and comparative
freedom from “crystallization” and treacherous breakage under long
continued vibration or sudden jar have fostered its application in such
products as stay bolts, chain-links, cable hooks and others where
failure might have serious consequences. Too, it is thought by many that
pipes, sheets and other articles of wrought iron resist the corrosive
influences of moist air, soil, etc., particularly well. The cinder films
in which the fibers are supposed to be encased are given credit for such
protective influence.
[Illustration:
“BOX PILES” READY FOR THE RE-HEATING FURNACE
]
Though the product has always been a favorite with iron users, the
industry has suffered during the last sixty years by reason of the high
cost of manufacture, which has very largely restricted the application
of wrought iron to certain uses for which first cost is not a main
factor.
The production of wrought iron rails has rapidly dwindled since the year
1880, when they began generally to be replaced by rails made from
Bessemer steel.
[Illustration:
THE BALL GOING INTO THE “SQUEEZER”
]
[Illustration:
SHEARING THE MUCK BAR INTO SHORT LENGTHS FOR “PILING”
]
[Illustration:
THE BLOOM FROM THE SQUEEZER GOES TO THE “MUCK” ROLLS WHICH ROLL IT
DOWN INTO “MUCK BAR”
]
[Illustration:
THE RE-HEATED “BOX PILES” BEING ROLLED INTO “SKELP” FOR PIPE, IN THE
FINISHING MILLS
]
[Illustration:
CROSS PILING GIVES CROSS FIBERS IN THE FINISHED BAR—A DESIRABLE
QUALITY
]
Of recent years, mild steel has been the great competitor of wrought
iron. With marvelous energy, skill, and much capital the manufacturers
of Bessemer and open-hearth steels have adapted their products very well
to the needs of the iron user, while the enormous tonnages turned out in
short time by use of ingenious furnaces and other devices has resulted
in a low cost of production.
Had any of the several mechanical puddling furnaces devised for the
manufacture of wrought iron proved really successful, things might be
more rosy commercially for this very excellent material.
According to the 1916 Statistical Report of the American Iron and Steel
Institute, the recent yearly production in this country of wrought iron
and steel merchant bars, plates and sheets, and skelp for pipe in gross
tons has been:
════╤═══════════════════╤═══════════════════╤═══════════════════
│ Merchant Bars │ Plate and Sheets │ Skelp for Pipe
────┼─────────┬─────────┼─────────┬─────────┼─────────┬─────────
│ Iron │ Steel │ Iron │ Steel │ Iron │ Steel
────┼─────────┼─────────┼─────────┼─────────┼─────────┼─────────
1905│1,322,439│2,271,162│ 72,156│3,460,074│ 452,797│ 983,198
1906│1,481,348│2,510,852│ 74,373│4,107,783│ 391,517│1,137,068
1907│1,440,356│2,530,632│ 74,038│4,174,794│ 444,536│1,358,091
1908│ 685,233│1,301,405│ 54,033│2,595,660│ 297,049│ 853,534
1909│ 952,230│2,311,301│ 76,202│4,158,144│ 370,151│1,663,230
1910│1,074,163│2,711,568│ 91,118│4,864,366│ 350,578│1,477,616
1911│ 835,625│2,211,737│ 89,427│4,398,622│ 322,397│1,658,276
1912│ 944,790│2,752,324│ 75,044│5,800,036│ 327,012│2,119,804
1913│1,026,632│2,930,977│ 64,729│5,686,308│ 312,746│2,189,218
1914│ 563,171│1,960,460│ 56,590│4,662,656│ 264,340│1,718,091
1915│ 657,107│3,474,135│ 20,253│6,057,441│ 262,198│2,037,266
1916│ 993,948│5,625,598│ 13,303│7,440,677│ 355,445│2,572,229
────┴─────────┴─────────┴─────────┴─────────┴─────────┴─────────
There is considerable so-called wrought iron on the market to-day which
is not truly wrought iron, by which we mean iron “puddled” from pig
iron.
Bushelled Iron
After heating a mixture of thin sheet or other soft steel and wrought
iron scrap to a welding heat in a reverberatory furnace, it may be
balled and put through the squeezer, muck rolls, etc., as was the true
wrought iron made by puddling. This manipulation of scrap is known as
the bushelling process and the product as “bushelled iron” or “scrap
bar.” By using scrap bar for the outer layers and old wrought iron bars
cut to length for the interior, box piles are made, which, heated and
rolled, make very good material, though not as good as puddled iron.
Certain grades on the market, e. g., common bar iron, contain more or
less of this bushelled iron, but the better grades, refined wrought
iron, double refined wrought iron, engine and stay-bolt iron, are
usually the pure puddled product.
As with the latter, repeated shearing, piling, and rolling improve the
quality.
Bushelled iron is largely the result of an endeavor to reduce the cost
of production of wrought iron. The material has a legitimate place and
considerable of it is used.
CHAPTER VII
CEMENTATION AND CRUCIBLE STEELS
In the early days practically the only steels recognized—certainly the
only ones desired—were of the high carbon or hardening variety. These
were required for the manufacture of swords and other implements of war,
for tools, etc., most of which had to have hard and sharp cutting edges.
When softer and less brittle metal was desired, wrought iron was
available, but in all probability high carbon steel was the material
most largely used.
Having but the two iron alloys and these of very different properties,
it was not difficult to distinguish between them. A piece of metal could
be heated to redness and plunged into cold water. If it became glass
hard when cooled in this way it was thereby proved to be steel; if still
soft, it was iron.
But the problem is not so simple to-day. Medium, mild and yet softer
steels, and other alloys which have steel characteristics have appeared
and are used in immense quantities. Their advent introduced considerable
complication.
It will be well, therefore, before taking up our subject, “Cementation
and Crucible Steel,” and the several steels which are to follow, to make
sure that we all understand, as well as we may, what is “steel” as
defined to-day, what are the best known varieties, and what are their
characteristics?
For a rough classification it is safe for us to divide the steel world
into four general divisions as follows:
1. The harder, high carbon steels used for tools, dies, etc.
2. The mild and medium steels of which wire, rod, bar, plate, pipe and
structural shapes for bridges, ships and “sky scrapers” are made.
[Illustration:
HARDENING A PIECE OF TOOL STEEL. READY TO QUENCH
]
3. Alloy steels, to which some metal such as nickel, manganese or
chromium gives definite properties and the name.
4. Those other modern materials which are known as “self-hardening” and
“high-speed steels.”
[Illustration:
BOWKNOT MADE FROM PIECE OF STEEL PIPE
]
The two classes last named are not simple iron-carbon alloys and their
properties are less directly derived from and do not so plainly depend
upon carbon. Metallurgically, then, they are not steels in the exact
former sense of the word; but as they do require carbon—though perhaps
in lesser amount, are made by regular steel processes, have most of the
characteristics of steel and are used for the same general purposes,
they are undoubtedly entitled to the appellation “steel.” However, to
distinguish, they are usually termed “alloy steels.”
We are just now concerned only with the steels of classes one and
two—the carbon steels. As explained in a previous chapter, these are
alloys of iron with not more than 2 per cent of carbon.
[Illustration:
HIGH CARBON TOOL STEEL (1.25 PER CENT C) AS CAST
(Magnification 70 diameters)
]
[Illustration:
LOW CARBON TOOL STEEL (.50 PER CENT C) ANNEALED
(Magnification 70 diameters)
]
[Illustration:
LOW CARBON TOOL STEEL (.50 PER CENT C) AS CAST
(Magnification 70 diameters)
]
[Illustration:
MEDIUM CARBON TOOL STEEL (.86 PER CENT C) AS CAST
(Magnification 400 diameters)
]
Carbon is the element the presence of which confers upon iron the
ability to harden when _cooled suddenly_ from a cherry-red heat, as by
quenching in water or oil. If the steel contains less than four-tenths
of one per cent of carbon it has little or no hardening power under this
treatment; but steel with six-tenths of one per cent or more of the
element, has the wonderful property of being slightly malleable when in
the annealed state, but extremely hard and brittle after this sudden
cooling—leads a dual life, so to speak.
[Illustration:
LOW CARBON TOOL STEEL (.50 PER CENT C) HARDENED
(Magnification 100 diameters)
]
At any time, hardened steel may be returned to its former condition of
softness by the well known process of annealing, wherein it is reheated
to the same cherry-red heat and _slowly cooled_.
At the will of the blacksmith or metal worker alternate hardening and
softening may be repeated a great many times without apparent
deterioration.
Various degrees of hardness also, may be obtained according to (1), the
percentage of carbon in the steel, and (2), the completeness and
suddenness of the cooling.
As considerable brittleness and internal strain in the metal necessarily
follow hardening, the hardness is usually “tempered” or “let down” by a
careful reheating to a much lower temperature, usually 425 to 550
degrees Fahrenheit. From this temperature a second quenching “fastens”
the temper at whatever of the original hardness the steel retains at the
temperature chosen by the smith for the second quenching. Much of the
brittleness is in this way relieved. The smith calls it “toughening” the
steel. Tools so treated are much less liable to break.
The steels that will harden (we will call them “carbon tool steels”),
range ordinarily from the .60 per cent carbon variety, used for hammers,
cold chisels, etc., to those containing 1.50 per cent of carbon which
are selected for razors, scalpels, and other tools requiring high
temper. Each one of these many grades is susceptible of a wide variety
of temper in the hands of a capable man, who must select his steel and
give to it the most desirable temper for the work for which the tool is
designed.
[Illustration:
MILD STEEL PIPE (.10 PER CENT C)
(Magnification 70 diameters)
]
Blacksmiths and other tool makers become extremely proficient in judging
steels and the proper temperature at which each should be hardened and
“drawn” (tempered). They judge temperatures solely by the color of the
steel when heated. Every five or ten degree change imparts a slightly
different shade as the steel grows hotter in the forge fire or cooler
when about to quench.
Observation of a good blacksmith at work and a few minutes’ conversation
with him about his “art” will give one greater knowledge and
appreciation of the carbon tool steels than volumes of writings
concerning them. Along with it will come more respect for the skill of
these clever men whose handiwork is never exhibited in salons and about
whom the world hears little, though indebted to them for a great measure
of its civilization and prosperity.
[Illustration:
HIGH CARBON TOOL STEEL IS EXTREMELY BRITTLE WHEN HARDENED AND HAS VERY
LITTLE MALLEABILITY WHEN ANNEALED
]
What and how much would be possible without machines and proper tools?
[Illustration:
QUARTER-INCH MILD STEEL PLATE WITH DOUBLE FOLD. FOLDED COLD WITHOUT
SLIGHTEST CRACK
]
About sixty years ago steels of much lower carbon content appeared. They
have been made softer and softer until we have what we now know as the
“mild” steels and even the almost or practically carbonless material
which we called “open-hearth iron” or “ingot iron” in a former chapter.
These have not the hardening property but they possess softness,
ductility and freedom from brittleness which the higher carbon steels
always lack. For such real evidences of our Twentieth Century
civilization as the great bridges, ships, buildings, etc., they are
indispensable, for they are easily cut, bent and otherwise worked into
shape, and they combine pliability with sufficient strength for the
service intended. Such steels are desirable, for when overloaded they
bend before they break, thus giving warning of the danger.
These mild and medium steels are of immense importance industrially. Of
the 31,000,000 tons of steel made in the United States during 1912
probably 99 per cent was of the soft and medium varieties.
It has been said that “the exception proves the rule.” Cementation steel
is the exception to the rule which we gave in Chapter VI that steel is
always melted during its manufacture.
If a thin piece of bar iron be packed in powdered charcoal and heated at
low red heat for some time, the metal, after cooling, will be found to
have acquired the hardening property. In other words by _absorption of
carbon_ it will have become steel with all of the characteristics of
that material. Neither the iron nor the carbon by which it was
surrounded have melted, yet in some way carbon has penetrated into the
iron and if the heating has been sufficiently long, carbon will be found
at the center of the bar. But _always_ there will be more carbon in the
outer layers of the bar than in those farther inside, i. e., it will be
found in diminishing amounts as we approach the center.
[Illustration:
SHELBY SEAMLESS STEEL TUBING CRUSHED ENDWISE
]
Just how and when the cementation process for making steel, to be now
described, was discovered is not known. It may have been the result of
the non-uniform working of the larger blast furnaces which were
developing in Continental Europe during the Thirteenth century. From the
German “natural steel” which was probably the steely product too rich in
carbon for the wrought iron which they intended to make and much too
poor in carbon to be the fluid cast iron which with the growing height
and heat of the blast furnace they later did make, may have come the
idea. More likely, a piece of thin wrought iron was accidentally left
imbedded in glowing charcoal until it had absorbed some carbon.
[Illustration:
A CRUCIBLE MELTING ROOM. “MELTING HOLES” ARE BENEATH THE SQUARE COVERS
ON THE FLOOR AT THE LEFT. NOTE THE NEW CRUCIBLES DRYING ON THE
SHELVES, AND THE INGOT MOLDS AT THE RIGHT
]
The first mention of cementation steel appears to have been by an
Italian metallurgist, Vannuccio Biringuccio, who, in 1540, described the
making of steel by heating billets of soft iron for a long time in
molten cast iron. The modern method, the heating of wrought iron in
powdered charcoal, was certainly known in the sixteenth century and this
method of cementation has been practiced in France, England, Belgium and
Germany since the seventeenth century.
[Illustration:
A SHEFFIELD (ENGLAND) CEMENTATION FURNACE
]
Reaumur, the Frenchman, whose process of making cast iron soft by
annealing bears his name and is still used in Europe, was the first to
study and understand to any extent the cementation process. Publication,
about 1722, of his complete directions for cementing iron gave great
impetus to the manufacture of steel by this process. Fate, however, was
unkind and his own nation, France, by reason of her small production of
suitable iron for the work, was unable to profit greatly through his
discoveries. Sweden, England and Germany were benefited to a much
greater extent.
During the early years many were the secret and wonderful mixtures and
compounds offered for this work, but of them all carbon in some form was
the only necessary element.
Finely divided or powdered charcoal or bone dust has been mostly used.
[Illustration:
HUNTSMAN CRUCIBLE FURNACE—ORIGINAL TYPE
]
[Illustration:
ONE TYPE OF OIL-FIRED CRUCIBLE FURNACE
]
Sheffield, England, steel makers, have been very successful in the
manufacture of cementation steel. Their usual method is to pack flat
strips of best Swedish Walloon iron in charcoal in rectangular stone
boxes about four feet wide, three feet high and fourteen feet long.
Alternate layers of small-sized charcoal and thin iron bars are piled in
these boxes until they are filled, the bars not being allowed to touch
one another. When full, top slabs are luted on to the boxes to make them
airtight.
Fire is kindled in the firebox below and the heat gradually raised until
furnace and boxes are cherry-red in color. This heat is maintained for
seven to eleven or more days, depending upon the hardness desired, i.e.,
the amount of carbon they desire absorbed. The furnace is closed and
allowed to cool slowly, which requires another seven or more days.
Upon unpacking the furnace the bars are found to be brittle and of a
steely fracture instead of the soft malleable material which was put in.
They have become high carbon steel.
Expert workmen are able to judge very closely the hardness of the steel
by looking at the fracture and they sort the bars in this way, piling
bars of similar hardness together.
[Illustration:
HUNTSMAN COKE-FIRED CRUCIBLE FURNACE—MODERN TYPE
]
Bars thus made show many blisters on the surface and the steel became
known as “blister steel” on this account. The reason for these blisters
was not discovered until along about 1864, when the well-known English
metallurgist, Percy, proved that the blisters were caused by the
chemical action of carbon on the slag contained in the wrought iron. The
gases formed produced the blistering of the bar. That this is the
explanation is proved by the fact that bars of mild steel or iron
without slag do not blister.
Blister bars heated to a forging heat and drawn out under the hammer or
rolled into bar steel are known as “spring steel”, or “plated bars.”
As in wrought iron manufacture, a cutting to length, repiling, heating,
welding and again drawing down by hammering or rolling produces much
more homogeneous and reliable steel. Piled and reworked steel of this
sort became known as “shear” steel because blades of shears for cropping
woolen cloth were always made in this way.
Many of us will recognize in the cementation process an extended “case
hardening.” Case hardening is very largely resorted to by iron and steel
workers, who in a few hours can give a hardened and long-wearing thin
outer layer of steel to a piece of iron or soft steel after it has been
forged or machined into the desired shape.
[Illustration:
SIEMEN’S GAS-FIRED CRUCIBLE FURNACE—REGENERATIVE SYSTEM
One pair of Checker-work Chambers, k. h., is being heated by the hot
outgoing flame and waste gases while the other pair is heating
incoming gas and air. They are worked alternately.
]
This shear steel was largely made and was quite satisfactory, until, as
described before, Huntsman, a Sheffield clock maker, conceived the idea
of melting together in a pot or crucible blister bars or bars of shear
steel. This he did to equalize the carbon content and give uniformity of
product which had never been attainable through the cementation process
alone.
From that date (1740) to this the crucible process has undergone only
minor alterations and to-day it produces the highest grades of steel
which we have. Practically all of the high grade tool steels are
produced by this process.
Nor has Huntsman’s form of furnace been greatly changed, as the
illustrations prove. Though gas and oil as well as coal are, in many
cases, used as the fuel, the general design of the furnace has remained
the same.
For a century crucibles were made from clay molded to form, slowly dried
and very carefully burned. Usually each steel maker made his own
crucibles. They could be used but three times, becoming so thin and
tender after use for three batches of steel that they were not safe for
a fourth. Graphite crucibles are now very largely used. They withstand
the severe heat much better and can be used five or six times. The
expense item for either clay or graphite crucibles is a large one.
[Illustration:
THE STALWART MELTERS
]
After filling with small pieces of blister or shear steel the crucibles
are entirely surrounded by coal or coke in the furnace pit. The fire is
so regulated that the steel is not too quickly melted. Fresh coal or
coke must be put in around the crucibles two or even three times.
When he thinks the steel should be molten, the expert attendant known as
the “melter” quickly removes the tight fitting cover of the crucible and
with an iron rod determines whether any unmelted pieces remain.
After complete melting the steel must be “killed,” else it will boil up
in the mold upon pouring and leave a spongy or insufficiently solid
“ingot” or block of steel. This “killing” of steel is a rather peculiar
phenomenon. It is accomplished by allowing the steel to remain quiet in
the furnace for another half hour or so. Undoubtedly the quieting is the
result of the escape of the gases or impurities which are contained in
the charge, and absorption of the chemical element, silicon, from the
walls of the crucible.
We have met this element, silicon, before in our metallurgical journey
and we will likely meet it several times again. To the metallurgist it
is secondary in importance only to carbon.
[Illustration:
PULLING THE CRUCIBLE
]
When the steel has been properly melted and killed it is ready to pour.
An assistant lifts the cover from the melting hole, the “puller-out”
seizes the crucible just below the bulge with circular tongs and pulls
it from the coke which surrounds it. The slag is skimmed off the top and
the steel poured into iron molds forming small “ingots,” usually from 2
to 4 inches square and two feet or more long.
Every part of the process, even the pouring, must be done with extreme
skill and care or the product suffers.
After liberation from their molds, the ingots are heated and either
rolled or hammered down to the sizes desired for tools, etc.
As stated before, crucible steel necessarily is an expensive material
both on account of high labor and crucible costs. For this reason, many
have resorted to the process used in the very small way mentioned for
the manufacture of Wootz steel—the melting of wrought iron bar or soft
steel in a crucible with carbon.
In the Wootz process chopped wood and green leaves were used. Nowadays
charcoal is substituted or there is added the proper amount of cast iron
to give the desired amount of carbon to the wrought iron or soft steel
charged. During the melting the iron takes up the charcoal and alloys
with it.
[Illustration:
“TEEMING” OR POURING INTO INGOTS. THE INGOTS LATER ARE FORGED OR
ROLLED INTO BARS FROM WHICH THE TOOLS ARE MADE
]
Proper amounts of silicon, manganese, and other beneficial materials are
also charged, which become either part of the alloy itself or have a
cleaning or fluxing action upon it.
Steels made in this way are practically, though perhaps not quite, as
good as steels made by melting together the properly selected
cementation bars. The method has come to be very generally used on
account of its directness and because it eliminates the long and
expensive preliminary cementation process.
When Bessemer and open-hearth steels made their appearance in the market
an attempt was made to use them instead of wrought iron as the base for
high grade crucible steels. Though seemingly pure enough, apparently
purer even than wrought iron, these metals were not able to compete with
wrought iron for this purpose. For some reason, not yet satisfactorily
explained, these new materials which are made in 15, 35 and 50–ton
batches, when used as a base, do not give as high quality tool steel as
puddled wrought iron, which is slowly and laboriously made in 500–pound
lots. Considerable of these materials are utilized but it is for a
somewhat lower grade of crucible steel.
For many years mild steels for castings have been quite largely made by
the crucible process. They are among the best but the crucible and labor
costs are usually too great to allow crucible steel castings to compete
in present markets.
CHAPTER VIII
BESSEMER STEEL
The “Manufacture of Malleable Iron and Steel without Fuel” was the
startling title of a scientific paper read in 1856 before the British
Association for the Advancement of Science. This was the announcement to
the world of Henry Bessemer’s invention of the process for making iron
and steel which led to the greatest commercial development the world has
seen.
To those of us who have had little or no experience along manufacturing
lines the announcement seems strange enough, but metallurgists,
engineers and manufacturers who know how serious is the matter of fuel
bills realize at once how revolutionary the claim of Bessemer must have
seemed to men of those days.
As occurs with so many new things the idea was scoffed at; Bessemer’s
scheme was one purporting to give “something for nothing” and—well, it
could not be.
It was ridiculous!
And why should it not have seemed strange when we consider that up to
that time fuel had been required in all metallurgical processes. In the
old Catalan furnace and the types that preceded it, in the Finery Fire,
the Walloon and the several other refining furnaces fuel had to be
provided without stint. The lowest proportion that seventy years of
experiment and practice had brought about in Cort’s puddling process was
one ton of coal per ton of iron, while the blast furnace required at the
least four-fifths of a ton of coke for each ton of pig iron produced.
[Illustration:
KELLY’S FIRST TILTING CONVERTER
]
Whether Bessemer, an Englishman of French descent, or William Kelly, an
American of Irish descent, of Eddyville, Ky., first conceived the idea
of the “pneumatic” process is a moot question. Considerable evidence
substantiates the claim that the latter first hit upon the scheme and
during the ten years between 1846 and 1856 had considerable success with
its development. Perhaps Bessemer had heard of Kelly’s experiments.
There is no proof that he did. Whether he did or not, the fact remains
that he quite independently and very fully developed the process in
England, and with great business sagacity and energy made it the success
that it is.
As fortune has withheld from Kelly and from this country credit which
was deserved, it is desirable to tell briefly the part which he had in
the development of this process that with a single furnace converts pig
iron into steel at the rate of a thousand tons in 24 hours and first
made mild steel available as a building material.
In 1846 Kelly, with a brother, bought the Suwanee Iron Works, near
Eddyville, Ky. After about a year they encountered the same difficulty
that charcoal iron manufacturers usually have encountered—the failure of
the supply of fuel. This difficulty Kelly, a better inventor than
business man, apparently had not foreseen. His business was threatened
unless some other way of refining his iron was found.
[Illustration:
CRUCIBLE WITH WHICH BESSEMER’S FIRST EXPERIMENTS WERE CONDUCTED
]
One day while watching the operation of his Finery Fire he noticed that
the blast of air from the tuyère made the molten iron where it impinged
very much whiter and apparently hotter than the rest. Like other iron
makers, he had always supposed that a blast of cold air chilled molten
iron.
It appears that Kelly was not long in surmising the truth. In a few days
he had rigged up a crude apparatus and made soft iron from which a
horseshoe and a horseshoe nail were fashioned by a blacksmith.
[Illustration:
FIXED CONVERTER OF 1856 WITH SIX TUYÈRES ABOUT THE SIDES
]
Being conservatives, Kelly’s customers were not slow in informing him
that they did not want iron made by anything other than the “good old
process” and he was obliged to accede to their demands or lose their
trade.
Like Galileo, however, he had not really surrendered. In the woods near
by he built and experimented with seven successive “converters,” as the
furnaces are called in which Bessemer steel is made.
Upon learning that Bessemer of England had been granted a United States
patent (1856), Kelly came before the patent office and proved that he
had several years before used the same process. The priority of his
invention was acknowledged, and a patent was granted to him also (1857).
[Illustration:
BOTTOM BLOWING TILTING CONVERTER
]
Financial troubles and finally bankruptcy handicapped him. However, the
Cambria Steel Co., of Johnstown, Pa., became interested and let him
experiment with his process at the company’s plant. Here in 1857 he
built his first “tilting” converter. His first public demonstration
resulted in failure and ridicule, but a few days later he was
successful. Steel makers bought interests in his patent, which at its
expiration in 1870 was renewed by the United States Patent Office, while
renewal of Bessemer’s patent was refused.
[Illustration:
IN 1858 BESSEMER ERECTED HIS FIRST CONVERTER OF THE FORM GENERALLY
USED TO-DAY
]
The Kelly Pneumatic Process Company, which was organized to operate
under Kelly’s patents, built a converter at an iron works at Wyandotte,
Michigan. Here the first pneumatic process steel ever made in this
country in other than an experimental way was “blown” in 1864.
Meanwhile Alexander L. Holley, an American engineer, had obtained for
another American company the right to manufacture steel here under
Bessemer’s patents. He built a plant at Troy, New York, which began
making steel in 1865.
[Illustration:
EVEN THE DETACHABLE BOTTOM—TO FACILITATE REPAIRS—WAS THOUGHT OF AND
PATENTED BY BESSEMER—1863
]
It was soon decided to merge the interests of the two companies and in
1866 this was done, the process thereafter being known as the Bessemer
Process. During the early years of the process here Holley became very
well known. As consulting engineer he designed practically all of the
Bessemer plants which were built during the first ten or fifteen years.
To the majority of the people of the United States to-day Kelly and his
parallel part in the great invention are practically unknown, and thus
not only he but the United States is without credit which should be
ours.
Fortunately Kelly did not entirely fail to profit financially as so many
times is the case with inventors. He received a total of about $500,000.
Bessemer’s return from his process is said to have approximated
$10,000,000 and he was knighted by the British sovereign.
More intimate details regarding Kelly and his work may be found in
_Munsey’s Magazine_ for April, 1906, where H. Casson gives information
which he received direct from several of the men who knew and worked
with Kelly.
While apparently not the originator of the process, Bessemer is without
any doubt entitled to most of the credit he received. There is no proof
that he had heard of Kelly’s experiments when he began his own or that
he was aided by Kelly’s discoveries. He worked out the details of the
process independently, as had Kelly, and it was Bessemer who put it on a
commercial basis.
As has occurred with other new processes Bessemer’s first licensees were
not particularly successful. When those who had bought the right to use
his process had failed in their efforts to use it, and become
discouraged as most of them did, he quietly bought back their rights and
went ahead with his development of the process. Perhaps no man ever
exhibited more perseverance in continuing experiments and development
under very discouraging conditions than did Henry Bessemer. He had
faith.
[Illustration:
SECTIONAL VIEW OF A MODERN CONVERTER SHOWING AIR DUCT AND TUYÈRES
]
He had a genius for invention and was thorough in his experimental work.
Practically no type of converter has since been brought out that he did
not think of and try, and the process has been modified in but one or
two important particulars in the years that have passed.
The essential part of the Bessemer process is the blowing of air through
molten cast iron to remove the metalloids by which cast iron differs
from steel and wrought iron, as has been explained before.
This being the essential point, and at first thought the lack of fuel
seeming so peculiar, we must describe what happens during the Bessemer
“blow.”
[Illustration:
POURING CHARGE OF MOLTEN PIG IRON INTO CONVERTER
]
Technically speaking, the metalloids are “oxidized.” Oxidation is the
chemical uniting of oxygen, generally from the air, which has 21 per
cent of this element, with another element or material such as iron,
silicon, carbon, wood, coal, etc. If the oxidation is slow as in the
“rusting” of iron, the resulting heat dissipates as fast as it is
generated and the change is hardly noticeable. If, however, the reaction
occurs rapidly and with vigor enough, we say that the material “burns.”
The latter sort of oxidation is what we call “combustion.”
The affinity between the metalloids and oxygen has been noted by us
before, but in those cases most of the oxygen came from a different
source.
In the wrought iron process most of it was furnished by the iron ore or
scale which was stirred into the metal, or by the slag which covered the
“bath.” In the Bessemer, or as it was first known in America, “Kelly’s
air blowing process,” the oxygen of the air blown through the molten
metal directly oxidizes or burns out the carbon, silicon, and manganese.
The extremely rapid oxidation of these furnishes the heat.
The iron, then, furnishes its own fuel and _no outside combustible is
needed_.
How can this be?
In every ton of molten cast iron there are approximately 70 pounds of
carbon, 25 pounds of silicon, and 15 pounds of manganese or a total of
about 2000 pounds of these metalloids in the fifteen-ton charge of
molten metal which goes into the ordinary steel plant converter.
We know that if burned in a furnace this ton of high grade fuel would
generate much heat. Burned _inside_ of the mass of molten metal it
generates exactly that same amount of heat and the heat is applied with
such rapidity, directness and efficiency that the molten iron which had
a temperature of 2300° F., say, when charged, in nine or ten minutes has
become steel with a temperature of about 3000° F. simply through this
rapid oxidation of its 4 to 6 per cent of metalloids.
How the blast under 15 to 30 pounds per square inch is applied through
little nozzles in the bottom of the modern “converter” and the several
types of vessels with which Bessemer experimented in the course of his
investigations are shown in the illustrations.
Nor is it necessary that the air be blown through the metal. Air blown
upon its surface accomplishes practically the same purpose, and in many
of the steel foundries of to-day smaller converters of this
“surface-blown” type are used for producing steel for castings. The
large steel plants, however, use the larger “bottom-blown” converter.
Two or three of these vessels, working with proper metal from the
“mixer,” produce an immense tonnage of steel each 24 hours.
The “mixer” is quite necessary. It is a large vessel or furnace holding
and keeping hot from 75 to 300 or more tons of metal from the blast
furnace. It mixes and equalizes irons of various compositions, so that
the converters have the advantage of uniform and hot metal with which to
work.
In addition it is made to perform a “refining” service. By mixing into
the metal a quantity of manganese, considerable of the sulphur present
(a deleterious substance) is removed.
The fifteen or twenty minute blowing of 15 tons of metal in the big
egg-shaped converters of a steel plant presents a spectacle which, when
once observed, will never be forgotten.
One sees a little “dinky” engine come shooting into the converter
building with its ladle of molten iron from the “mixer.” With America’s
time saving routine not a single minute is lost while emptying the metal
into the converter, now in a horizontal position. Almost before the
ladle is out of the way, the converter swings to the upright position
with the blast already on, for otherwise the metal would flow into the
tuyère holes at the bottom.
[Illustration:
COMPARISON OF INGOTS
A. From Four Pot Crucible Furnace: Each Heat 400 Pounds in 4 Hours or
100 Pounds an Hour, Each Heat Pours 4 Ingots 3 × 3 × 36″. B. From
Fifteen Ton Bessemer Converter: 30,000 Pounds in 20 Minutes or
90,000 Pounds an Hour, Each Heat Pours 6 Ingots 19 × 20 × 62″. C.
From Fifty Ton Open-Hearth Furnace, 100,000 Pounds in 8 Hours or
12,500 Pounds an Hour, Each Heat Pours 6 Ingots 24 × 32 × 72″.
]
Reddish-brown smoke and a shower of sparks come from the converter.
These gradually develop into a flame.
The blast shows considerable partiality in selecting for its first
attention the metalloids silicon and manganese, in preference to the
iron itself or any other of the metalloids present. After from three to
five minutes half of the silicon and manganese have been burned out. If
the temperature of the metal and other conditions have become right the
carbon then begins to burn. This gives a change in the nature of the
flame which becomes large and of a dazzling whiteness.
The metal is hot—very hot—so much so that pieces of cold steel often
must be dropped in to cool it somewhat. This is known as “scrapping” the
charge.
An experienced blower can judge through every period of the operation of
the condition of his metal and just how things are progressing.
After some minutes the flame begins to waver and later “drops”; i.e.,
there is scarcely a flame at all. This signal, which is very definite to
an experienced man, cannot be lightly disregarded. Oxygen has affinity
for iron as well as for the metalloids and it is only because of its
greater love for silicon, manganese and carbon that it has thus far
largely neglected the iron. With the metalloids mentioned out of the
way, as they are when the drop occurs, the iron will begin to burn. Were
the “blowing” continued we would shortly have no iron left, but in its
place a mass of iron oxide and slag.
Thus we see that during the first minutes of the blow, more than
one-half of the silicon and manganese are burned. The remainder of these
and all of the carbon are removed in the subsequent five or six minutes.
At the end of this short blowing period we have practically pure iron.
[Illustration:
TWO CONVERTERS IN OPERATION AND A THIRD POURING
]
The metal is not yet in condition to pour well, however, largely because
of the dissolved air and gases which it holds. Something akin to the
“killing” of the steel which we observed in the crucible process must be
accomplished or ingots from it will be spongy. And, having practically
no carbon, it is not yet “steel.”
Bessemer, knowing that the finished steel should contain carbon, tried
to stop the blow long enough before the drop of the flame to leave
exactly the desired amount of this element. He found this difficult to
do and therefore uncertain. It was found to be far better to blow until
the drop of the flame and then put back sufficient carbon to give the
proper composition.
An English metallurgist named Mushet discovered that addition of
manganese ridded the metal of injurious gases and oxides and what is
known as “red-shortness.” After a period of difficulty without it
Bessemer acknowledged the necessity of manganese and adopted its use. It
had before this been used in crucible steel.
Upon turning down the converter at the drop of the flame, the blast is
turned off and a smaller ladle is run in on a track above. This brings a
molten mixture of irons, usually known as “spiegel” or “spiegeleisen”
which contains just enough carbon, manganese and silicon to give to the
whole of the molten metal in the converter the metalloids needed to make
of it steel of the composition desired. This addition also accomplishes
the “deoxidation” of the metal. By deoxidation we mean that the iron is
relieved of the oxygen and gases which have remained as a result of the
blast. This is necessary in order to give proper fluidity for pouring
and the best physical properties to the finished steel.
After “recarburization,” as this addition of manganese-silicon-carbon
metal is called, the steel and slag are quickly poured out into a ladle
waiting below from which the steel is “teemed” (i.e., poured), through a
“nozzle” or hole in the bottom into ingot molds arranged on trucks on
the railroad track which runs through the building.
[Illustration:
TEEMING THE FINISHED STEEL INTO THE INGOT MOLDS
]
When the molds have been filled and a strong crust develops on the steel
the cars are pulled to the “stripper” where the molds are removed,
leaving the white-hot ingots standing on the cars.
The ingots mentioned in the chapter on Cementation and Crucible Steel
were usually small enough that one pot of 100 pounds of metal filled the
mold. A four pot furnace therefore produced 400 pounds. Now for the
first time, we are talking in _tonnage_ figures. Instead of a batch of
steel making four 3″ × 3″ × 36″ ingots of 100 pounds each, the ordinary
“heat” of Bessemer steel from the 15–ton converter gives six or seven
ingots about 18″ × 20″ × 60″ in size. Each of these weighs about two
tons. The total is 30,000 pounds.
From the stripper the ingots go to the gas-fired soaking pits where the
molten interiors of the ingots gradually solidify by cooling while the
outer crusts are reheated. After equalizing the temperatures of
exteriors and interiors in this way, the ingots are white-hot again and
ready for rolling.
[Illustration:
MOLDS BEING STRIPPED FROM INGOTS
]
The purpose for which the steel is intended, of course, determines the
shapes and sizes into which the ingots are rolled. For rails they are
rolled down directly, each ingot making about six rails, of
thirty-three-foot length. For most other purposes the ingots are rolled
in the slabbing mill into billets or slabs which are of intermediate
shapes and sizes which are reheated and further rolled down into axles,
bars, shapes, wire or other products.
Meanwhile the converter which we saw emptied has not been idle. The
American steel engineer has genius for mechanical efficiency and all
parts of a great steel plant are so co-ordinated that enormous
quantities of material can be handled with not a moment lost between
trips. Almost before the ladle of steel had swung away from the
converter’s mouth, any remaining slag was dumped from the converter by
further tipping, the vessel returned to receiving position and the ladle
car, back again from the mixer, poured in the next charge.
Thus blow after blow is made without loss of time.
Repairs are allowed to take no longer than is absolutely necessary. When
the lining around the tuyères gets too badly cut by the action of the
air and metal the bottom is removed, another one is quickly substituted
and the steel making goes on.
Blowers, ladlemen, cranemen, pourers, patchers, vesselmen, sample boys
and the other workmen are relieved by their “partners” at the end of
each shift, each man of necessity working until relieved—twelve,
twenty-four, or even thirty-six hours, for there must be no delay. So
day and night, through the entire week from Monday morning at six, when
they begin, until the next Sunday morning at six, when the plant shuts
down for a brief spell, the converters go on turning out three heats per
hour or four to five hundred per week each.
It has been mentioned that most of Bessemer’s first licensees failed
with the new process. The reasons for this were various, but one in
particular was the attempt of many to use metal of high phosphorus
content. Bessemer soon discovered that no phosphorus was removed during
the “blow” and that, as phosphorus in quantity over one-tenth of one per
cent was detrimental to steel, it was necessary to use raw material
which had little of this element.
This could be done, but it barred many pig irons otherwise good.
Fortunately Swedish and many English irons had low phosphorus. Germany’s
vast beds of high phosphorus ores, however, were useless for the
purpose.
For twenty years this situation existed, during which time many
metallurgists endeavored to make the process applicable to irons which
contained high phosphorus. After long study and many experiments the
problem was solved by Sidney Thomas, an English metallurgist. With a
cousin, Percy Gilchrist, he made hundreds of blows with a toy converter
holding only eight pounds of iron.
Bessemer’s linings had been of sand, clay and other earths which are
known chemically as “acid” materials. By using “basic” materials such as
limestone, dolomite, etc., for the converter lining and additions of
limestone or burnt lime to the charge before and during the blow to make
and keep the slag “basic,” Thomas was able to make the phosphorus burn
after the carbon had been removed. Therefore, a three or four minute
“after blow” following the “drop” of the carbon flame took out the
phosphorus,—again, with generation of heat.
So there are two varieties of the process—the acid Bessemer and the
basic Bessemer, but the former, only, is used in this country as we have
few high phosphorus ores. The analogous open-hearth processes, which are
next to be described, are both used in this country with the basic
open-hearth greatly in the lead.
However, the basic Bessemer process of Thomas and Gilchrist is credited
with making Germany’s great industrial development possible.
════╤════════════════════════════════════╦═══════════════════════════════════
Year│ TABLE NO. 1 ║ TABLE NO. 2
│ Materials Used for Rails[6] ║ Total Steel Made by Processes[6]
────┼──────────┬────────────┬────────────╫─────────────┬────────────┬────────
│ Wrought │ Bessemer │Open-Hearth ║ Bessemer │Open-Hearth │Crucible
│ Iron │ Steel │ Steel ║ Steel │ Steel │ Steel
────┼──────────┼────────────┼────────────╫─────────────┼────────────┼────────
1849│ =21,710=│ │ ║ │ │
1850│ =39,360=│ │ ║ │ │
1855│ =124,000=│ │ ║ │ │
1860│ =183,000=│ │ ║ │ │ No Data
1865│ =318,000=│ │ ║ │ │
1867│ =410,000=│ 2,280│ ║ =2,679=│ │
1868│ =445,970=│ 6,450│ ║ =7,589=│ │
1869│ =521,370=│ 8,620│ ║ =10,714=│ 893│
1870│ =523,000=│ 30,360│ ║ =37,500=│ 1,339│
1875│=448,000=↘│ 260,000│ ║ =335,000=│ 8,080│ 35,180
1880│ 441,000│ ↘=852,000=│ 12,160║ =1,074,000=│ 110,850│ 64,660
1885│ 13,000│ =959,000=│ 4,280║ =1,515,000=│ 133,000│ 57,600
1890│ 14,000│ =1,868,000=│ 3,590║ =3,689,000=│ 513,000│ 71,200
1895│ 5,810│ =1,300,000=│ 700║ =4,909,000=│ 1,137,000│ 68,700
1900│ 695│ =2,384,000=│ 1,330║ =6,685,000=│ 3,398,000│ 100,500
1905│ 318│ =3,192,000=│ 183,000║ =10,941,000=│ 8,971,000│ 102,200
1906│ 15│ =3,391,000=│ 186,000║ =12,276,000=│ 10,980,000│ 127,500
1907│ 925│ =3,380,000=│ 253,000║=11,668,000=↘│ 11,550,000│ 131,000
1908│ 71│ =1,349,000=│ 572,000║ 6,117,000│↘=7,837,000=│ 63,600
1909│ │ =1,767,000=│ 1,257,000║ 9,331,000│=14,494,000=│ 107,400
1910│ 230│=1,884,000=↘│ 1,751,000║ 9,413,000│=16,505,000=│ 122,300
1912│ │ 1,100,000│↘=2,105,000=║ 10,328,000│=20,780,000=│ 121,500
1913│ │ 818,000│ =2,528,000=║ 9,546,000│=21,600,000=│ 121,200
1914│ │ 324,000│ =1,526,000=║ 6,221,000│=17,175,000=│ 89,900
1915│ │ 327,000│ =1,775,000=║ 8,287,000│=23,679,000=│ 113,800
1916│ │ 440,000│ =2,270,000=║ 11,059,000│=31,415,000=│ 129,700
────┴──────────┴────────────┴────────────╨─────────────┴────────────┴────────
Footnote 6:
In United States—long tons of 2,240 pounds.
The well-known “Thomas Slag” which is in demand as a fertilizer on
account of its phosphorus content is the by-product of the basic-lined
converter.
An idea of what the invention of the Bessemer process meant to railroad
development alone may be gained by studying for a moment Table No. 1.
Wrought iron was our first material for rails, but, being very soft, it
did not give long service. But a short time was required for Bessemer
steel to displace it for rails when steel became available. The greater
uniformity, strength and hardness of the alloy gave such excellent
wearing properties that few rails of iron were laid after the year 1880.
During recent years rails have been made of greater and greater strength
and hardness to keep pace with the fast increasing weight, speed and
frequency of railroad trains, steel being susceptible to much
modification of properties.
Now it appears that Bessemer steel is giving way to other products which
show even superior properties.
What happened in the railroad world to a great extent has happened
elsewhere, as the figures of Table No. 2 show. They are a barometer
which indicates what has been our industrial development and our advance
in civilization.
CHAPTER IX
THE OPEN-HEARTH PROCESS
Bessemer’s was a wonderful process, but the time seemed to be ripe for
great development along metallurgical lines, and the method of
converting pig iron into steel which he devised soon had a competitor
which was destined eventually to take the lead in steel production. Many
years passed before the tonnage turned out annually by Bessemer’s
process was equaled by that of the new, but as shown in the last chapter
the Siemens-Martin or open-hearth process in 1907 produced the greater
tonnage. It has since retained its lead and probably will continue to do
so.
As far back as 1845 John Marshall Heath took out a patent for a process
for making steel patterned after the old puddling process. In a way he
may therefore be said to have devised or forecast the open-hearth
process, but because of the great obstacles that had to be surmounted in
getting a furnace that would fulfill the requirements he was unable to
carry out his scheme. You will remember that in the puddling furnace the
purified metal became pasty because of its high melting point. Because
of the great heat required it was not until the invention of the
regenerative system by C. W. Siemens in 1860 that the open-hearth
process was possible. Siemen’s furnace was the first one that could keep
the iron molten. It was in Birmingham, England, that the first
successful open-hearth furnace was used.
[Illustration:
EARLY TYPE OF GAS PRODUCER, REGENERATORS, AND OPEN-HEARTH FURNACE.
COURSE TAKEN BY AIR, GAS, AND PRODUCTS OF COMBUSTION ARE PLAINLY
SHOWN, AS ARE THE VALVES THAT REVERSE DIRECTION OF FLOW
]
While not as speedy nor as prolific a producer as the Bessemer process
and far less spectacular, the open-hearth has several advantages.
The acid Bessemer was always handicapped because pig iron with less than
0.1 per cent of phosphorus was necessary. The majority of ores carry
more than this amount. The basic Bessemer requires pig iron containing
not less than 2 per cent of phosphorus. The vast quantities of material
which contain percentages of phosphorus between these limits are useless
as far as the Bessemer process is concerned.
To be successfully used the pig iron must be further limited as to
composition. It must have sufficient silicon, manganese, and carbon to
give the heat required for Bessemerizing, as the burning of these
metalloids has to be depended upon for the conversion to steel and to
give proper fluidity to the finished alloy.
Then, too, the large amount of air forced through to a certain extent
“over-oxidizes” the bath and some of the gases are mechanically retained
by the steel no matter how complete the deoxidation. There also is loss
of metal due to unavoidable “spitting,” for the rapid streams of air
mechanically carry some metal and slag with the flame out of the vessel.
On the other hand, for the open-hearth process can be used pig iron of
widely varied character and composition and, further, large percentages
of low-priced steel scrap can be utilized in the charges; as no air is
blown through the metal and little comes in contact with it, the
conversion takes place quietly and smoothly and with much less loss by
oxidation, the yield of steel usually being from 90 to 97 per cent of
the metal charged as against 83 to 87 per cent which is the yield by the
Bessemer process; besides giving less over-oxidation and gases in the
metal, the slowness of the conversion is an advantage, as control is
very easy, and, when desired, samples for test may be taken. From his
tests the melter can be quite certain when he taps out the steel that it
is of the composition desired.
[Illustration:
SECTION THROUGH TYPICAL STATIONARY OPEN-HEARTH FURNACE, SHOWING
CONSTRUCTION OF FURNACE, LINING, BATH AND AIR AND GAS PORTS
]
The melting in an open-hearth furnace is done largely by indirect or
radiated heat, and it is not intended that the flame shall impinge too
directly upon the surface of the bath.
[Illustration:
BOXES OF STEEL SCRAP AND ELECTRIC CHARGING MACHINE IN FRONT OF
CHARGING DOORS AT REAR OF FURNACES
]
Except during the melting down of the pig iron and other materials
charged in the furnace, the flame and air take little part in the actual
elimination of the metalloids. Their main function is to furnish the
heat necessary. Being used so indirectly—mostly by radiation from the
roof and walls—very great heat must be used and much would be wasted if
special precautions were not taken to save it. The bath must be kept hot
enough to remain molten after purification of the metal, which we were
unable to accomplish in the wrought iron puddling furnace.
Under each end of the rectangular furnace are two chambers built up with
checker-work of fire brick. These sets are in duplicate and each has one
chamber for air and one for gas.
[Illustration:
CHARGING MACHINE WITH BOX OF SCRAP HALF WAY INTO FURNACE
]
Thus an open-hearth furnace will be seen to occupy a sort of hollow
square, the furnace proper forming one side, the regenerative chambers
two sides, with the chimney and flues the remaining side. “Reversing”
valves force the incoming gas and air to travel each through its
respective hot regenerating chamber up through the ports and into the
furnace where they unite and burn with a very hot flame. The hot gases
leave through similar ports in the other end of the furnace and on their
way to the chimney heat the checker-work in the regenerative chamber.
Every fifteen or twenty minutes the valves are reversed and the
direction of flow is changed. In this way the incoming gas and air are
preheated and in the furnace burn with a very much hotter flame than
would cold gas with cold air. No blast is required, the draft caused by
the chimney being sufficient.
For protection of the roof from the great heat developed and the metal
of the bath from too great oxidation, the air ports usually are located
above the gas ports. The streams of air, while protecting the roof from
the flame, at the same time are prevented from directly impinging upon
and too strongly oxidizing the metal of the bath.
The diagrammatic sketches given show roughly a furnace, regenerative
chambers, ports, etc.
[Illustration:
CHARGING “HOT” METAL
]
The original intention was to melt pig iron and reduce it; i. e., burn
out the silicon, manganese, and carbon by action of the flame and
addition of iron ore. This was the process worked out by Siemens in
England. In France, P. and E. Martin altered the method by diluting
molten pig iron in the Siemens furnace by melting and dissolving in it
steel scrap. It was soon found that a combination of the two methods was
better than either one alone and the open-hearth process acquired its
name—the Siemens-Martin—in this way.
In the United States about 20,000,000 tons of steel are made annually by
the basic open-hearth process while only 1,100,000 tons are produced by
the acid open-hearth process.
The two processes are practically the same except that by the basic
process the phosphorus as well as the silicon, manganese, and carbon are
reduced or eliminated. In order to take out the phosphorus, additions of
lime (i. e., calcium oxide or calcium carbonate) are made just as
occurred with the basic Bessemer process.
Should we use lime in a furnace having an acid lining, much of the lime,
which is a “base,” would react with the “acid” (silica) bricks of the
lining, and, becoming neutralized, would not do its work. So, as in the
basic Bessemer process, we here have to use either “basic” or “neutral”
lining.
The material generally used is burnt magnesium carbonate which is known
as “magnesite.” Dolomite, which is a combination of the carbonates of
calcium and magnesium, is sometimes used. Chrome bricks, the usual
neutral material, are rather too expensive for extensive use. The best
magnesite comes from Austria and is usually not very cheap. As acid
materials (those of silica or clay) are cheaper and mechanically
stronger, a compromise is ofttimes effected by using basic materials for
the furnace bottom and acid bricks for the sidewalls and roof. A few
rows of chrome bricks may be put in to form a neutral dividing line just
at and above the edge of the bath where the action of the slag is the
most severe. It also serves to keep the basic and acid materials apart
and from reacting with each other.
At the commencement of charging, limestone or sometimes burnt lime is
shoveled in upon the bottom or “hearth” of the white-hot furnace.
When cold metal is charged, the pigs of iron are conveyed into the
furnace by the melter and his helpers by means of long handled flat iron
tools called “peels.” This is followed by charging some or all of the
scrap or iron which is to be made a part of the charge.
Even in the smaller 15 or 25–ton furnaces hand charging takes a great
deal of time, sometimes as much as six or eight hours, and the labor
cost as well as the heat loss is therefore excessive. Modern machine
charging which requires not more than an hour is therefore highly
desirable.
[Illustration:
ROW OF OPEN-HEARTH FURNACES SHOWING PIT OR TAPPING SIDE
]
During the melting down of the pig iron with the scrap that has been
charged, the air and flame burn out about half of the silicon and
manganese of the metal. To remove the remainder of these and the carbon
of the charge, additions are made from time to time of sufficient ore to
keep the bath “boiling.” This phenomenon results from the giving off of
carbon monoxide gas formed from the oxygen of the iron ore and the
carbon of the metal, just as happened when the puddler in the
manufacture of wrought iron used iron ore in his furnace. The covering
of slag which forms and protects the bath from the flame undoubtedly
transfers oxygen from the furnace gases to the bath and this helps to
burn out the carbon.
The lime charged unites with the phosphorus of the iron and takes it
into the slag which covers the bath. If necessary, further additions of
lime may be made from time to time during the melting and the “working
down” (elimination of the metalloids) of the charge. As long as the slag
is kept basic it retains the phosphorus, but should it turn acid the
iron of the bath would take the phosphorus back again.
These reactions are all chemical, just as much so as are the burning of
wood and coal and the thousands of reactions which are brought about in
chemical laboratories.
Additions of ore are made from time to time and the bath rabbled.
Samples are taken now and then with a long handled iron spoon or ladle
and these are poured into molds to form small bars of steel, which,
after quenching, are broken.
[Illustration:
OPEN-HEARTH FURNACE “TAPPING”
]
The melter has become very proficient in judging the composition of the
metal of the bath from the fracture of these broken test pieces. By
means of the samples taken he watches the elimination of the metalloids.
When he thinks the reactions have progressed far enough he takes a last
sample which is rushed to the chemist who makes a hurried “control”
analysis for carbon and phosphorus, the metal being held in the furnace
meanwhile. If the results of this analysis show the bath to have the
desired composition the steel is poured. If the reactions have not been
complete, the chemist’s report shows that the carbon and perhaps the
phosphorus are still too high, in which case the charge must be still
further worked down.
Some melters are able to make fairly uniform and satisfactory steel
without a chemist, but for best results a chemical laboratory is
desirable.
When ready to tap, the big ladle is suspended from a crane under the
spout of the furnace. With a tapping bar the plug of clay is removed
from the tap hole and the molten steel gushes out into the ladle. The
slag which has covered the bath is the last to drain out. Many times
this will overflow the ladle, making a beautiful cascade as it pours
over the sides all around to the floor beneath. Especially at night is
this a glorious sight.
[Illustration:
TEEMING THE STEEL INTO INGOT MOLDS
]
Recarburization is not done to the same extent as it is in the Bessemer
process. As the open-hearth elimination of carbon is slower and under so
much better control, the furnace usually is tapped when the carbon has
been reduced to the percentage desired in the finished steel. When it is
necessary to add carbon it is done sometimes by adding pig iron to the
bath and sometimes by throwing a weighed amount of coal or coke in the
ladle as the steel is going in. Molten iron and steel have strong
appetites for carbon and dissolve it very readily. Ferro-manganese is
used to prevent red-shortness and to deoxidize the metal. This also is
usually put into the ladle as too much loss would occur were it added in
the furnace.
While the furnace is again being charged through the charging doors at
the rear, the steel is teemed through the nozzle of the big ladle into
the waiting ingot molds. These go to the stripper, to the soaking pits,
and then to the rolls of the blooming mills just as did the Bessemer
ingots.
[Illustration:
AT THE “STRIPPER”
]
In the acid-lined furnace no attempt is made to reduce the phosphorus.
It would be futile. Therefore the materials charged must be very low in
phosphorus and sulphur. No lime additions are made, the flame simply
melts down the pig iron and scrap, the iron oxide later is added from
time to time to keep up the boil until the test bars show that the
carbon as well as the silicon and manganese have been eliminated as
fully as is desired. The metal is then tapped as described above.
Three or more hours are usually required to melt down cold charges. The
elimination of the remainder of the silicon, manganese, and the carbon
requires about four or five hours more. So for each heat the open-hearth
furnace requires from eight to twelve hours, depending largely upon the
speed of charging and melting.
Of late years the difficulties attending the use of molten metal from
the blast furnace in place of cold pig iron have been largely
surmounted. The use of uniform metal from the “mixer,” which was
described in the article on the Bessemer process, has aided the
open-hearth process also. Of course, when molten metal is added none of
its silicon and manganese is reduced by the flame as occurred with the
cold metals during the melting down, so the molten metal charged is
usually low in these elements to compensate. By use of “hot” (molten)
metal the time necessary to produce a “heat” of steel is considerably
shortened.
The first and perhaps the majority of furnaces yet building are
“stationary.” Some have found it advantageous to construct furnaces that
can be tipped to pour the metal into the ladle. Such are known as
“tilting” furnaces. One furnace designer has even gone so far in a
smaller type used for steel castings as to make the furnace removable,
thus doing away with a ladle entirely. The big crane simply lifts the
whole furnace out from between the housings which contain the ports. It
is taken bodily to the molds which are poured directly.
Open-hearth furnaces have been built of larger and larger capacity. A
great many fifty-ton furnaces have been built and furnaces which produce
eighty or more tons at a heat are now not uncommon.
Furnaces of the Talbot type are built for as much as 200 and even 300
tons of metal, but from these only part of the finished steel is tapped
at a time, the remainder being left to help work down the additions of
new material which is added to replace the steel tapped out.
The rolling mill industry is so intimately connected with and dependent
upon the steel-making methods and equipment that each is designed with
reference to the other.
Bessemer steel has been largely used for the manufacture of rails, rod,
wire, pipe, merchant bar, etc., while open-hearth steel has gone into
plate, boiler tubes, structural shapes, billets for axles, etc. Recently
it is being used for rails and very many of the products which were
formerly made from Bessemer steel.
[Illustration:
LOWER HALF OF A “BATTERY” OF MODERN GAS PRODUCERS
]
It should not be inferred from this that Bessemer steel is no longer in
demand or that it is not good steel. As you will notice from the table
given in the last chapter, the production of Bessemer steel has not
declined appreciably, if at all. The fact is that open-hearth steel
production has been increasing at a great rate, while the production of
Bessemer has remained stationary. With the growing scarcity of ores
suitable for pig iron for Bessemerizing, the open-hearth process is
becoming able to compete with the Bessemer process in the matter of
cost. For some purposes the steel is considered to be a little more
desirable, but, as is the case with many good things, the pendulum
swings too far and there is no doubt that open-hearth steel is often
demanded and used for purposes for which Bessemer steel would be just as
good and perhaps better.
For many years it has been said that the Bessemer process is “doomed.”
This, of course, was because of the scarcity of low phosphorus ores.
Just how “doomed” it is, it is perhaps impossible to say. Certainly it
is still a very live process and the combining of processes, such as
“duplexing,” will probably prolong its life.
By the “duplex” process, molten blast furnace iron from the mixer is
“desiliconized” in the Bessemer converter. Before too much of the carbon
has been burned, the metal is transferred to a basic open-hearth furnace
where the remainder of the carbon and most of the phosphorus is removed.
By this method the advantages of the open-hearth and much of the speed
of the Bessemer process are combined. The output of the open-hearth
furnace is thus greatly increased.
To-day all kinds of combinations of Bessemer, open-hearth, and electric
furnace are being projected and it is difficult and likely impossible
for any one to predict the future of any of the processes.
Lest the metallurgical facts scattered through several chapters escape,
let us summarize a little. Roughly speaking, the capabilities of and
materials required for the processes are as follows—the chemical symbols
for silicon, manganese, carbon, phosphorus, and sulphur being used for
brevity:
PROCESS REFINING CAPABILITY MATERIAL REQUIRED
Crucible Process. Removes no metalloids, Very low Si. P. S. and
but simply remelts. C.
Acid Bessemer Process. Takes out Si. Mn. and Very low P. and S.
C. (under 0.1%).
Basic Bessemer Process. Takes out Si. Mn. C. P. Very high P. (2% and
and some S. over).
Acid Open-Hearth Takes out Si. Mn. C. Very low P. and S.
Process. (under 0.1% of each).
Basic Open-Hearth Takes out Si. Mn. C. P. Wider Variety.
Process. and some S.
Electric Furnace Takes out Si. Mn. C. P. Wider Variety.
Process. and S.
In further explanation of the competition in quality of Bessemer and of
open-hearth steels it should be understood that in both the acid
Bessemer and the acid open-hearth furnaces we get out in quality just
what we put in. While for some purposes phosphorus and sulphur of 0.1
per cent is allowable, for other purposes they should not be over 0.025
or 0.03 per cent. To produce steel of the latter high quality, material
containing slightly less than this of sulphur and phosphorus must be
charged, and these are usually much higher in price than are pig iron
and scrap containing greater percentages of these metalloids.
[Illustration:
CHARGING FLOOR OF THE BATTERY OF GAS PRODUCERS SHOWING ROCKING ARMS
FOR GRADUAL FEEDING OF THE COAL
]
Where materials of 2.5 to 3 per cent of phosphorus are obtainable, as,
generally speaking, they are not in this country, the basic Bessemer
should make as low phosphorus steel as does the basic open-hearth.
The great advantages of the basic open-hearth process, then, are that
for it can be utilized a much wider variety of raw materials than is
possible with the acid open-hearth or either of the Bessemer processes,
and, particularly that here, at least, the proper materials are readily
available.
The fuels used vary, of course, according to what is most available,
considering quantity, quality, and price. Natural gas has been a
favorite fuel, as also has oil. But in many localities natural gas never
was available and in others which were thus blessed, the supply has been
exhausted. By-product coke oven gas and tar are being experimented with
with some success.
Largely because of the great size of the open-hearth furnace solid fuel,
such as coal which can be used for puddling furnaces, is not adaptable.
As far back as 1839 attempts were made to gasify coal by burning it to
ash and utilizing the gaseous products for industrial purposes. These
attempts succeeded and the process has been brought to quite a high
state of development. There are to-day a large number of efficient types
of “gas producers” which furnish gas for general industrial use and it
is with this “producer gas” that a great deal of the steel nowadays is
made.
While endeavoring to leave out of these articles most of the chemistry
and as much of the technical detail as is consistent with clearness, the
chemistry of combustion and the “gas producer” is so interesting that it
will be well to explain that carbon (coal, coke, wood, etc.) can burn
either in one or two stages. Nearly every one has noticed the blue flame
with which coal burns in the parlor coal heater or in other furnaces
where little draft is used and most of us remember that the gas which is
given off from such a fire has asphyxiated many who were unfortunate
enough to be sleeping in a closed room, when through insufficient
chimney draft or a leaky stove some of the unburnt gas filled the room.
This gas, which is carbon monoxide, is labeled CO in books on chemistry.
It is the result of burning the coal with insufficient air. Chemically
it is explained by the second of the chemical “equations” which follow.
The third equation explains the second stage of the burning which would
occur were further air or oxygen admitted to the upper part of the
furnace.
_The usual one-stage combustion with plenty of air_:
1. C (carbon) + 2O (oxygen) burns to CO_{2} (carbon dioxide).
Non-poisonous.
_The two-stage combustion with insufficient air_:
2. C + O burns to CO (carbon monoxide). Poisonous.
3. CO + O burns to CO_{2}. Non-poisonous.
Carbon monoxide asphyxiates by forming a chemical compound with the
hæmoglobin of the blood, which therefore is prevented from supplying the
body with the oxygen that is required for the sustenance of life.
Carbon dioxide is no such poisonous product, as may be inferred when we
remember that it is the gas with which our carbonated waters are charged
and which is so commonly served with ice cream in ice cream soda.
Now in a gas producer, by maintaining a sufficiently thick bed of
glowing coal and admitting only such amounts of air as will produce
mainly carbon monoxide gas, a product of high burning value is obtained.
A kilogram (2.2 pounds) of carbon in burning from C to CO generates only
2450 calories or heat units, whereas its complete burning to CO_{2}
would give 8080 calories. So by conducting the carbon monoxide gas—the
product of the first stage of the combustion—through brick-lined pipes
to the furnace, and in the latter by addition of air allowing it to burn
to CO_{2}, the greater amount of heat (i. e., 8080 minus 2450 or 5630
calories) is evolved in the furnace. Of course, some of this theoretical
two-thirds which is in this way made available at the furnace is lost
because a little CO_{2} is formed, and always the nitrogen of the air
used greatly dilutes the gas. But there are gains, notably the great
heat which is carried over by the hot gas from the glowing bed of coal
and that from the water-gas which is formed from steam used in the
producer. So, all in all, the gas generated in a “battery” of gas
producers, all of which discharge into one large main or header to
maintain gas of average composition, is quite a satisfactory fuel.
CHAPTER X
CAST IRON
From the preceding chapters we now know pretty well the place which cast
iron occupies in the iron family. In the chapters which have succeeded
the one in which we discussed the blast furnace and pig iron, every one
of the products except crucible steel has been produced through some
“refining” operation which greatly changed the composition, structure
and properties of the product. Cast iron is not the result of a refining
operation in this sense of the word. It is produced through simple
mixing of pig irons of various compositions, usually with some admixture
of iron castings of similar composition which have outlived their
usefulness in the industrial world and have been returned as scrap to be
remelted.
When we say that cast iron is not produced through a refining operation,
it must not be inferred that no change in composition occurs during the
remelting. There is some change, notably a loss through oxidation from
the air blast of a little of the silicon and manganese. Aside from this
there usually will be absorption of enough, or sometimes more than
enough, carbon from the coke used in melting to make up for the carbon
which is oxidized. Usually some sulphur also is taken up from the fuel.
There is, however, no such actual or intended alteration of composition
through burning out of the metalloids as is necessary for the production
of wrought iron and steel.
But from this we must not assume that the manufacture of cast iron for
chilled rolls, car wheels, machine parts, valves and fittings, etc., is
an easy proposition. As we will soon see, accurate regulation is
required of metal for proper depths of “chill” for rolls, car wheels and
castings which must have high resistance to wear. Too, the metal for
valves and fittings and other more or less complicated castings for high
steam, air, ammonia, water, etc., must be uniform, of close grain,
strong, yet soft enough to machine easily at the extremely high speeds
which modern efficient tools and methods demand. The production of the
best metal for such work requires the use of properly selected
materials, judicious mixing, and clever operation of the cupola furnace,
that the molten metal delivered to the foundry for the pouring of the
molds may be hot and fluid and of the right composition for the
particular work in hand.
[Illustration:
SAMPLING CARS OF HIGH SILICON PIG IRON
]
It is always interesting and instructive to follow the materials through
their course from the “raw” state to finished products, and, therefore,
we are going to take you on a little trip from the receiving yard of a
firm making cast iron goods where we see the cars of pig iron just in
from the blast furnace and where the materials are sampled and held
pending analysis, to the laboratory where the samples are analyzed, then
to the storage bins where the materials are unloaded, and, later, with
the weighed charges, to the cupolas which convert them into molten cast
iron of the proper composition and quality for high grade castings.
[Illustration:
SAMPLING OTHER PIG IRONS
Pig irons of lower silicon content cannot be broken easily with a
sledge but usually are thrown from a height across an iron block.
]
Twenty years ago it looked as if the iron foundry would be one of the
last strongholds of “rule-of-thumb” to give way to scientific methods.
It does not look so to-day, though there are many foundries which yet
buy and use their pig iron on the basis of _fracture_; i.e., the
foundryman guesses by judging of the color and closeness of grain and
other characteristics of fresh fractures of the pig irons how suitable
they are for his purpose and in what proportion to mix them. A skillful
man can get fair results in this way only so long as he uses the small
number of brands of pig iron with which he is perfectly familiar, and
even then there must be but little fluctuation in composition of the
irons used and he must be allowed considerable latitude in the quality
of the iron which he produces.
Success by this method is even more difficult now than it was ten years
ago, for the advent of many new blast furnaces and their greater variety
of products have made this rule-of-thumb mixing a much more uncertain
matter than it formerly was. Machine-made pigs, which are so generally
on the market now, give fractures which tell little regarding their
compositions.
[Illustration:
DRILLING THE SAMPLES
No oil or other lubricant is allowable and the drillings are taken up
with a magnet that no sand or other impurity may get into the sample
for
analysis.
]
While some foundries still attempt to accomplish this difficult and
sometimes impossible feat, the majority are now applying more scientific
methods to their manufacture of cast iron.
Though the eye cannot tell surely from the fracture the composition or
quality of the iron which is used in making up the charges, chemical
analysis does definitely give this information. Therefore, every car of
pig iron purchased by this firm is sampled and analyzed, the composition
of all other materials used in its mixtures is determined, and,
irrespective of fracture, which may or may not tell the truth regarding
their composition, the raw materials are charged with respect only to
their actual content of the metalloids. The resulting molten iron each
day is analyzed to confirm the correctness of the mixture and to furnish
analysis of the “sprues” which next day are to be used as a part of the
day’s charge. Physical test bars, too, are cast each hour or so, and the
tensile, transverse strengths, hardness, shrinkage, etc. are accurately
determined in testing machines and recorded. In this way absolutely
nothing is left to chance or to guess work, and, as you may surmise, any
slight deviation from the composition desired is shown at once and the
mixture immediately changed to the extent necessary to bring the iron
back to normal. It is surprising within what narrow limits of variation
compositions and physical properties can be held, with furnace
operations continually under such surveillance.
[Illustration:
WEIGHING OUT PORTIONS FOR ANALYSIS
The finely divided mixed drillings are shaken from a thin-bladed
spatula on to the balance pan. Drillings are added or taken off
until the long needle attached to the beam of the balance swings
over an equal number of divisions on each side of the center mark of
the white scale in the middle. Accuracy is 1/453,000 of an
avoirdupois pound; this is approximately the weight of the lead of a
“pencil mark” one inch long.
]
As the basis for its cast iron, many thousands of tons of pig iron are
each year used direct from the blast furnaces. The raw materials come in
railroad cars or by boat. The inspector who represents the metallurgical
department enters each car and inspects the materials, taking from each
a representative sample for analysis. In the case of pig iron this will
be from four to eight half pigs, it having been found by experience that
these represent very well the contents of the car. So each car of
material is held without unloading until it has been determined by
inspection and analysis that it is fully up to the specifications upon
which the iron was purchased.
Arriving at the laboratory, the half pigs from each car are drilled,
equal amounts of the drillings being taken and mixed in an envelope
which bears the name of the brand of iron, the number of the car, the
date, etc. The sample pigs from each car are treated in this same way,
each car being treated individually.
The envelopes containing the drillings then go to the chemists.
Frequently samples from fifteen or twenty cars of pig iron, with as many
other samples of various derivation, are being analyzed at the same time
for the four or six different constituents which it is necessary for the
metallurgists to know and control in order that a highly satisfactory
product may result. Though a hundred different determinations may be in
progress at the same time, spelling “chaos” in the mind of one not
entirely familiar with the details of the work, it will be interesting
to single out and explain briefly how the samples are analyzed.
[Illustration:
CLOSER VIEW OF THE WEIGHING
]
In such analytical work everything is based upon weight; i.e.,
constituents are determined and reported in percentages by weight. In
chemical laboratory work everywhere the metric system is used, the
cumbersome English system of weights and measures being practically
impossible. Thus, the metric system is the international scientific
standard. The unit taken is the gram, which is equivalent to ¹⁄₄₅₃ part
of an avoirdupois pound. One gram of pig iron drillings is such an
amount as could be held on an ordinary ten-cent piece.
Working with such small amounts of the sample, exactness and skill are
extremely necessary. The balances used are necessarily very
delicate—just as delicate as were the scales upon which the jeweler
weighed your diamonds—you remember, of course. On these balances we can
weigh an inch-long mark made by an ordinary lead pencil.
[Illustration:
DISSOLVING IN ACIDS
This is done under a hood that the irritating fumes given off may be
kept from the room.
]
As the results of the analysis have to be known inside of three or four
hours that the cars may be quickly unloaded in order to avoid demurrage,
which is the penalty for holding cars longer than the allowable time,
separate portions of each sample are weighed out for determination of
the silicon, manganese, sulphur, phosphorus, graphitic carbon, and
combined carbon. These are necessary in order to determine that the iron
is up to the quality specified in the purchase contract and also to
provide for its most efficient use in the manufacture of iron castings.
The exactly weighed portions are put into clean, numbered beakers, which
are small pieces of high grade glassware that will stand sudden changes
of heat and cold. Some of these portions are dissolved in nitric acid,
some in hydrochloric acid, others in combinations of acids. In each case
the drillings go into solution in the acids, and after various
treatments of boiling, evaporating, filtering, etc., well known to those
of the chemical profession, the desired results are obtained. In some
cases it is by actually weighing a constituent which has been filtered
out and burned to ash of a constant known composition, in others it is
by comparison of color with standards of known composition, and
sometimes it is by other means.
[Illustration:
FILTERING SILICONS
After evaporating the excess acid, baking dry, cooling, and
redissolving in weaker acid, the silicon compound formed may be
filtered out. The iron and other soluble constituents, now in
solution, pass through the filter, which is of pure, porous,
unglazed paper.
]
In all of this analytical work the chemist must take care to lose not
one drop of the solution or one grain of the ash from the burned
“precipitate,” as the “filtered out” constituent is called.
The pig iron is always bought upon guarantee that it will contain a
certain percentage of silicon—the element which in cast iron is known as
a “softener.” But this is not the only thing necessary in the iron that
is purchased. It must also show proper specified quantities of
manganese, phosphorus and carbon, which also are very desirable elements
in iron castings, and as little of that undesirable element, sulphur, as
possible. Therefore they pay in proportion to the content of silicon,
manganese, phosphorus, and carbon—and penalize the seller for sulphur.
[Illustration:
“BURNING OFF” THE SILICONS
The paper and contents, in a little crucible, are placed in a red-hot
muffle furnace. The paper is such pure cellulose that it leaves no
weighable ash. That which remains after burning is silicon oxide,
which is a perfectly white, fine sand. This is very carefully
weighed. (Ordinary sand is silicon oxide usually slightly colored
with iron.)
]
The laboratory holds copies of the contracts upon which these materials
were bought. If, upon comparison, the analysis obtained complies with
the terms of the contract, an O. K. unloading slip is made out and the
receiving department is given directions into what raw-material bin in
the receiver building it shall be unloaded. If not fully up to the
standard called for in the contract, the purchasing department is
notified and the car is either rejected or accepted upon some proper
terms of adjustment if it can be used without detriment to the product
in which it is to be utilized.
Cars of coke, limestone, fluorspar, etc., are inspected, analyzed and
treated in the same way, so that nothing is left to guess work. The
compositions as determined by the laboratory serve not only as the basis
for acceptance or rejection, but the analyses of accepted materials are
forwarded at once to the metallurgists, who from them figure the
mixtures to be used in the cupolas.
Having great stocks of analyzed raw materials in the labeled bins in the
receiver building, the metallurgists who supervise the mixing and
melting of the iron determine by mathematical calculation just what
irons and how much of each must be taken to give molten iron of the best
composition and properties for the castings.
[Illustration:
TITRATING THE SULPHUR
Sulphur is evolved from the drillings as a gas (hydrogen sulphide).
This is absorbed in a solution of chloride of zinc. The amount of
sulphur is measured by slowly running in from a burette a solution
of iodine of very accurately known strength. The iodine unites with
the sulphur compound as long as any of the latter remains, but the
first drop added thereafter turns blue the whole solution because of
the reaction of the excess of iodine with starch paste that has been
added previously as an indicator. Accuracy is about .005 per cent of
sulphur.
]
The total iron materials charged must have a definite amount of silicon,
of manganese, of phosphorus and of carbon. For a 4,000–pound charge for
soft cast iron, for instance, the total silicon in the materials which
make up the charge must be somewhere near 118 pounds, the manganese and
the phosphorus about 30 pounds each. The usual losses of these materials
through oxidation are known, of course, and sufficient excess has been
allowed that the desired final composition will result.
[Illustration:
TITRATING PHOSPHORUS
A yellow precipitate containing the phosphorus is filtered out on
filter paper. It is redissolved in alkali and titrated with a
standard solution of nitric acid, similarly to the sulphur. The
solution in the flask turns pink with the first drop added after the
phosphorus has been measured.
]
On several scales which are regularly inspected and kept carefully
adjusted, the weighers weigh out the prescribed quantities of the raw
materials. “Buggies” holding nearly one ton each are loaded in turn with
coke, with proper amounts of pig iron, cast iron scrap, sprues from the
foundry castings of the preceding day, and a proper weight of limestone
flux. Each charge of two tons of iron requires four buggies for its
transportation from the raw-material bins and scales to the cupolas.
The old-time way was for laborers to dump the charges into the cupola
and spread the materials by hand, but in modern foundries better ways
have been provided. Here a charging machine operated by compressed air
lifts into the furnace, one by one, the buggies of coke, and of the
other materials, whence, after the dumping of their contents, the
buggies are returned to the receiver building to be filled again.
Thus many charges per hour pass through the yawning charging doors of
the cupolas, being dumped in fast enough to maintain the level
approximately even with the bottom of the charging door.
[Illustration:
READING THE CARBONS
The higher the combined carbon the darker the nitric acid solution of
the iron or steel. The solution is diluted with water or weak acid
until the color matches that of a “known” sample or standard.
Accurately graduated comparison tubes are used.
]
In starting, a wood fire has been made on the sand bottom of the cupola.
This is covered with coke in such a way and in such amount that, when
ready for charging of the metal, a column of glowing coke extends to a
distance of one foot or two above the tuyères. Upon this “bed” in
alternating layers are piled the weighed amounts of pig iron, sprue,
scrap and limestone as described above. Following each charge is a layer
of coke sufficient in amount to replace the “bed” coke which is burned
away in melting the iron charge, thus maintaining the top of the bed of
coke throughout the day at approximately the same height.
[Illustration:
WEIGHING THE GRAPHITIC CARBON
The graphite is filtered out on an asbestos pad in a perforated
platinum crucible. After drying until all moisture is gone it is
weighed, ignited, and weighed again. The loss of weight equals the
weight of the graphite of the sample.
]
Ever since 7 o’clock A.M., when the twelve to sixteen ounces of blast
pressure was put on, the charges have been descending gradually from the
charging door. Encountering the intense heat in the “melting zone” at
the top of the bed of coke a little above the tuyères, the iron melts
and trickles down through the three to five-foot bed of glowing coke on
to the sand cupola bottom or hearth where it accumulates. The tapper,
with his iron bar and “bod stick” with its little ball of moist fire
clay, alternately opens and plugs the tap hole at the bottom of the
furnace as occasion requires, but throughout the day of ten or more
hours there is almost constantly a full stream of iron flowing from the
spout. The big “bull ladle” which receives it, in turn gives it up to
smaller or “shank” ladles, in which it is conveyed along trolleys to
still smaller ladles from which it is poured into the sand molds to form
the castings.
[Illustration:
ANOTHER CLOSE-UP VIEW
]
As in the blast furnace, limestone is added as flux to make liquid and
dispose of sand, dirt, scale, etc., which are detrimental. The liquid
slag formed from union of limestone with these impurities floats upon
the molten iron in the cupola hearth, as it is less than half as heavy
as the iron itself. It flows almost continuously from a higher hole
called the “slag hole,” in the rear of the furnace and just beneath the
tuyères. The slag has little value except as material for filling
purposes, etc. So-called “slag wool” can be made by blowing air through
it. Sometimes the blast from the cupola blows it in such a way that this
pure white “wool” is formed and blows out of the slag hole of the
cupola. About Christmas time some of the workmen take quantities of it
home for decoration and for fireproof whiskers for “Santa Claus.”
These operations go on continuously throughout the day, each cupola
making the particular grade of cast iron or “semi-steel” which is best
adapted to the particular castings to be poured, size, shape and purpose
of
[Illustration:
DETERMINING CARBON BY DIRECT COMBUSTION
In an electric furnace with pure oxygen passing over them, the
drillings burn as would splinters of wood. From the gas given off
the total carbon is determined with great accuracy.
]
[Illustration:
WEIGHING PIG IRON FOR THE CUPOLA
From three to six varieties of pig iron are used in each charge.
]
[Illustration:
THE RECEIVER BUILDING
Here pig irons of different compositions are separately kept in
numbered and labeled bins. The magnet which is used for unloading
and handling the iron may be seen. The grab bucket for sand is at
the right.
]
[Illustration:
WEIGHING THE CHARGE OF COKE
Close weighing is necessary, for a variation of ten or fifteen pounds
may affect the running of the cupola.
]
the castings being the three main determining factors.
[Illustration:
SECTIONAL VIEW OF A CUPOLA
]
When nearing the end of the day, charging ceases, the charging door is
closed and the last charge, gradually descending, melts and flows into
the bull ladle about an hour later. As soon as the bull ladle is emptied
it is run out of the way, the cupola is drained of all iron and
considerable slag through the tap hole, and the bottom doors of the
cupola are dropped by pulling from under them the “props” which have
held them in position. The great mass of bed coke follows with a great
burst of heat, light and flame. This is quickly subdued with a stream of
water and removed. When cool, the slag and accumulations are chipped
from the cupola lining, burned areas are patched with bricks and stiff,
fire-resisting mud, the bottom doors are raised and fastened in
position, and the eight-inch sand bottom is packed in ready for the next
day’s run. After building a fire and getting a good “bed” of glowing
coke, the cupola is ready again for the charging of iron.
[Illustration:
A CUPOLA IN OPERATION
The stream of iron from the cupola spout flows into a “bull ladle” and
from that into “shank-ladles.” The bull ladle serves as a reservoir
and mixer.
]
[Illustration:
CASTINGS OF CAST IRON AS THEY COME FROM THE MOLDS
The “sprues” have not been removed.
]
CHAPTER XI
CAST IRON (_Continued_)
Unlike the modern Bessemer, open-hearth, and other steel products which
are reworked, i.e., rolled, forged, etc., cast iron is a comparatively
old alloy dating back over several centuries. It cannot be rolled,
forged, or otherwise reshaped, so its final form must be given to it at
once by pouring or “casting” the molten metal into a mold. Its castings
serve exceedingly well in the hundreds of places for which they are
adapted. They are comparatively cheap, can be readily duplicated in
small or large quantities, and those from the softer grades of cast iron
may be machined easily.
These cast iron alloys have only from one-third to one-half the strength
of steel or wrought iron and are, comparatively speaking, very brittle.
Where resistance to severe shock must be withstood they should not be
used. Also, some varieties have a “habit” of growing larger upon
repeated heating and cooling. This “permanent growth” is particularly
noticeable when the alternate heating and cooling is at red heat or
over. Pieces of cast iron have been made to gain 15 per cent in linear
dimensions, and it is quite common knowledge among machinists that a
piece of cast iron which is slightly too small can be permanently
expanded by heat.
Nevertheless, the cast irons have large and legitimate fields in which
they are very serviceable. From most of their important present uses
they are not likely soon to be displaced.
[Illustration:
BECAUSE OF THEIR LARGE SIZE MOLDS FOR VERY LARGE CASTINGS HAVE TO BE
MADE ON THE FLOOR OF THE FOUNDRY, OR PARTLY IN PITS
]
Cast irons in considerable variety of compositions and physical
properties are available, as was indicated by alloys Nos. 14 to 19 which
were given in the table on page 83, part of which is here reproduced. In
alloys 3 to 13 the carbon exerts the great influence on the physical
properties, and this is true also of the cast irons. But all of the
latter have a total carbon content of more than 2½ per cent, and, under
certain conditions, some of the carbon assumes a different form from
that which we encountered in the steels. This modified form is
“graphite,” well known to us as a flaky, black, greasy-feeling material,
which is soft and very fragile. Graphite in the iron alloy naturally
weakens it and, as it is itself such a good lubricant, it makes cast
iron machine easily if sufficient amount is present.
A FEW OF THE CAST IRONS
════════════════════════╤════════╤═════════╤════════╤═══════╤══════════
│Silicon,│Graphite,│Combined│ Total │
│Per Cent│Per Cent │Carbon, │Carbon,│
│ │ │Per Cent│ Per │
│ │ │ │ Cent │
────────────────────────┼────────┼─────────┼────────┼───────┼──────────
No. 14. White Cast Iron│ .70│ .10│ 2.65│ 2.75│Very Hard
No. 15. Annealed │ .70│ 2.70│ .05│ 2.75│Machinable
Malleable Iron │ │ │ │ │
No. 16. Cast Iron for │ 1.00│ 1.00│ 2.00│ 3.00│Very Hard
Chilled Castings │ │ │ │ │
No. 17. Semi-steel │ 1.75│ 2.80│ .40│ 3.20│Machinable
No. 18. Gray Cast Iron │ 2.00│ 3.10│ .30│ 3.40│Machinable
No. 19. Soft Gray Cast │ 2.50│ 3.30│ .15│ 3.45│Machinable
Iron │ │ │ │ │
────────────────────────┴────────┴─────────┴────────┴───────┴──────────
Now, the above must be understood as being _typical_ compositions only.
There are, of course, irons of all intermediate compositions, also, and
while the total, graphitic and combined carbons, typically, are about as
indicated, there may be wide variation.
To illustrate what a variety of chemical and physical properties may be
produced, let us assume that the total carbon in a certain cast iron is
3.25 per cent. If this carbon is all in the _chemically combined_ form
(i. e., combined with the iron to form the very hard compound which is
known to the metallographist as “cementite”) the fracture will be white
and the alloy extremely hard. If none of this carbon is combined, but
all is in the form of graphite flakes throughout the alloy, the fracture
will be “gray” and the alloy soft and machinable. It is possible to
produce either of these two conditions or practically any intermediate
stage; i.e., we can almost at will split up the 3.25 per cent of carbon
into varying percentages of graphitic and combined carbon—the total
always equaling 3.25 per cent.
[Illustration:
No. 30. VERY SOFT CAST IRON. NOTE LARGE GRAPHITE FLAKES
]
[Illustration:
No. 31. MEDIUM HARD CAST IRON
The “combined carbon” is in the roundish, dark parts. It is the
“combined carbon” that increases the strength of cast iron and
steel.
]
The “precipitation” of graphite which is necessary for softness is
brought about mainly through the influence of silicon, which we before
termed the “softener.” Other conditions being equal, the higher the
silicon (if not above 4 per cent), the higher will be the graphite and
the lower the combined carbon; and vice versa, the lower the silicon the
lower will be the graphite and the higher the combined carbon. It is
mainly due to the “combined carbon” which is left after precipitation of
the graphite that the alloy has greater strength, hardness, and closer
grain. So, just as the steels are stronger and harder as the carbon
increases (in steel all the carbon is combined), so, other conditions
being equal, the strengths and hardnesses of the cast irons, within
usual limits, increase as the combined carbon increases.
[Illustration:
No. 92d. SEMI-STEEL. A CLOSER GRAINED AND YET STRONGER CAST IRON
]
[Illustration:
No. 33e. MOTTLED CAST IRON
So-called because it is a mixture of white and gray iron.
]
Just here it is interesting to remember that from the standpoint of
metallography _cast irons are simply steels in which there is what we
might call an impurity or an adulterant, graphite crystals_. It will be
seen at once that could these graphite crystals be removed from the cast
irons shown in photomicrographs No. 74, No. 92d, No. 30 and No. 31, we
would have alloys quite similar in appearance to the steels shown in
photomicrographs No. 3b and No. 22c which appeared on pages 77 and
78.[7]
Footnote 7:
Magnification 70 diameters.
[Illustration:
No. 7. WHITE CAST IRON
]
So the softer cast irons which are used for valves and fittings, machine
parts, radiators, hollowware, etc., have high silicon. Parts that do not
have to be machined can be of “harder” iron; i. e., made of iron having
lower silicon content.
Manipulation of the silicon content is not the only method by which the
hardness of cast iron can be influenced. Graphite can “precipitate”
(i.e., separate throughout the casting) only if sufficient time is given
it to do so. That is, the cooling of the casting after pouring must be
sufficiently slow. In a sand mold the iron remains molten for a time,
and after solidification it cools slowly enough that the greater portion
of the carbon separates as graphite. Therefore, castings of proper
composition made in sand molds are soft and machinable.
If the iron is poured into a mold the surfaces of which are made of
iron, the molten metal upon entering becomes solid almost as soon as it
takes the form of the mold, and it cools with great rapidity. Under such
conditions the carbon of the alloy is denied the time necessary to
change into the graphitic form and the casting has a white fracture and
is so hard that it cannot be machined.
[Illustration:
SECTION OF CHILLED CAR WHEEL
Showing white iron rim.
]
There are many purposes for which the alloy should have extreme hardness
and the great resistance to wear which accompanies such hardness. The
wearing faces of gears, brake shoes, rolls, and car wheels, for
instance, must be hard. For such products, white cast iron, the
extremely hard condition of the alloy, just referred to, is utilized.
Such castings are usually produced with a white cast iron face, but with
a gray iron interior, gray iron being less brittle and less likely to
break under shock or strain. A car wheel, for instance, has
approximately an inch in depth of white iron on the surface which lies
next to the rail on which it runs.
[Illustration:
CHILLED CAST IRON
The white edge resulted from the more rapid cooling against an iron
chill, as did the white rim of the wheel shown above.
]
Such are known as “chilled castings.” Molds for them are usually made of
sand, with pieces of iron (called “chills”) imbedded where white iron is
to be produced. The molten iron next to the sand surfaces cools in the
usual way and is gray and soft, while that which lies next to the
“chill” is white and extremely hard. The “depth” of the chilled layer
can be increased or diminished according to the thickness of the iron
“chill” used, its temperature, and by the composition and temperature of
the molten cast iron with which the mold is poured.
The sulphur and total carbon of the molten cast iron also have
considerable influence on the depth of “chill.”
[Illustration:
TWO-PART MOLDING FLASKS
]
There is a cast iron alloy which is familiarly known as “semi-steel.” It
is simply a high grade and stronger “gray” iron and must be classed as a
cast iron, as our table on page 180 shows. While it could undoubtedly be
made from materials which are commonly used for cast iron, it is
practically always produced by charging with these a certain amount of
steel scrap to bring about the lower silicon, phosphorus and total
carbon desired.
[Illustration:
A HOLLOW CYLINDER
The casting which we are about to make.
]
Because steel has a higher tensile strength than has cast iron, many
have inferred that it was the steel addition which made semi-steel
stronger than the ordinary soft cast iron alloy. The rather unfortunate
name, “semi-steel,” apparently was given because of the steel used and
the intermediate strength which the resulting product possessed.
However, during the melting down of the charge the steel scrap becomes
molten and its constituents merge with those of the other iron materials
charged. We get out of the cupola, then, a mixture which, disregarding
the losses and gains due to the air of the blast, the fuel, etc., is an
average of the materials charged. We, therefore, no longer have any
steel, but a cast iron which has a somewhat lower silicon, phosphorus
and carbon than the softer cast irons. The greater strength of the alloy
is due to its composition and only indirectly to the fact that steel was
used in its production. The physical properties of the steel charged
have been entirely obliterated in the melting process.
[Illustration:
SPLIT PATTERN OF WOOD, SURFACE-COATED WITH SHELLAC VARNISH
]
This view that semi-steel only indirectly gets its increase in strength
from the steel charged is confirmed by its structural appearance under
the microscope, as was shown in numbers 74 and 92d which were given on
page 79, and the photomicrographs given here, and by its extreme
brittleness under hammer blows. Under such shock it is but little more
resistant than cast iron.
[Illustration:
CORE THAT MAKES HOLE IN CASTING
]
This weakness under “shock” was shown by tests from which the table
which follows was compiled. Bars one inch square and thirteen inches
long laid on supports exactly twelve inches apart, were struck at the
center by a twenty-five pound weight. It took seven blows to break the
cast iron bar, the semi-steel bar required eleven, while cast steel
withstood ninety-two blows. Even this does not adequately express the
great resistance of the cast steel (another alloy not yet discussed),
for the height of the “drop” was being increased one inch with every
blow, and the cast steel bar, on account of its bending, had to be
regularly turned. The total foot-pounds exerted by the blows are given
in the table which follows:
[Illustration:
DRAG, OR BOTTOM HALF OF MOLD, AFTER PATTERN IS WITHDRAWN
]
[Illustration:
DRAG WITH CORE IN PLACE AND COPE, OR TOP HALF OF MOLD READY TO CLOSE
]
[Illustration:
TRANSPARENT MOLD, SHOWING RELATIVE POSITIONS OF CORE, CASTING, SPRUE,
ETC.
]
══════════════════════╤════════╤══════╤══════
Alloy │Tensile │Number│Total
│Strength│ of │ Foot
│ │Blows │Pounds
──────────────────────┼────────┼──────┼──────
Cast Iron │ 23,400│ 7│ 102
Semi-Steel │ 35,050│ 11│ 206
Malleable Cast Iron[8]│ 37,140│ 22│ 1,580
Cast Steel[8] │ 72,120│ 92│10,112
──────────────────────┴────────┴──────┴──────
Footnote 8:
On account of bending, the malleable iron and the steel bars had to be
turned several times.
Semi-steel is a very close-grained alloy of ten or twelve thousand
pounds per square inch greater strength than cast iron. It is a most
satisfactory material for medium and larger sized castings for which
cast iron formerly was used.
[Illustration:
CAST IRON TEE, COCK PLUGS, AND RETURN BENDS, WITH SPRUES AND GATES
ATTACHED
]
Did we say that cast iron was very brittle? So it is, comparatively
speaking. But just as the chemist will tell you that there are no
substances which are absolutely insoluble, just so does cast iron appear
to be extremely brittle only when compared with the iron alloys of
considerably less brittleness.
The three pictures of the hooped and twisted casting illustrate how
unwise it might be to speak with absoluteness. A few years ago the
casting illustrated on pages 190 and 192 was brought to this country by
a visitor to Europe, with an expression of regret that cast iron
produced in this country did not have such qualities of elasticity as
had cast iron made abroad. Whereupon, without any change whatever in his
iron mixture or cupola practice, the superintendent of a well-known
foundry made castings which were exact duplicates of that submitted. The
three photographs shown further on (Figs. A, B, C) were of one of these
castings. The ability to bend without breaking is, of course, largely
due to the shape.
[Illustration:
PLUGS AND WHEELS AS THEY COME FROM THE MOLD
Occasionally as many as 200 castings can be made in one mold.
]
[Illustration:
TYPE OF MOLDING MACHINE
]
As a matter of fact, such castings were not at all new in this country,
having been furnished by American foundries for electrical work for many
years. Cast iron springs, piston rings, and many other articles of cast
iron are regularly made, which show such elastic quality.
We have said considerable about “castings.” In general we know what
castings are, but in the minds of some there may be a little uncertainty
as to the manner in which they are produced.
[Illustration:
ANOTHER OF THE MANY KINDS OF MOLDING MACHINES
]
There are few lines of human endeavor which require greater judgment and
skill than does the making of molds for castings. Sound judgment based
on long experience, knowledge of conditions under which the work
immediately in hand must be done, observation, and accurate, deductive
reasoning as to the causes of failure are absolutely necessary for
success.
[Illustration:
CASTING, WHICH BECAUSE OF ITS LENGTH AND SMALL CROSS SECTIONS,
REQUIRES VERY FLUID CAST IRON. (FIG. A)
]
In general, molding may be said to be done in “pit” or on the “floor”
for large work, on the “bench” for smaller work, or by “machine.” Pit,
floor, and bench molding are applicable for production of castings of
all sizes and descriptions and this general type which we might term
“hand molding,” is the form that has been practiced longest. Molding
machines are more or less recent inventions which have enabled certain
standard shapes and sizes of castings which are in sufficient demand to
be produced in great numbers by unskilled workmen and therefore at less
cost than is possible by the older hand method.
Each design for casting may be said to demand individual treatment, and
the molder must select that method out of the many which alone, perhaps,
can be successful. The subject is such a broad one that little will be
here attempted further than to give by description and illustration the
predominant points of the making of molds and castings. A simple,
typical case of bench molding will be taken, that the relation of
pattern, mold, core and casting may be clear.
The molding sands used are usually natural sands which contain greater
or lesser amounts of clay, which, when moist, acts as a “binder” of the
grains of sand. When used without drying, the mold is said to be a
“green sand” mold; if dried, a “dry” or “baked” mold, as the case may
be. The majority are “green sand” molds.
For the usual casting of which only a few or several duplicates are
wanted, the “split” pattern is generally the most convenient.
The two halves of the mold, the “cope” (top) and the “drag” (bottom),
are separately made in the two parts of the “flask” or molding box by
“ramming” properly selected and “tempered” (moistened, mixed, and
sieved) sand over the halves of the pattern. Of these, the drag is made
first over the lower half of the separable pattern placed flat side down
upon a bottom board. After “ramming,” i.e., packing the sand, just hard
enough but not too hard, this half mold is reversed and the top half of
the pattern placed upon the lower half, now at the upper face of the
drag and flat side up. A little “parting powder” or fine, dry sand is
sprinkled over the fresh surface of the half mold so that the upper
half, next to be made, will not stick to the lower half, but can be
lifted off at the proper time.
[Illustration:
IT MAY BE BENT DOUBLE READILY WITHOUT BREAKING. (FIG. B)
]
The cope half of the two-part “flask” is now put on, filled and rammed
with sand as was the drag. Any extra sand is scraped off with a straight
edge and at the proper place a hole is cut with the “sprue cutter”
straight down through the cope to the “parting.” More commonly, perhaps,
this “sprue” hole is made by withdrawing a “sprue” stick (of wood) about
which sand had been packed during the making of the cope. It is through
this hole that the molten metal will be poured into the mold.
[Illustration:
BECAUSE OF ITS ABILITY TO WITHSTAND BENDING AND 180–DEGREE TWISTS IT
IS OFTEN JOCULARLY REFERRED TO AS THE “RUBBER CASTING.” (FIG. C)
]
Lifting the cope or top half, it is turned upside down, and, after
cutting in the drag the “runner” or “gate” connecting the “sprue” hole
with the casting, the halves of the pattern are carefully drawn that the
sand may not be disturbed. Now in the cavity left in the drag, to make
the hole in the casting, is hung the baked “core” of sand, held together
by flour or rosin or a “drying” oil. The cope is carefully replaced upon
the drag, thus “closing” the mold.
As will be noted from the drawings, there is left between the core and
the mold a space all around, which will be filled by the metal of the
casting when poured. Therefore the surface of the core shapes the
inside, and the mold itself the outside and ends of the casting.
The molten metal, entering through the vertical “sprue” hole, flows
along the “runner” and into the mold through the more or less
constricted entrance called the “gate.” The gases formed during pouring
and the air with which the mold was filled are driven out through the
porous bodies of sand of the mold and core. Had the mold been rammed too
hard the gases could not escape through the sand and an imperfect
casting would result.
The poured mold is allowed to stand until the metal has solidified and
cooled sufficiently, when the casting is “shaken out.” The sand is
returned to the molder to be used again. The sprue and runner are broken
from the casting, which, after cleaning by “tumbling” with others in a
revolving mill, or otherwise, goes to the machining and assembling
shops.
Some form of the above general method is everywhere used for the
production of all kinds of castings, except for those which can be made
by machine at a lower cost for molds.
This kind of molding, which we have termed “hand work,” requires expert
molders and is too slow and expensive for the hundreds of standard
shapes and sizes of castings which are in great and constant demand. The
latter are made on cleverly devised molding machines working with
compressed air or by hand power applied through a lever. The pattern is
attached to the machine, set and very accurately adjusted by a skilled
mechanic. Thereafter the sand is rammed, the runner formed and the
pattern drawn by the machine itself, all of these very critical
movements being therefore rapidly and unerringly duplicatable any
desired number of times by unskilled labor, which has but to put on the
parts of the “flasks,” feed in the sand, set the cores, close and remove
the mold, and begin the next.
Sometimes there is but one, but for the smaller sizes there are often
ten or twenty, and, occasionally, as many as two hundred pieces or
castings in a single mold.
CHAPTER XII
MALLEABLE CAST IRON
It almost goes without saying that the capacity to withstand distortion
without breaking was the meaning of and the reason for the use of the
term “malleable.” But wrought iron is malleable as also is mild steel,
and, in Europe fifty years ago (though in general not now) by the term
“malleable iron” was meant and understood what we know as wrought iron.
You will remember that Bessemer’s paper announcing his great process was
entitled “The Manufacture of Malleable Iron and Steel without Fuel.” The
first reference was to wrought iron. Bessemer did not succeed in making
this by his process but his success in the manufacture of steel was
immense. Therefore, while in ordinary conversation such definiteness is
not necessary, perhaps, and not usual here, to be safe one should say
“malleable cast iron,” and not simply “malleable iron,” for by the
latter, many Europeans still understand wrought iron.
Like “Topsy” of “Uncle Tom’s Cabin” fame, the various members of the
iron family “just growed.” Therefore a strictly logical classification
and nomenclature is hardly to be expected.
It was mentioned in a former article that a process for making malleable
the brittle white cast iron was discovered, or at least described, by
Reaumur, a Frenchman, about 1722. It is likely that his discovery or
acquaintance with it came about through his extended experiments with
cementation steel.
[Illustration:
MALLEABLE CAST IRON BARS
]
[Illustration:
MALLEABLE IRON CASTINGS
]
The publicity which Reaumur voluntarily gave to his researches forms a
notable exception to the customs of those days when it was the usual
thing for manufacturers jealously to guard all trade secrets. These were
handed down from father to son or to others of close interest in the
business. So aside from Reaumur’s announcements concerning malleable
iron, few details of its manufacture came to light during the eighteenth
century. Even during the hundred years which have just passed there have
been few lines in which greater secrecy has been maintained both in
Europe and America. During the last thirty years, only, has real
scientific work been done to make known the reactions which occur during
annealing and the real causes of the malleability.
The father of malleable iron in this country was Seth Boyden, of Newark,
New Jersey, a very ingenious man who well deserves the monument erected
in his honor by the citizens of the city, which is pardonably proud of
him.
[Illustration:
TEST SPRUES, SHOWING WHITE, SLIGHTLY MOTTLED, MEDIUM MOTTLED, AND GRAY
FRACTURES
]
Boyden apparently had no knowledge of the existence in Europe of the
malleableizing process, but after noticing that a piece of formerly
brittle cast iron had become rather malleable, apparently through the
action of heat, he set about making experiments to produce a malleable
material which could be produced more cheaply than wrought iron. By
melting in a forge pieces of pig iron and then annealing in a small
furnace in his kitchen fireplace the bars which he cast from the melt,
he had worked out by 1826 a process that produced cast iron which was
malleable. In 1831 he started a foundry and made a thousand or more
different articles for which there was demand, and, from this beginning,
an immense industry developed in this country.
[Illustration:
TEST BARS WITH ONE EDGE CAST AGAINST A “CHILL”
The composition of the mixture is regulated according to the depth of
the chill as well as by appearance of test sprues.
]
We must not forget that the malleable cast iron as produced in this
country is an entirely separate and distinct thing from the European
malleable iron, as will be shown later. So our immense industry is our
own and not a copied one.
It is only certain members of the cast iron family that can be made
malleable by proper heat treatment. Alloys No. 14 and No. 15 represent
one of these alloys before and after the annealing process. While No. 14
was given as a typical analysis for white cast iron for malleableizing
it must be understood that compositions can vary considerably without
detriment from that given.
There is one thing, however, which is absolutely necessary and that is
that all or practically all of the carbon of the alloy must be in the
_combined_ form previous to the annealing process. This means that the
alloy shall be white cast iron and have no free graphite, for any
graphite flakes will remain through and after the annealing process and
weaken the alloy just as it weakens gray cast iron.
[Illustration:
SKETCH OF A COAL-FIRED AIR FURNACE
]
For producing this white cast iron two processes are in general use—the
“cupola” and the “air furnace.” The latter predominates.
Operation of the cupola for malleable iron requires great skill and very
close attention to detail, for, to malleableize easily and with the best
results, the composition of the alloy must be regulated within narrow
limits, very much narrower than for gray cast iron. However, this is
entirely possible and cupolas are operated continuously for malleable
cast iron for ten or more hours with very slight fluctuation.
In general, operation is very similar to that described for cast iron
except that the composition of the charge is necessarily different, much
lower silicon being required, and more coke has to be used for the
melting.
Most malleable iron castings are made in sand molds, and, as stated, the
iron poured must be of such composition and temperature that the
castings so made will be white of fracture. It is possible to get a
quick indication of the condition of the iron for pouring by making test
pieces, every one in the same way, which, after cooling and breaking,
will show by fracture the approximate composition of the metal.
According to these test pieces, called “sprues,” which, at times, may be
cast as often as every five or ten minutes, the mixture is regulated to
produce a uniform product.
[Illustration:
FIRING AN AIR FURNACE
]
To illustrate: The fracture of a round sprue, or test piece, always ⅞
inch in diameter, when poured in the sand, cooled there to low red heat,
quenched in water and then broken, should be white with only a few
flecks of dark constituent. A gray iron fracture indicates too high
silicon content and such iron is usually termed “low” iron. Castings of
medium or heavy section, which, therefore, cool slowly in the sand, if
poured of too high silicon, i.e., “low” iron, might precipitate a little
graphite during cooling, even though thinner-sectioned castings which
cool so much more rapidly would come white from the same iron.
While iron giving nearly white sprues is necessary for particularly
large castings, to make sure that the usual run of malleable castings
will come white in the sand requires very slightly mottled test sprues.
Test blocks also, with one side cast against an iron “chill” are poured
to determine the depth of chilling, and test bars of various shapes are
regularly made, to test after annealing, for tensile strength, torsion
and other physical properties.
“Air furnaces” are much like longer puddling furnaces. They vary in
capacity from ten to forty-five tons while occasionally small ones of as
little as three or five tons capacity are met with.
[Illustration:
TAKING OFF THE SLAG
]
The usual fuel is soft coal. The long flame passes from the grate at one
end over the bridge wall and is deflected by the roof down upon the bath
beneath. A chimney at the outgoing end furnishes draft. The furnace bed
is usually of brick upon which is fritted (slightly fused) a mixture of
sand with a little lime. In order to facilitate charging of the
materials to be melted the roof is usually removable in parts, called
“bungs.” These have frame work of iron which hold in place the fire
bricks that come in contact with the flame. During charging these bungs
are lifted off one at a time, and the iron materials are dumped through
the openings. Small doors in the sides just above the bath allow
“rabbling” or mixing of the charge and skimming of the slag which forms,
and one or more spouts lined with fire bricks and clay provide for
tapping out the metal when it is ready to pour.
[Illustration:
TAPPING
]
Unlike puddling furnace and open-hearth no burning out of the silicon,
manganese and carbon is desired, though, of course, some occurs and has
to be allowed for in calculating the mixture. The intention is simply to
melt together with the least possible loss a mixture of such materials
as will give the average final composition which long experience has
shown to give the proper qualities to the finished product.
Charges usually are of certain percentages of pig iron with not too much
phosphorus, sprues from previous melts, more or less good malleable iron
scrap and small amounts of steel scrap. These are melted down as quickly
as possible. Occasionally the slag which accumulates is skimmed off and,
after rabbling, test plugs are poured from the fractures of which the
composition of the iron is judged.
When the silicon content is deemed proper or has been adjusted through
longer action of the flame if too high or addition of more silicon in
the form of a high silicon alloy if too low, the iron is tapped,
provided it is hot enough.
Malleable iron is largely used for very small castings. These require
very hot and fluid metal. So, even if it is of proper composition, the
metal must be held in the furnace until it is of a high enough
temperature to pour properly. Through prolonged and strong heating the
iron may easily become oxidized or “burnt” and much skill is necessary
for proper operation of the furnace.
After tapping, the iron must be got into the molds with the least
possible delay.
As has been mentioned with former processes the melting of iron in
contact with coke or coal results in more or less contamination with
sulphur. For this reason cupola malleable has considerably higher
sulphur than has malleable cast iron made in the air furnace. Cupola and
air furnace each has certain advantages and certain disadvantages. While
strength and elongation are somewhat greater in air furnace than in
cupola malleable, both anneal well and give materials which are
satisfactory for the purposes for which they are intended.
Cupola metal has an advantage in that the temperature and the
composition can be closely maintained the same throughout the heat,
perhaps more so than with the air furnace. With the latter the metal at
the top of the bath is hotter than that underneath, and, through action
of the flame and air, silicon is somewhat lowered before all of the heat
can be poured, especially with air furnaces of large size. The metal can
easily be “burnt” unless extreme care is taken. In the cupola we can get
very hot iron continuously so that it is unnecessary to prolong the heat
with the danger of burning that occurs with the air furnace.
Air furnace iron anneals rather more readily than does the product of
the cupola, and the strength and malleability are usually greater. The
former requires a temperature of about 1350° F., while the latter must
have 1500° F., a difference of about 150° F. Whether this results alone
from the somewhat higher sulphur of cupola malleable is not definitely
known, but it is probable that, also, the slightly higher total carbon
gives the iron-carbon chemical compound a tendency to persist more
strongly.
The open-hearth furnace is sometimes used for making malleable cast
iron. It melts much more quickly than does the air furnace which
requires from three and one-half to nine hours per heat, depending upon
the size. The quality of the product which the open-hearth furnace
produces is of the best, but on account of the continuous operation
necessary, this type of furnace is not largely used. Malleable iron has
also been made in the Bessemer converter, and, occasionally, in the
crucible furnace, but in this country the practice is not at all common.
In Germany a great deal of malleable iron is made in the crucible
furnace.
When dumped from the molds the castings are extremely brittle. The
sprues are knocked off and the castings go to the “tumbling” mills where
they are tumbled, either with the sprues, with hard iron (white iron)
shot or star-shaped pieces of iron which quickly clean the sand from
them and give smooth, clean surfaces.
At the chipping and sorting benches any remaining pieces of gates and
other excrescences are removed while the castings are being handpicked
and sorted. White iron, because of its brittleness, breaks easily and
small protruding parts can more readily and cheaply be removed before
annealing than after the castings have been thereby toughened.
[Illustration:
ANNEALING FURNACES, SHOWING SETS OF POTS OR “SAGGERS” IN WHICH THE
CASTINGS ARE ANNEALED
]
Having the cleaned castings of white iron of proper composition,
malleability is given to them by the heat treatment known as
“annealing.” Through the influence of a cherry-red heat continued over a
sufficiently long period the iron and carbon, which in the white iron
are chemically combined, gradually become divorced, and, after complete
annealing, the casting will be found to consist of free iron in which
are imbedded throughout very small particles of coke-like carbon.
Castings that before this heat treatment were so brittle that they broke
into many pieces under a blow and so hard that they might scratch glass,
are now found to be capable of withstanding considerable distortion
without fracture and so softened that a needle may scratch them.
Not only is a proper temperature necessary for best annealing, but, as
stated, a sufficient time must be allowed for the separation of the
carbon and iron. The separation requires many hours and the cooling from
the annealing temperature must be slow in order that the carbon and iron
may not again unite, as they certainly would do were the castings
chilled in water or otherwise cooled too fast.
Most manufacturers produce tonnage enough that castings have to be
annealed in quantity. As iron at red or higher heat wastes very rapidly
on account of scaling (in fact would take fire in air if hot enough),
and the quality of castings deteriorates somewhat if even a small amount
of air is allowed to come in contact with them during the annealing
process, they are generally protected by enclosing in iron containers on
which tops are luted and cracks filled with stiff, fire-resisting mud
which keeps out the air.
These iron drums are of suitable size and shape that they can be stacked
one upon the other to a height of from four to six feet. The stacks or
“sets” of “saggers” as they are called, are run into large brick-lined
retort chambers which are heated either with coal from a grate, by
powdered coal, oil, producer gas or other fuel. The larger the furnace
and the greater the tonnage of iron which must be heated, the longer
will be the time necessary for bringing the furnace and castings up to
the cherry-red heat which is necessary for the annealing. Therefore, the
larger furnaces require a somewhat longer time than those of smaller
size, though the time required for the annealing of the castings
themselves is no longer. In this country with ordinary-sized furnaces
the usual time for the annealing operation is approximately one week.
This includes the heating of the furnace and castings and the cooling to
a black heat again.
[Illustration:
No. 109. PHOTOMICROGRAPH OF WHITE CAST IRON
]
Some manufacturers, however, anneal without pots but they aim to have
the castings protected from the flame and air.
[Illustration:
No. 110. PHOTOMICROGRAPH OF NO. 109 AFTER SEVERAL HOURS OF ANNEALING
]
Handling devices have been designed which facilitate loading and
unloading the furnaces. With these the many sets of pots or saggers
which the furnace holds can be very quickly charged or removed.
After shaking the castings out of the cooled pots, the dark coating is
removed from them by “tumbling” with iron shot, pieces of leather or
other polishing material in tumbling mills after which they are ready
for any machining which may be necessary.
Photomicrographs Nos. 109, 110, 113 and 35 which show the samples at 75
diameters magnification show the course of the annealing process. No.
109 was taken from an unannealed casting. No. 110 was of the same iron
after approximately thirty hours in the furnace. No. 113 shows that
after about forty-five hours nearly all of the iron-carbon chemical
compound has been broken down into black patches of free carbon,
surrounded by the white areas of pure iron. After about sixty hours all
of the iron and carbon have been divorced and the annealing operation is
complete, as is shown by photomicrograph No. 35.
[Illustration:
No. 113. PHOTOMICROGRAPH OF NO. 109 WHEN NEARLY ANNEALED
]
[Illustration:
No. 35. COMPLETELY ANNEALED MALLEABLE CAST IRON
]
While heat alone effects the divorcing of the carbon and iron, which is
the essential part of the annealing process, in the greater number of
cases aid is given by what may be termed chemical means. Reaumur, who
about 1722 discovered the annealing process, used iron oxide for the
purpose. The white iron was packed in iron ore or mill scale. At the
high temperatures employed the oxygen of the ore in some way not yet
definitely known, gradually removed the greater amount of the carbon
from the casting. It has always been a scientific conundrum how a solid,
iron oxide, surrounding another solid, a piece of white iron, could
remove from the latter its carbon when neither of them melts nor mingles
with the other. Whether some of the oxygen from the ore penetrates the
iron and burns out its carbon or whether the carbon of the casting
itself migrates is not yet definitely settled. Certain it is that the
carbon is gradually removed from the casting, from the surface first and
with increasing length of time from greater depths.
[Illustration:
No. 390b. PHOTOMICROGRAPH SHOWING DECARBONIZED OUTER LAYER
The photomicrograph also shows that this casting was not fully
annealed.
]
In European practice malleable iron castings are still malleableized in
this way, i. e., by burning out the carbon. The castings are made as
thin as possible and the annealing in “packing” (iron ore or mill scale)
is continued for from one to two weeks. At the expiration of this time
the castings have a white, steely fracture which is entirely unlike the
fractures of malleable iron castings which are made in this country.
Photomicrographs of such malleable iron show few or none of the black
spots which No. 35 exhibits, and analyses of castings annealed in this
way give very low results for carbon.
[Illustration:
MALLEABLE CAST IRON IN WHICH PRACTICALLY ALL OF THE CARBON HAS BEEN
REMOVED BY REAUMUR PROCESS ANNEALING
]
While in this country the Reaumur process of annealing is not followed,
a “packing” of ore or scale is generally used. Some use an inert packing
such as sand, and as first mentioned, some use no packing at all.
Really, one of the main purposes of the “packing” as now used is the
prevention of warping of the castings in the pots while annealing. The
annealing temperature is not so high as in Europe nor is the annealing
continued so long, but when packing is used for the shorter time only,
some surface carbon is removed and the carbon throughout the center
portions of the castings is precipitated in the coke-like form which is
known as “temper carbon” to distinguish it from graphite which is shown
in photomicrograph No. 35. To the eye, then, fractures of such castings
show black centers and white rims. They are known as “black heart”
castings and these form the bulk of the malleable cast iron made in this
country.
[Illustration:
FRACTURE OF BLACK HEART IRON
Note the white rims and black coke-like interiors. The majority of
American malleable iron is of this “black heart” variety.
]
We may say, then, that there are in general three varieties of malleable
cast iron: the “all black” which is annealed without “packing,” the
“black heart,” annealed in “packing” and the most common kind in this
country, and in Europe, but very rarely here, the “whiteheart” from
which practically all of the carbon has been burned during the “anneal.”
[Illustration:
MALLEABLE CAST IRON SWIVELS OF WHICH PARTS NO. 2 ARE CAST TIGHTLY
AROUND NO. 1 AND LOOSENED ONLY UPON ANNEALING.
]
Comparison of photomicrographs No. 35 and No. 30 given on page 181, will
show at once one of the reasons for the much greater malleability of
malleable cast iron. While the total carbon present is very nearly the
same in the two irons, the difference in physical form causes great
difference in the malleability of the two. In the gray cast iron, No.
30, the carbon is crystalline and in the form of long brittle flakes
which cut through and separate the grains of iron. Thus “planes of
cleavage” are formed which make the alloy unable to resist severe shock
and cause it to be anything but malleable. It is not so with annealed
malleable cast iron. Here the carbon is in the form of small pellets
which are imbedded among the grains of pure iron, the malleability of
which is not seriously impaired largely because of the continuity of the
“pure iron” structure. A second reason for the ability of malleable cast
iron to withstand shock is that in the burning out of the carbon of the
outer portions of the casting very small cavities are left. These allow
the surface to become considerably deformed and battered under
successive shocks without great strain on the casting itself.
Nothing has been said so far concerning one trait of all of the irons
and indeed of most metals and alloys which are used for casting
purposes. This is the tendency to “shrink” during the solidification and
cooling of the metal of the casting. On account of the freezing of the
outer portions of the casting before the metal of the inside, there must
result certain hollow places or cavities after the inside metal has
cooled _unless_ some channel is kept open through which fluid metal can
pass inside to keep cavities from forming. We will not here go into the
matter of shrinkage with its great worry to the molder nor the ingenuity
and strategy through which he produces castings without shrinkage
cavities. One of the methods taken to overcome the trouble will be
explained in the chapter on Cast Steel which is to follow.
There is, however, another type of shrinkage—that exhibited by the
contraction of the entire piece of metal as it gradually cools after
solidification. This presents a rather curious and interesting case.
It is well known among founders and pattern makers, that gray cast iron
shrinks during cooling about ⅛ inch per foot, white iron ¼ inch per foot
and cast steel ⁵⁄₁₆ inch per foot. That is, a bar cast exactly one foot
long will be found when cold to be ⅛ inch short if of gray cast iron, ¼
inch short if of white cast iron and ⁵⁄₁₆ inch short if of cast steel.
The patterns have to be made larger than the castings desired to allow
for this shrinkage.
But, during annealing, white cast iron loses one-half of its ¼ inch per
foot shrinkage and the resulting malleable cast iron is found to have a
net shrinkage of but ⅛ inch per foot which is the same as that of gray
iron.
It appears that the precipitation of the temper carbon expands the bar
throughout to practically the same dimensions which it would have had if
flake graphite had been allowed to precipitate through slow cooling, as
is the case with gray cast iron.
This is cleverly taken advantage of by manufacturers of swivels of
malleable iron, such as those shown. The inner portions are separately
cast first and thoroughly cleaned after which they are imbedded in
another mold. The outer portions are then cast around them, shrinking so
tightly upon the inner portions that they cannot be turned at all.
However, upon annealing they loosen enough that they can readily be
turned yet remain tight enough that they cannot be separated.
Malleable iron from which the carbon has not been removed can be
hardened and given a steely fracture by sudden cooling from a red heat
even if it has previously been annealed. Decarbonized malleable iron,
also, can readily be recarbonized by the cementation process. These
characteristics are often taken advantage of for the manufacture of
tools from malleable iron. Hammers, wood working chisels, gears, etc.,
are quite largely made. Where they are sold at a cheaper price than the
better steel tools and without misrepresentation, there can be little
objection, but sometimes they pass for steel.
Ofttimes malleable iron castings are made in what are known as
“permanent molds” of iron. They are really “chilled castings.” Annealing
of these is accomplished in the regular way. Such castings have very
smooth and beautiful surfaces but as the iron molds have high first cost
they can be used only for castings for which the sales warrant the
expense.
While much less malleable than is wrought iron or mild steel, annealed
malleable cast iron has considerable malleability. It will resist great
shock and can be severely battered and bent without breaking. It has
about 75 per cent or more of the tensile strength of mild steel and
because of the cheapness of its castings the malleable iron industry has
developed wonderfully. About a million tons of this product are produced
here each year.
Naturally malleable iron castings are used where a material with
properties intermediate between cast iron and steel will suffice. Such
are castings for railroad cars, for reapers, binders, and other
agricultural machines, pipe fittings, and the cheaper grades of tools.
CHAPTER XIII
CAST STEEL
We have seen how primitive man hunted and fought with no implements and
weapons better than clubs, bows and arrows, and stone hatchets, and how
his wife cracked and ground the corn between flat rocks or in mortars of
stone. In the succeeding “Bronze Age” we found ornaments, idols and
tools being made of copper or the copper alloy, bronze. It was only
after the next great advance that we found man utilizing iron for his
purposes of civilization. This metal, which with us is so common, was in
those days very expensive, so much so that it could be used only for
purposes of war and as the gifts of kings.
But the world was traveling fast and it was not long before the
iron-carbon alloy, steel, was produced. Even so, many hundreds of years
elapsed before the present wonderful age was ushered in through the
great inventions of Henry Bessemer and the Siemens brothers. And while
fine steels for swords and tools have had an incalculably great
influence upon the development of the human race, it was the mammoth
production of Bessemer and open-hearth steel which permitted its general
use as a material for construction of ships, bridges, buildings, and for
railroads, that made this the “Age of Steel.”
Speaking in terms of the power house, it is also the “Age of Cast
Steel.” Twenty-five years ago the manufacturer and power house man were
quite content with their “saturated” steam temperatures and pressures.
With cast iron valves and fittings their plants were well equipped.
[Illustration:
STEEL CASTINGS SHOWING THE RISERS ON THE FLANGES
Castings for use on steam, ammonia, water lines, etc., must be of very
close-grained metal and require much larger risers than castings for
less exacting service.
]
But the world did not stand still. It became known that by heating the
steam out of contact with the water in the boiler it lost the moisture
which it carried and became dry and then could be charged with as much
additional heat as it was desired to give to it. This “superheated”
steam, of course, would do more work and it had also certain other
advantages which the old-fashioned “saturated” steam had not.
But while cast iron fittings gave satisfactory service up to
temperatures, say, around 450° F., they faltered when forced to work
under the new conditions which meant decidedly higher temperatures and
pressures. And, too, the repeated heatings and coolings which were often
necessary, disclosed a disadvantage previously unknown—a so-called
“permanent growth” of the cast iron which was attended by loss of
strength, and altogether it was soon found out that when superheated
steam was to be used, higher types of materials were advisable than
those which had been used under old conditions.
[Illustration:
STEEL CASTINGS IN THE ANNEALING OVEN
]
Superheated steam has rapidly come into general use. Some of the new
locomotives and most of the modern power plants are now built for as
much as 200° superheat, i.e., a total temperature of approximately 600°
Fahrenheit.
Valves and fittings of cast steel not only are the articles “de luxe”
for such service but they have come to be considered the necessary
articles and their advantages have only fairly begun to be appreciated.
Though our most august scientific societies are proposing and debating
upon systems of classification which shall include and satisfactorily
define all of our ferrous metals, a satisfactory one has not yet been
evolved, and, considering the intricacy of our ferrous metallurgy and
the discoveries which are being made almost daily, the outlook for a
strictly logical classification is not yet flattering.
With “Cast Steel” our metallurgical nomenclature is again faulty. Before
what we now call the “steel casting” was known, crucible steel was
poured into ingots, “forged” into tools just as it now is and often went
under the name “Cast Steel” to distinguish it from the contemporaneous
material, wrought iron. So to-day we buy many tools and implements which
bear the name cast steel, which we know to have been forged in bringing
them into their final shape.
But it is not these which we mean by the term, cast steel, but rather
those steel products which get their final form by being “cast” from a
fluid condition into a mold. These are what are rapidly coming to be
understood when the term “cast steel” is used.
Satisfactory metal for steel castings may be made in any of three or
four types of furnaces, but, as was suggested before, the making of
molds for castings is a fine art, as is the preparation of the metal
which is to go into them. Further, the making of that special class of
castings which are to withstand water, steam or air pressure is a very
different thing from the making of steel castings for other purposes,
and this is too often forgotten.
For the former are necessary particularly close-grained castings, free
from flaws or spongy spots. Under the great pressures applied such
defects would certainly allow leakage.
[Illustration:
FLANGES AND FITTINGS OF CAST STEEL
]
Whatever the method of production of steel for castings the metal is
poured into molds to receive its final shape. Because of the intensely
high heat of the steel only sands of great refractoriness (resistance to
heat) can be used as material for the mold. White silica sand is such a
material and is generally used, mixed with enough clay and
molasses-water to give it “bond.” While molds for some steel castings
are made in “green” (i.e., undried or unbaked) sand, baked molds are
preferred for fine finish and surest results. After the making of the
molds in the usual way they are sprayed with very finely powdered white
sand or quartz mixed with a little molasses-water. They are then
thoroughly dried in an oven.
Cast steel shrinks during cooling even more than malleable iron and the
pattern and mold must be made to allow for this. Upon the freezing of
the surfaces of the casting with consequent attainment of rigidity, the
interiors, which freeze last, may have cavities unless means for
avoiding them is provided. For this purpose heavier pieces, which later
can be cut off, are cast upon such parts of the casting as tend to have
“shrink holes.” These may be likened to receptacles filled with fluid
metal, which being larger than the parts of the castings which they
“feed,” hold excess metal in fluid condition until the casting itself
has become solid throughout. Such are usually called “risers” or, in
Europe, “lost heads,” and the molten metal in them flows down into the
interior of the casting and fills the shrink holes which are forming.
Not only must the risers be large enough that the metal in them is the
last to solidify but they must be built high enough above the casting
that sufficient pressure is exerted on the steel entering the shrinking
parts to make its entry sure.
[Illustration:
GRAIN OF STEEL CASTINGS AS THEY COME FROM THE MOLD
]
[Illustration:
GRAIN OF STEEL CASTINGS AFTER
ANNEALING
]
(Magnification 60 diameters)
Baked molds, of course, are comparatively rigid. As the risers which
stand on top of the flanges and other high parts of castings aid in
resisting the natural shortening of long castings during and after
“setting” of the metal, there is great liability that the still red-hot
casting will crack somewhere along its length. It is therefore necessary
to loosen with bars the sand of the mold as soon as the metal of the
casting has set, particularly between the risers, and to break out the
sand of the core inside, around which the shrinking metal might crack
were the sand left in its hard packed condition.
After the casting is shaken out from the mold, it is cleaned and the
risers cut off either by sawing or with the more modern oxy-acetylene
torch flame.
[Illustration:
OTHER TYPICAL STEEL CASTINGS
]
Steel castings should be annealed in order to “refine,” i.e., make finer
the grain of metal and to equalize “strains” which are set up in the
castings during cooling. Coarse grain and internal strains tend to make
the castings brittle. No such extended annealing, however, is necessary
as is the case with malleable cast iron, for no divorcing of carbon from
the iron with separation of free carbon is possible. The castings are
carefully heated to a temperature of about 1600° or 1700° Fahrenheit and
allowed to cool slowly.
After annealing, they are cleaned and excrescences removed by chipping,
after which the castings are tapped, drilled or otherwise machined
according to the purposes for which they are intended.
[Illustration:
CAST STEEL VALVES, STEAM SEPARATOR, AND DIRECT-RETURN TRAP FOR USE
WITH SUPERHEATED STEAM
]
While more costly in manufacture and installation than are those of cast
iron, valves, fittings and other cast steel products are, so far as we
now know, practically permanent. Their notable shock resisting quality
is well shown in the following table which is reprinted from page 188.
[Illustration:
POURING STEEL INTO MOLDS FROM A BOTTOM-POUR LADLE
]
It is to be noted that while malleable cast iron far surpasses
“semi-steel” in this property, though their tensile strengths are
ordinarily somewhere near the same, cast steel, in turn, offers more
than six times the resistance of the malleable iron to shock and has
nearly double its tensile strength. It is this great strength and
resistance to shock, heat and pressure, with freedom from “permanent
growth” under alternate heatings and coolings that make cast steel such
a valuable material for the many purposes for which castings are to-day
employed. Millions of steel castings annually find varied application.
═══════════════════╤════════╤══════╤══════
Alloy │Tensile │Number│Total
│Strength│ of │ Foot
│ │Blows │Pounds
───────────────────┼────────┼──────┼──────
Cast Iron │ 23,400│ 7│ 102
Semi-steel │ 35,050│ 11│ 206
Malleable Cast Iron│ 37,140│ 22│ 1,580
Cast Steel │ 72,120│ 92│10,112
───────────────────┴────────┴──────┴──────
In modern power houses and other commercial steam and hydraulic
installations particularly, steel castings have come to be the materials
usually specified and approximately the only ones which satisfactorily
serve under the severe conditions of to-day.
[Illustration:
POURING FROM A LIP-POUR LADLE
]
Undoubtedly the first steel castings were poured from crucible steel,
though we must remember that the crucible is a melting and not a
refining furnace. This was only natural. In the crucible the metal can
be made very hot and fluid, and if of proper composition and properly
“killed” crucible steel makes very fine castings. Crucible steel
castings, however, are not in as fortunate a position as are other
products of this high grade material. Tool steels ordinarily bring high
enough price that there remains a profit to the manufacturer though his
manufacturing cost is necessarily high. In the steel casting line,
however, there is much keener competition and crucible steel has had
considerable difficulty in maintaining its place. It seems to be a
matter of price alone.
Open-hearth steel is very largely used for steel castings, more than
two-thirds of all made in this country being of this material. About one
half of these are poured from basic open-hearth metal, and the other
half from acid metal. It is generally considered that the product of the
acid-lined furnace is a little freer from over-oxidation.
[Illustration:
TAPPING SIDE OF TWO-TON OIL-BURNING OPEN-HEARTH FURNACE FOR STEEL
CASTINGS
]
Open-hearth steel cannot generally be as hot and fluid as are the steels
made in other types of furnaces. For this reason as well as because of
the larger size of the usual open-hearth furnace, small castings are not
generally poured from this steel. It is for steel castings of
considerable size and where there are sufficient orders to warrant a
steady and large output that the open-hearth has its place. True,
smaller open-hearths are now built, some of them of only two or three
tons capacity, but, in general, the standard open-hearth for steel
castings is of fifteen tons or more capacity and of the style of the
open-hearth furnaces which were described in Chapter IX.
In their proper sphere they are highly satisfactory, but they are
“inelastic” in that they must be run continuously day and night and
should not be allowed to cool until extensive repairs are imperative.
It was mentioned that in the open-hearth process the furnace is always
hotter than the metal which it contains and that the heat which can be
put into the steel is limited by the ability of the refractories of
which the roof and side walls are made to withstand melting. In the
Bessemer process the metal is hotter than the furnace because the heat
is generated by combustion of certain of the metalloids contained in the
metal itself. As metal for castings must be very hot and fluid the
Bessemer process is very satisfactory for the making of steel for
castings.
[Illustration:
A 30–TON BASIC OPEN-HEARTH FURNACE TAPPING
The overflow from the ladle into the pit is slag.
]
It has, also, the advantage of “elasticity.” The supply of metal is
practically continuous and one furnace can make from one to eighteen or
even more heats on day turn only and be shut down for the night turn or
longer and then started again without such loss as would result from the
shutting down of an open-hearth furnace with regenerators.
For the making of metal for steel castings, very small-sized Bessemer
converters are used which make from one to three tons of metal per blow.
Some converters of as little as one-half ton capacity are being used.
While some are of the “bottom-blown” type already described, the
majority are what are called “surface-blown” or “side-blown.” In these,
from four to eight round tuyères, about one and one-half inches in
diameter each, pierce the brick or ganister lining just above the
surface of the bath. They slope downward a little toward the bath so
that when the converter is tipped to its upright or blowing position the
air blast will strike the adjacent edge of the metal and blow across its
surface. This three or four pounds per square inch of air blast keeps
the metal in circulation, meanwhile burning out its silicon, manganese,
and carbon, just as it does in the larger bottom-blown converters.
Surface-blown give hotter metal than do bottom-blown converters and very
fine steel castings are made from their metal. For these converters,
which are practically all acid-lined (i.e., with silica or clay brick or
ganister), metal low in phosphorus and sulphur is regularly drawn from a
cupola specially run for the purpose.
[Illustration:
SMALL SIDE-BLOWN CONVERTER MAKING STEEL FOR CASTINGS
]
The remaining recognized type of furnace for steel for castings is the
comparatively new electric furnace.
Commercial melting of metals by the electric current has been sought for
half a century. In 1879 the first furnaces of promise were patented by
Sir William Siemens, one of the Siemens brothers who became so well
known through their great work with the open-hearth furnace, the gas
producer and many other things metallurgical. While Siemens melted as
much as twenty-two pounds of iron per hour in his furnace, the cost of
the electric current at that time was so high as to be practically
prohibitive for the manufacture of steel in competition with the
open-hearth, Bessemer and crucible processes.
[Illustration:
DRAWING OF SIDE-BLOWN CONVERTER IN BLOWING POSITION, SHOWING EDGE OF
METAL EVEN WITH ROW OF TUYÈRES
]
Little of great moment in the electric furnace line developed during the
nineteen years which followed. Then, in 1898, Stassano in Rome, Italy,
constructed a furnace in which three carbons gave an electric arc above
the surface of the bath. About the same time, Heroult, a Frenchman, was
developing the electric furnace which to-day has become so well known in
this country, and which bears his name. Other well known furnaces of the
arc type are the Gronwall-Dixon, the Snyder, the Girod and the
Rennerfelt.
In general, electric furnaces have more or less round steel shells with
shallow brick, magnesite or sand-lined hearths, and sidewalls and
removable roofs of brick. Heat, of course, is furnished electrically. In
most of them long carbon electrodes are lowered through holes in the
roof until the lower ends strike an arc with the metal on the hearth.
The number of carbons may be from one to four or more depending upon the
style and size of the furnace and the manner in which electrical
connections are made. All of the furnaces mentioned have been used for
the production of steel for castings and the Heroult and Girod are in
use in larger sizes for electric steel for rails and miscellaneous
products. The steel is first cast into ingot molds and is later rolled
down into bars, rods, etc.
[Illustration:
A GRONWALL-DIXON 5–TON ELECTRIC FURNACE TAPPING
]
[Illustration:
FIRST EXPERIMENTAL ARC ELECTRIC FURNACES PATENTED BY SIR WILLIAM
SIEMENS IN 1879
]
All of the above use carbon electrodes and are known as “arc” furnaces.
There is a distinctly different type of furnace which, also, is in use
in commercial sizes. This is the “induction” furnace. In this, what is
known to electricians as a secondary current is “induced” in the bath
itself and heats the metal. Of this type the Kjellin and the
Rochling-Rodenhauser are the best known in this country. While they are
in use in the larger sizes for production of steel for ingots, these two
furnaces do not seem to have been used to any extent for metal for steel
castings.
The details of construction of the furnaces which are used for metal for
castings are more or less different, but they are not of particular
interest to us. The working of all is similar and a general description
should suffice.
[Illustration:
DRAWING OF THE SNYDER ELECTRIC FURNACE
]
[Illustration:
SMALL SNYDER ELECTRIC FURNACE TAPPING
]
Whether starting with furnace cold or hot, materials in molten or in the
more usual “cold” form are charged on the shallow hearth of the furnace.
The charging doors are closed, the current is turned on and the carbon
electrodes are lowered until an arc is struck between the upper
electrodes and the metal on the hearth, which in some way is made to
connect with the negative electrodes. In one or two of the types
mentioned the arc plays between the carbons, all of which are above the
bath.
At first there are great fluctuations in the current intensity because
of the uneven surface presented by scrap steel on the hearth. In a short
time, however, the current steadies. The intense heat of the arcs soon
brings cold steel to a molten condition.
Occasional attention from the attendant is necessary to see that the
melting is even and that any outlying pieces of steel are pushed to the
center where they must melt.
[Illustration:
SKETCH OF THE HEROULT THREE-PHASE ELECTRIC FURNACE
There are three electrodes, all of them above the bath. Only two show
here.
]
In the basic-lined furnaces lime is usually charged with the cold steel.
With the iron oxide which is added from time to time this forms a highly
oxidizing slag, which, after it takes the phosphorus from the metal, is
skimmed off.
As you will remember, the other processes stop at this point, little
further refining being possible. In the electric furnace, however, the
sulphur, also, can be reduced to almost any desired amount by use of a
further addition of lime, and greater heat. Not only can the sulphur be
reduced to very small percentages but the over-oxidized bath can be
brought to neutral condition and the green or black slag made white with
return of its manganese and iron to the bath. This is accomplished by
addition of small amounts of powdered coke or coal. The whole process is
under very accurate control.
With a practically white slag, which is the signal that the deoxidation
of the bath is complete, and the sulphur reduced, the steel is ready to
pour provided it is hot enough. Tests of this are usually made either by
pouring a little of the steel from a small ladle and observing its
fluidity or by observing the quickness with which the end of an iron bar
is melted off when plunged into the metal in the furnace.
[Illustration:
THE HEROULT ELECTRIC FURNACE
]
Of all the metallurgical steel furnaces, the electric furnace is the
most susceptible of accurate control. With the heat applied directly to
the metal in the cleanest way possible, i. e., without the admission of
coal ash or gas or air of the blast, the atmosphere in the tightly
closed electric furnace can be made “oxidizing,” “neutral,” or
“reducing” at will. The metal can be held in the furnace and additions
made, samples taken, and the operations conducted with regulation and
certainty.
This newly devised metallurgical apparatus is coming to be largely used
in the production of tool steels. While it has not displaced the
crucible method for the production of steels of the very highest
qualities, it has proceeded far enough in this direction in the very
limited number of years since its introduction, that it is certain that
the crucible, even for tool steels, is to have a keen competitor. Tool
steels in considerable variety are to-day being quite satisfactorily
made in the electric furnace and it is not at all unlikely that steels
of the very highest grade will shortly be produced by this method.
CHAPTER XIV
THE ALLOY STEELS
We have learned that steel, fundamentally, is an alloy of iron with
carbon, i.e., carbon is the characteristic element. We are now to note
what often seem to be exceptions to this rule. While in reality steel is
just this iron-carbon alloy, there are alloys known as steels to which
such strong characteristics are given by elements other than carbon,
that carbon seems not to be the defining constituent at all. Indeed, in
some of these, the carbon content may be small enough that, judging from
our experience with the carbon steel series, we would not expect any
such physical properties as some of these alloy steels show.
You remember that in olden days they distinguished between wrought iron
and steel by quenching the piece in water from a cherry-red heat. If the
piece was hardened and made brittle, by this treatment, it was thereby
proved to be “steel.” Also, it is generally known that by annealing a
piece of hardened steel, which usually means holding at a cherry-red
heat for a time and then cooling slowly, it is made soft.
Then what shall we say concerning a certain one of these new alloys,
Hadfield’s manganese steel, which is made very much less brittle and a
little softer by quenching, but which refuses absolutely to soften under
annealing treatment—in other words, is almost the opposite of what we
know as steel in these chief defining traits? The nickel steel which
contains 15 per cent of nickel, also, exhibits just these
characteristics, being softened by quenching but not by annealing.
Again, while iron, the carbon steels, and even the magnetic oxide of
iron which contains only about 72 per cent of the metal, are strongly
magnetic, manganese steel which has 85 per cent of iron is so
non-magnetic that it is sometimes used in place of brass or bronze where
an entirely non-magnetic material is required. The nickel steels with 24
per cent or more of nickel are also non-magnetic though both constituent
metals, alone, are strongly attracted by the magnet.
These are some of the things which make a logical classification of the
iron family so difficult. Though derived from the steels which we knew
and made from the same materials with the exception that a greater
amount of one constituent, manganese, is added, or perhaps, in other
cases, another element or two, the resulting alloys have markedly
different and often contradictory properties.
However, we must not be led astray. In all probability carbon is still
the necessary constituent, but much less of it is needed to produce
results when the other elements are present. There is no doubt, however,
that in “manganese steel” or in “nickel,” “chrome,” “tungsten,”
“silicon,” “vanadium,” “titanium,” and other alloy steels, the added
element or elements exert very strong modifying influences, and
sometimes obscure the influence of the carbon.
In the first place, we better at once dispose of certain of these steels
by terming the added element a “scavenger” only. Such usually are
“titanium” and “aluminum” steels. These are generally ordinary carbon
steels in which a very small amount of titanium or aluminum has been
used to rid the alloy of certain gaseous or other deleterious elements.
Upon analysis, steels so treated often show no trace of the element
which has been added to do the work, all of it having passed into the
slag, carrying with it the obnoxious substances, which, had they
remained would have injured the quality of the steel. Manganese and
silicon which were spoken of in the discussion of the Bessemer process
as deoxidizing the metal, also exert just this same influence, though
there is usually added of these enough that a certain percentage remains
in the finished steel. Vanadium and titanium have a particular affinity
for oxygen and nitrogen, and aluminum for oxygen. By chemically
combining with these gases in the metal, and through possible other
influence, they help to produce sound steel having very good physical
properties. Vanadium, however, is much more than a “scavenger” as will
be seen later on.
Manganese Steel
Manganese steel was discovered and highly developed by Robert Hadfield
of Sheffield, England, along about 1882. His 11 per cent to 14 per cent
manganese steel with about 1 per cent of carbon has such great hardness
that it cannot be drilled or cut with tools. In forgings and castings it
is used for milling machinery for ore treatment; manganese steel rails
inserted around sharp curves and for “frogs,” etc., under severe service
conditions outlast ordinary steel rails three or four times; it goes
into various rolls and crusher parts, steam and dredge shovels, grab
buckets, sand pumps, gears, pinions, etc., which have to resist heavy
wear. It is much used, too, as a material for burglar-proof safes. The
alloy is far too hard to drill and too tough and strong to be broken. It
is said that no manganese steel safe has ever been drilled or forcibly
entered.
In forming irregular shapes, manganese steel must be cast and finished
by grinding but for ordinary bars and rails it can be rolled. In the
“raw” condition it is quite brittle and extremely hard. Quenching from a
cherry-red heat greatly toughens it and makes it ductile. Though now it
can be dented by a hammer blow and marked with a file, it always is so
tough that it cannot be machined with any tool. Ordinary annealing
treatment has no softening effect on the alloy.
Silicon Steels
When alloyed in the steel in certain quantities, silicon gives desirable
properties. Steels with from one to two per cent of silicon in the
tempered condition are very tough. For this reason the leaves of
automobile springs are often made from it. Steels with from 3 to 5 per
cent of silicon are much used in electrical appliances because of their
improved magnetic properties.
Molybdenum Steels
To a certain extent steels containing 3 or 4 per cent of the metal
molybdenum, and 1 or 1½ per cent of carbon are used in the construction
of permanent magnets. It is said that molybdenum is used in some modern
guns, which longer resist the corrosive effect of the powder-gases
because of it. A certain amount goes into the high-speed steels where it
replaces part or all of the tungsten. Here, however, it has been a
disappointment and the amount so used seems to be decreasing rather than
increasing.
Tungsten Steels
Steels with about ½ per cent of each of carbon and tungsten are
occasionally used for manufacture of springs, and with greater amounts,
e.g., ¾ per cent of carbon and 5 or 7 per cent of tungsten, for
permanent magnets for which they are claimed to be the best material
known. The use of tungsten in the tool steels (other than the high-speed
steels) is considerable.
Nickel Steel
Nickel steel is much used on account of its high strength. The most
usual alloy, perhaps, is the one which contains about 3½ per cent of
nickel. This is in addition to the carbon which may vary between .15 per
cent and ½ per cent. This 3½ per cent of nickel adds several thousand
pounds per square inch in strength to the steel, and when tempered, both
the strength and toughness are greatly improved.
Nickel steels of these compositions can readily be forged and rolled.
They are used for drop forgings, machine parts, engine and automobile
parts, in seamless tubes and for bridge members of great span.
Nickel will not rust so it does not surprise one that with 22 per cent
or more of nickel the steel is almost immune from ordinary corrosion.
Steels containing from 25 to 46 per cent nickel are variously used for
resistance wire, for valve stems, valves for motors, etc. The 36 per
cent nickel steel is the alloy, “Invar,” which has such slight expansion
and contraction with heat and cold that it is used for clock pendulums,
watch parts and for parts of measuring instruments.
Forty-six per cent nickel steel is called “Platinite.” It has
practically the same rates of expansion and contraction with heat and
cold as glass and for this reason it finds extensive use in incandescent
electric lamps. Wires of the alloy are fused into the glass bases and
connect with the filaments in place of the expensive platinum which
formerly was used.
As remarked above, the 13 per cent to 15 per cent nickel-iron alloys
soften with quenching but not with annealing. The 15 per cent nickel
steel has the highest strength of the nickel-iron-carbon series. Though
nickel and iron are each strongly magnetic, alloys of the two which
contain 24 per cent or more of nickel are not magnetic.
Chrome and Nickel-Chrome Steels
While “simple” chrome steel is pretty well known as a material for
products which require great hardness, such as balls, roller bearings,
files, rolls, five-ply safes, stamp shoes, projectiles, etc., and
heat-treated chrome-vanadium steels are now extensively used in forged
frames and shafts of automobile and other machines, a combination of
nickel and chromium gives steels which have been great favorites. With 2
to 3½ per cent of nickel, not over 3 per cent of chromium and ½ per cent
of carbon, these steels, when expertly heat-treated, can give “elastic
limits” anywhere between 40,000 and 250,000 pounds per square inch, with
good freedom from brittleness. They are very largely used for automobile
gears, axles, and other parts, for armor plate, for projectiles and for
many other purposes.
These alloys are also used for castings.
Chrome-Vanadium Steels
Within a comparatively short time the chrome-vanadium steels have come
to be very largely used, often in place of the chrome and nickel-chrome
steels. As vanadium is a “deoxidizer,” whereas nickel is not, the
chrome-vanadium steels show fewer imperfections than the nickel-chrome
steels and they also roll, forge and machine better.
They are used for automobile frames, shafts, for miscellaneous forged
and rolled articles and for heat-treated armor plates. Of this
comparatively new material about 90,000 tons were made during 1913,
according to a recently issued bulletin of the Department of the
Interior.
It is impossible, of course, to even begin to impart any adequate
conception of the qualities and great importance of the alloy steels for
purposes of construction. As has been shown they are special steels for
special purposes and their application is wide. Incorporation of the new
element in the alloy imparts peculiar and valuable properties: for
example, 12 per cent of manganese, great hardness and toughness; 23 per
cent of nickel, non-corrosive properties and great strength; chromium,
nickel with chromium or chromium with vanadium, strength and high
elastic limit (resistance to distortion) as well as great hardening
power when desired, this, of course, the usual hardening through
quenching from a cherry-red heat.
Very often instead of the single defining element, a combination of two,
three or even four of them is used. Such, of course, are rather
complicated steels having combinations of properties as might naturally
be expected, though very often these resulting properties are not those
which are expected. In fact no one can tell in advance what properties
any new combination of metals in an alloy will produce and often new
proportions of the same constituent metals give entirely different and
unique results.
The only certain method of ascertaining what characteristics and
properties a new alloy will have is to develop it and in that way find
out.
Description of a special and extremely important class of these alloys,
the “high-speed steels,” will serve to show how laborious, slow and
expensive a process development of new alloys may be and what
unlooked-for results are sometimes obtained.
CHAPTER XV
THE HIGH-SPEED STEELS
During early centuries the art of metal cutting made little progress
except in-so-far as the application of greater driving power and the use
of better machines were concerned. Lathes had been known since the sixth
century B. C., at least, but, of course, little was or could be
accomplished, comparatively speaking, before the invention of the steam
engine by Watt, which was the first contrivance to give sufficient power
for machining purposes. During the century which followed Huntsman’s
time, steel makers and smiths became very expert in the manufacture and
tempering of carbon steels. Lathe tools were made of these carbon
steels, the only available material, but even with the better machine
shop practice of the 19th century they were capable only of what we now
consider to be inefficient results. The trouble was that since carbon
tool steel gets its hardness from quenching from a red heat and then
“drawing” the temper by reheating and slowly cooling from 400° or 500°
F., tools made from it could not retain their hardness if their cutting
points became much heated, as occurred if the lathe was run too fast.
The usual cutting speed, therefore, was 20 or 30 linear feet per minute.
Speeds in excess of this took the temper out of the tools and soon made
them useless.
About 1868 Robert F. Mushet, a metallurgist of Sheffield, England, made
a momentous discovery. He found that a piece of tool steel which had
cooled in the air was as hard as some of those which he had quenched.
Being an investigator he set about discovering the reason for this
experience which was without precedent. Analysis showed that beside the
usual constituents of tool steel this particular bar contained tungsten,
a comparatively new metal. He experimented with some hundreds of
mixtures and evolved an alloy, which, in tools, would stand up under
machine speeds double those which could be used with carbon steels.
These new alloys became known as “air-hardening” or self-hardening tool
steels because they required no quenching.
The principal application of these new steels was in the cutting of
harder metal than it had before been possible to cut, and little
attention, apparently, was paid to the getting of greater outputs by
increase of machine speeds.
At this point Frederick W. Taylor of “efficiency” fame appears upon the
scene. While manager of the Bethlehem Steel Works in the nineties of the
last century, he was working upon the efficiency investigations for
which he later became so famous. During his investigations, with Maunsel
White he experimented with many air-hardening steels to determine the
best grades to use for their shop work. Getting some inconsistent
results they determined upon and made what was a most extended and
systematic investigation, one so thorough and complete that Taylor and
White have become part of the history of high-speed steels.
They produced new compositions the quenching of which could be from
temperatures greatly in excess of those which tool makers for centuries
had held to be ruinous and which really are ruinous to carbon tool
steels. The best results were obtained when the new steels were quenched
by plunging in oil from close to their melting points—a dripping or
“sweating” heat as it is called. This was something entirely new but it
developed that after proper drawing, steels quenched from these extreme
temperatures, would stand up under lathe speeds as great as 200 or 300
feet per minute.
Compare these with the miserable speeds of 20 or 30 feet per minute,
which were the average performances of the best carbon steels.
The secret of Taylor and White’s treatment was not long in coming out
and soon high-speed steel makers in Europe and America were vying with
each other in production of finer and finer high-speed steels.
The progress which has been made during the last twenty years and
particularly during the past ten has been astounding. Improvement has
followed improvement in composition, manufacture and heat treatment, so
that to-day, instead of the cut at 30 feet per minute, which was a high
figure with carbon steel tools, the modern lathe or shaper tool often
works at 300 or 400 or more feet per minute, and, with sufficient power
behind it, at somewhat lower speeds plows out ½ inch deep and ¾ inch
wide chips so fast that their removal to keep the machine clear is no
mean problem. Often 2,000 pounds of the material per hour can be thus
cut away with one tool.
As suggested, the new steels do not suffer such loss of temper from the
heat generated by the friction of the tool in the metal as occurs with
carbon steel tools. In fact, tools of high-speed steel work best after
“warming up” and they can run for a considerable period of time with the
point of the tool red-hot, though such is not advisable.
As is readily seen the essential property of the high-speed steels is
the so-called “red hardness” which is the ability to retain hardness at
red heat. This is several hundred degrees in excess of the temperatures
at which the carbon tool steels quickly lose their “temper.”
As forecast by Mushet, the essential constituent of the new steels is
the metal, tungsten. But tungsten alone cannot give the desired
property. Mushet, it will be remembered, was the metallurgist whose
patents for the use of manganese in steel Bessemer was obliged to
recognize to make his process a success, though the metal had earlier
been used in crucible steel. The air-hardening property of Mushet’s
steel was contributed by a happenstance combination of tungsten and this
same metal, manganese. It later developed that tungsten and chromium
were the best hardening elements and these have maintained their place,
though refinements of the past few years have made use of vanadium, and,
more recently, cobalt in addition. Usual amounts may be said to be
tungsten 14 to 25 per cent, chromium 2 to 7 per cent, with vanadium ½ to
1½ per cent, and cobalt up to 4 per cent, perhaps. The carbon content is
usually .6 to .8 per cent. Sometimes another comparatively rare metal,
molybdenum, is used in high-speed steels in place of part of the
tungsten, but its use does not seem to be on the increase.
Manufacturers differ considerably in formulas.
It will be noticed that at best there is left room for only 70 or 80 per
cent of iron in the alloy. From certain standpoints, the high-speed
steels might not at first thought be called “steels” at all since carbon
seems to be of so little importance. They might be considered to be low
carbon alloys somewhat similar to the newer “stellite” (an alloy from
which tools are made), which contains little or no carbon and no iron
but is made up mainly of cobalt and chromium. They fit in, however, with
the general and very comprehensive scheme of classification of the
iron-carbon alloys which has been developing over a period of twenty
years and there is no doubt among metallurgists and metallographists
that, as is the case with the alloy steels described above, they are
iron-carbon alloys—in other words, steels—the properties of which have
been greatly modified through the presence of the other elements.
Carbon, therefore, is an essential, though it is much less in amount
than in the carbon tool steels. The hardening and softening properties,
also, very definitely classify these alloys with the “tool steels.”
Stellite cannot be softened.
As with the carbon tool steels, most of the high-speed steels are made
by the crucible method, though a small but increasing amount is of late
being produced in the electric furnace. After careful pouring into small
ingots and cooling, the ingots are removed from the iron molds and
“topped” to remove any “pipe” or unsound portion. Then, if without
defect and satisfactory as to analysis, they are slowly and carefully
heated to forging temperature and are hammered out into bars. By this
method they are taken nearly down to the final size desired. The bars
are finished by rolling to size. After careful annealing they are ready
for shipment to the tool maker.
When taking heavy cuts a tool of to-day may exert as much as ten tons’
pressure against the metal it is cutting and the advent of this
wonderful material for tools necessitated the building of immensely
heavier and stronger lathes and other machines, which, alone, were
capable of giving them power to do their work. The high-speed steels,
therefore, have revolutionized metal-cutting practice and shop methods
and have very largely aided efficiency.
CHAPTER XVI
THE MECHANICAL TREATMENT OF STEEL
Molten steel is practically always poured into upright molds of cast
iron which shape it into long slightly tapering blocks of metal of
square or rectangular cross-section. After the ingot mold has been
stripped off, the still red-hot ingot cannot well be taken directly to
the rolls, for, while the exterior parts may have the proper temperature
for rolling, the interior of the ingot may still be liquid. The ingot,
throughout, should be uniform in temperature when it is rolled. It is
therefore put into a closed pit or furnace of proper temperature where
the center of the ingot can be cooling while the outer portions are kept
hot or are reheated if necessary, until all is ready for the rolling
operation.
It would take a “steel man” a long time to tell you all of the
unfortunate things that can and do happen to such blocks or ingots of
steel which influence their applicability to the purposes for which they
are intended. You must have learned of the most serious of
these—“pipes,” “cracks,” “segregation,” etc., through reports of
investigations of broken railroad rails and accidents caused thereby. A
word or two regarding these:
In the ingot mold the outside of the steel ingot is, of course, the
first to solidify. It may be hours after the freezing of the outer crust
before the interior is able to cool sufficiently that it, too, can set.
As steel, like most other metals and alloys, occupies less space when
“frozen” than it does when molten, there must occur a hollow space in
the interior since the crust is solid and cannot contract much. This
hollow space usually takes the form of a more or less elongated cavity
extending along the axis of the upper quarter of the ingot. It is called
a “pipe.”
[Illustration:
PIPE AND BLOWHOLES IN AN INGOT OF STEEL
]
Then, too, the metalloids of the steel do not always stay where they
belong. Even if the steel has been of a uniform chemical composition
when poured, the interior portions of the ingot after cooling will be
found to have a greater amount of sulphur, phosphorus and carbon than
parts which are nearer the surface. Such gathering together of
constituents of the steel is known as “segregation.”
With the development of the steel industry and the demand for greater
and greater tonnages, ingots have been made larger and larger. Piping,
segregation, etc., are very naturally accentuated in the large masses of
steel.
Much “gray matter” has been expended in attempting to overcome these and
other defects to which large steel ingots are liable. Covering the
molten ingot top with charcoal; filling in before complete
solidification with additional molten metal; and keeping the ingot top
molten by application of powerful gas flames have been, perhaps, the
most useful methods.
But, even so, piping and segregation have not been completely prevented,
though great improvement has resulted.
The usual way around the difficulty is to make certain that only the
bottom (or best half) of each ingot is used for the most important
products, such as locomotive and car axles, firebox and boiler plates,
rails, etc. The next or third quarter or a little more is utilized for
products which go into less exacting service. These may be plates for
ordinary water tanks, for flooring, for ship plates, etc. The top part
which contains the pipe is cut off and goes back to the furnace to be
remelted. It is termed “discard.”
The big steel makers themselves shape most of their steel into such
finished products as rails, plates, rods, and wire. Some of it is by
them reduced from the ingot into intermediate “blooms,” “billets,”
“bars,” etc., and sold in this form for the manufacture of axles, drop
forgings and the hundreds of products which we each day see.
It is a very fortunate circumstance that at a cherry-red or white heat
the carbonless irons and most of the steels can be quite easily
fashioned into products. As is well known to us the most usual methods
of mechanically shaping these metals while hot are by hammering, by
rolling and by forging in a press.
With sufficient power and proper appliances, soft and medium steel to a
considerable extent can be fashioned cold, but, of course, in this
condition its resistance to reshaping is immensely greater. The cold
treatment of these metals is usually some form of tube or wire drawing.
Certain other methods such as extrusion, spinning, etc., are also in
use, and, through them, some otherwise difficultly formed products are
made.
In one of the earlier chapters we saw that annealing refines (make
finer) the grain of a steel casting and improves its physical
properties. Annealing for refining purposes is practiced with other
steel products also, and with just as effective results.
However, the mechanical shaping of steel while at cherry-red or at a
white heat much more materially refines the grain while helping the
strength and greatly increasing the ductility of the alloy. Steel which
has been hot-forged or rolled is said to have been “hot worked.” Steel
usually is “hot worked,” for “cold-working” methods are not so generally
applicable and the product is more liable to suffer under the more
drastic treatment. The amount of “hot-work,” at proper temperatures,
that low and medium carbon steels will stand with improvement of the
grain and physical properties is considerable.
As we must anyway shape the metal into useful implements and other
products, it is fortunate that the quality of the metal is benefited by
the process.
[Illustration:
No. 69a. PHOTOMICROGRAPH OF COLD-DRAWN STEEL WIRE SHOWING DISTORTION
OF THE CRYSTALS FROM COLD WORKING
Hot Working does not produce distortion
but makes the grain finer. Annealing relieves
this distortion to a great extent.
(_Magnification 70 Diameters._)
]
Forging
[Illustration: [Man]]
Undoubtedly the earliest shaping of ferrous (iron) metals was by
hammering the small balls of metal into bars, spears or swords.
Presumably it was done with stone hammers which later had to give way to
hammers made of iron. These had sufficient hardness to serve the purpose
well.
For hundreds of centuries the shaping of iron, steel and the other
metals into tools and weapons must have been done by such forging
methods. It is not difficult for us to picture the early smiths at their
work, laboriously and yet very skillfully hammering into spear-heads and
sword-blades the lumps of iron or Wootz Steel which they had made in
their crude furnaces.
[Illustration:
THE OLD OLIVER FOOT-POWER HAMMER
]
[Illustration:
AN OLD FORGE HAMMER
]
In much the same way, though on a considerably larger scale and with
heavier and better hammers and tools, was the same work done up to the
time of the invention of Cort’s rolling process—about 1783. Various
styles of hammers were used, some with a spring pole attached to raise
them for the next stroke which was delivered by foot power, others known
as “helve” or “shingling” hammers gave periodical blows as teeth on a
revolving wheel lifted and allowed the hammer heads to fall. The heavier
ones often gave as many as seventy-five and the lighter ones which were
used for “tilting” (forging) shear steel into bars or implements as high
as three hundred blows per minute.
[Illustration:
THE SQUEEZER WAS SOMETIMES USED IN PLACE OF THE HAMMER
]
[Illustration:
THE OLD TILT HAMMER
]
Though Cort’s rolls very materially aided in the shaping of balls of
iron from the puddling furnace into bars, the hammering or forging
method remained the one by which finished iron and steel articles were
made.
About 1835 it happened that a very large propeller shaft for a new ship
was desired. Being so large, no one was found who could forge it until
the matter was put before an English iron-worker named James Nasmyth,
who had a reputation for ingenuity. Nasmyth roughly sketched out an
immense hammer which he proposed to operate by steam. There was no
opportunity to build it, however, for the propeller shaft never was
ordered. But the idea of the steam hammer got to certain French
engineers, who constructed one which Nasmyth came upon during a visit to
a French iron works. Nasmyth realized the importance of his invention,
which, luckily, the Frenchmen had not attempted to patent. A patent was
granted to Nasmyth.
[Illustration:
A BELT-DRIVEN POWER HAMMER OF TO-DAY
]
To most of us the steam hammer, still little changed in essentials, is
quite well known and some of us have witnessed the cracking of an egg
without breaking the egg cup which held it. The adjustment and
regulation of these mammoth hammers is so nice that with almost
successive blows a skillful operator can flatten a piece of iron and
then break the crystal of a watch without otherwise injuring the
timepiece. Needless to say, the steam hammer has proved to be the only
efficient hammering device for forging large pieces.
[Illustration:
TWO BOARD HAMMERS AND TRIMMING PRESS
]
But whether made in the small way of the village blacksmith, by the
larger helve, tilt, Bradley, or by the monster steam hammer, each
forging, unless made in a die, must be considered to be _specially
formed_ and no two pieces, when finished, are exactly alike. They are
always “hand made” articles.
Drop Forgings
Many years ago, what are known as duplicate or interchangeable parts,
therefore, were quite unknown and it is related that parts of the famous
English Enfield rifle were made in various parts of the civilized world,
shipped to the Tower of London and there assembled. But during assembly,
the various pieces had to be filed and carefully adjusted by hand
because no two parts were exactly alike. But the “Yankee tool makers” of
New England solved the problem by forging the pieces of which many
duplicates were necessary in a “die” or impression in a block of steel.
The forged pieces, of course, took the exact impression of the “die” and
successive pieces thus made were alike in size and shape. From finished
duplicate parts which went to London from the New England states, the
Enfield rifle was assembled with very little final finishing of the “cut
and try” variety.
Done at first with the die on a blacksmith’s anvil and with a light
hammer, this promising method soon developed expert “die-sinkers” (die
makers), also ingenious men of whom the term “Yankee Tool Makers” is
self-explanatory.
In connection with this work what are known as “drop hammers” came to be
largely used. Of these an important type were the “board hammers,” in
which the heavy steel hammer-head was attached at the bottom end of a
vertical board set between pulleys. As the pulleys squeezed and revolved
against the board it was carried up between them and dropped, when the
pulleys loosened it at whatever height was desired. Rapidly and
periodically ascending and dropping upon the anvil beneath, it quickly
forced the white-hot iron into the “die” upon the anvil, forming what
have since been known as “drop forgings.”
[Illustration:
NASMYTH’S STEAM HAMMER REVOLUTIONIZED STEEL WORKING
]
Commonly the hammer face itself carries the impression of the upper part
of the article to be formed, i.e., there is an upper “die” on the hammer
and a lower one on the anvil.
[Illustration:
MODERN FORGING OF AN AUTOMOBILE CRANK SHAFT
]
“Fins” were of course, left all around where the excess metal was
squeezed out from between the upper and lower dies. It shortly developed
that a second pair of dies shaped for trimming could clean the forging
of this excess metal, which is so well known under the appellation,
“flash.”
Nasmyth’s steam hammer, also, has been used very largely for
drop-forging work.
A “cast” metal is not and cannot be as dense, free from holes,
sponginess or other defects or as strong as “worked” metal. While often
not as cheap as castings as far as cost of production goes, “drop
forgings” are usually considerably superior to them and are to be
preferred. However, it does not pay to make dies unless for many pieces.
One or several “castings” can be made without great expense.
Forging of Large Pieces by Hydraulic Press
Of late years much forging has been done, not by the hammer which gives
such a sudden, superficial blow with shallow working of the piece, but
by hydraulic or other press, which very slowly squeezes the hot piece to
smaller and longer shape. Sir Henry Bessemer was one of the first to
realize the advantages of and make use of the press for steel working.
Unlike the hammer the press exerts a deep working of the piece which can
be seen to flow throughout under the stress rather than in surface only
as occurs under the hammer. This is very desirable as the interior,
which is known to have much coarser grain than outer parts, particularly
needs to be “worked.” In plainer terms the press seems to knead the mass
much as the bread-maker kneads dough, while the hammering method simply
batters down the outside. At a glance an experienced eye can tell from
the appearance of the end of a forging whether it has been pressed or
hammered.
[Illustration:
A MODERN HYDRAULIC FORGING PRESS
]
Pressures as high as 8,000 pounds per square inch are used in hydraulic
presses, though much lower pressures are more common.
Forging vs. Rolling
Though we have not yet considered the rolling mill or its products, we
understand that, in general, only products of regular and uniform
cross-section and of considerable length can conveniently be rolled.
Where they can be obtained of satisfactory shape and size, steel
products formed by the rolling process are highly desirable and are
usually cheaper than those which are produced by the forging process.
Compared with those made by the rolling process, forged products are
usually quite costly in labor and time.
Rolling mills, however, cost immensely more to build and equip than do
plants installing even the steam hammering outfit, so the rolling
process cannot pay except for such articles as are demanded in great
quantities. Articles of irregular and odd shape must, of course, be
forged and here, especially for very small articles, the drop-forging
process is available and highly satisfactory where enough pieces of one
kind and size are wanted to pay for the requisite dies.
Forged articles have another advantage which we should not overlook. The
physical properties which are imparted during forging are somewhat
superior to those which the rolls bestow. The physical properties shown
by the latter are very satisfactory, however.
CHAPTER XVII
THE ROLLING PROCESS
[Illustration:
EARLY ROLLS
]
After invention of the puddling furnace with its rather large yield from
the standpoint of those days, Cort about 1783 found the hammering method
unsatisfactory for his purposes and rolls were devised by him to
facilitate working of the larger balls of iron which his furnace
produced.
His rolls were provided with a series of grooves which systematically
reduced the balls of iron to pieces of longer and longer length and
proportionally decreasing diameters. They were power driven and served
very well as long as iron and steel were made in quantities no larger
than those which were produced in the puddling and crucible furnaces.
Quite naturally there was little or no change in the essentials of
rolling mill design until it was forced by the invention of the Bessemer
steel-making process. With that occurrence trouble began. The
open-hearth process followed, and, with the increasingly large steel
outputs of mills using these processes, necessity after necessity
developed which resulted in the highly developed rolling mills of
to-day.
[Illustration: [Mill]]
The “Two-High” Mill
The mill as invented by Cort had but two rolls and these were actuated
by a fly wheel. Turning in one direction continually, the rolls allowed
the piece being rolled to go through in only one direction, i.e., it had
to be returned from the rear to the front side of the mill after every
passage, usually called “pass.” This was done by the “catcher,” a brawny
man at the rear of the mill, seizing the piece, lifting one end bodily
to the top of the upper roll over which it was carried back with more or
less difficulty and awkwardness to the “roller,” who, from the front,
seized and entered it again into the next succeeding or smaller groove
of the rolls.
[Illustration:
TWO-HIGH ROLLS AND EFFECT ON THE PIECE
]
The “Three-High” Mill
[Illustration:
ACTION OF THREE-HIGH ROLLS
]
In 1857 John Fritz was watching his men at their slow and fatiguing work
at the two-high mill of the Cambria (Pa.) Iron Works. The thought struck
him that by adding another or third roll at the top, the piece could
also be given a pass on every trip back to the roller in front. The
rolls would of course pull it through, the work would be less severe on
the men, and, receiving passes in both directions, the piece would
receive the full number in approximately one-half of the time which was
then required, and, more important than all else, it would not have
nearly so long a time to cool and could be finished at a more desirable
temperature—a great advantage.
Strange to say the idea was immediately pronounced impracticable when he
mentioned it and it was necessary for him to go through a long fight to
obtain permission to make a trial.
The experiment was from the first successful but the mill burned one
Saturday night, having supposedly been set afire by workmen who feared
loss of their jobs. Rebuilt and manned by new workmen it ran with
success.
The succeeding ten years saw the “three-high” type of mill come into
extensive use both in America and Europe. Elevating or tilting tables
have since been provided which mechanically raise the piece to the upper
rolls, thereby relieving the workmen of this duty, which, with the great
increase in size of ingots and pieces rolled soon became very arduous.
To-day the “three-high” mill is just as important as ever.
The Reversing Mill
Having a fly wheel for the storing-up of power, the rolls must keep
turning continuously in the established direction. In England,
Nasmyth—the same man who invented the steam hammer—suggested that the
fly wheel be dispensed with and the two-high rolls reversed after each
pass. The piece would go back to the roller’s side receiving work in a
regular pass on the way, just as in the three-high return. The idea was
developed by Mr. Ramsbottom of the London and Northwestern Railway
Company. By the use of powerful enough engines the desired end was
accomplished and this type is now quite generally used for very heavy
ingots which it is not economical to lift by tilting tables in the
three-high mill process. There are certain other advantages also.
The above are the three general types of mills.
“Breaking-Down” the Ingot
Whether it is to be sold in intermediate shapes or further rolled down
into a finished product, all ingots have to be “cogged” or broken down
into intermediate-sized slabs, blooms or billets, for an ingot contains
altogether too much steel for any single plate, rail, rod, or other
finished product.
The cogging or first rolling is accomplished in one of the three types
of rolls already described but now more generally in the reversing mill.
[Illustration:
INGOT COMING OUT OF THE “SOAKING PIT”
]
When the tongs of the big overhead crane have lifted the white-hot ingot
out of the soaking pit it is run back and forth through the rolls which
are forced nearer and nearer together by the “screw-down” man so that
the piece continually becomes thinner and much longer with each pass.
Tables made up of small rollers geared together receive the long piece
as it emerges from the rolls. After each pass they bring the piece back
to the rolls which are now turning in the opposite direction. The table
on the other side repeats the process, the piece being regularly turned
on edge or slid from one side of the rolls to the other by steel guides
which can be raised up between the rollers of the tables where desired.
The direction of these as well as of the rolls themselves is controlled
through levers by two or three men standing at one side of the mill.
Reversing after each pass, with the big ingot apparently turning and
sliding itself into the most advantageous position for the next entry,
the big engine, mill and roll-train seem almost human.
[Illustration:
INGOT IN THE ROLLS
]
The Rolling of Steel Plates
It is manifestly impossible in the space at disposal to give in much
detail the rolling of many of the better known products. Fortunately it
is not necessary, for after description of the making of plate, pipe and
tube, and of rod and wire, the rolling of other forms such as rails,
bars, and the structural shapes, I beams, channels, angles, Z bars,
etc., can well be imagined.
[Illustration:
STEEL BILLETS FOR FORGING OR OTHER PURPOSES
]
From the contract department to the mill clerk come the orders for
plate, with detailed list of sizes and thicknesses, and definite
specifications of quality in terms of chemical and physical
requirements, etc.
[Illustration:
ROLLING INGOT INTO SLABS
]
After studying these, the clerk makes requisition upon the open-hearth
furnace for such tonnages of steel of various compositions as he
estimates will give him sufficient stock for his purposes. As soon as
possible the steel is made and poured into ingots which are transferred
to the soaking pits of the slabbing mill there to await disposition as
soon as the chemical laboratory has made analysis of the sample taken
and has reported by telephone the result to the clerk who ordered the
material. If close enough to the composition he ordered, he sends to the
slabbing mill his requisition ordering them to roll and cut the four or
six ingots of the “heat” into slabs of definite weights, each one
designed for a plate on a customer’s order.
The clerk at the slabbing mill determines to what width and thickness
each ingot shall be rolled and in what varying lengths it is to be cut
to furnish slabs of the definite weights ordered in the requisition.
After rolling the ingot down to proper width and thickness, the “piped”
end is cut off and “discarded.” Slabs are cut and piled in regular order
on a little flat steel car on which they are pulled, still red-hot, by
the shrieking little dummy engine out from the slabbing mill, through
the yard and to the plate mill furnaces, into which they are charged in
proper order. Here they remain until they are again white-hot and the
plate mill roller is ready for them.
Meanwhile record sheets giving the heat number, the number of the ingot
and the weights of the slabs in the order in which they were piled come
to the plate mill clerk. From these and the results of analysis of the
steel he makes out the rolling orders for the plates to be manufactured.
[Illustration:
SLABS FROM WHICH PLATES ARE ROLLED
]
You have heard how difficult it is to get solid ingots and how the top
eighth (or sometimes more) of an ingot is usually “piped” and discarded.
Now slabs from the balance of the ingot were piled on the car and have
been charged in the plate mill furnace. Those from the upper part of the
ingot (next to the discarded part) are used for the less exacting
qualities of plate. Only the bottom half of the ingot, which of course
is the solidest and best, goes into the higher grades of plate, such as
“fire box,” the choicest grades of “flange” steel, etc. The third
quarter goes into flange stock, “ship” and “tank” plate, the latter
representing miscellaneous lower-priced plate which may be used for
water tanks, steel flooring, etc. Steel known as “fire box” of course
must be of very high grade. It is used for parts of locomotives, etc.,
which come in contact with and likely will suffer deterioration from
flame, smoke, etc. The best “flange” goes into boiler plates and other
products which have to stand considerable bending to shape.
In making out his rolling orders the clerk sees that each numbered slab
is ordered rolled only into a product for which it is well suited. He
has to take into consideration the chemical composition, the probable
strength and other physical properties which were definitely named in
the specifications of the customer’s order. And, as the physical
properties of such steel are mightily affected by temperature and speed
of rolling and by rapidity of cooling, he must know mill practice and
constantly keep in touch with the results which the physical testing
laboratory is getting from bars sheared from such of his plates as have
been “pulled” for customers or the inspectors who represent them.
When hot, the slabs one by one and in regular order come to the rolls
from the furnace. Following his rolling orders the roller and his
helpers put each slab back and forth through the plate mill rolls, first
drawing it out to a width a few inches greater than the plate to be
sheared from it, and then turning it a quarter around, they draw it out
in the rolls until it has come down to the proper thickness or “gauge.”
If the clerk’s computations have been correct the plate will now have
the proper length. However, he may have ordered a slab of insufficient
weight to make it, particularly if the rolls have become much worn.
It will hardly be realized how much the width and thickness of the plate
ordered have to do with the “percentage” of trimmed plate which the mill
will get out of the slab ordered. There is a “fish tail” on each end of
a rolled plate. On a thin, wide plate this becomes rather serious.
Wherever possible the clerk puts two or three plates end to end and
perhaps narrow ones side by side, but he must not exceed the width which
the “shears” can “split” nor give the mill such a long plate that it
will become too cold to roll or too long to be conveniently handled.
[Illustration:
AN 84–INCH PLATE MILL
]
For diversion the mill men take delight in throwing an extra amount of
salt upon the plate to rid it of scale when nervous visitors have come
as close to the rolls as their conductor through the mill will bring
them. The explosion which comes from the usual amounts is much
intensified and it is not at all out of the ordinary to hear shrieks
from the women and to see surprised and somewhat dismayed men among the
visitors.
Plate mills are usually three-high with tables of small rollers on each
side which tilt to feed the plate into the rolls and to receive it on
the other side from which it is fed in again, either above or below as
the case may be. As the plates must be flat, perfectly plain rolls are
used. For plates which are very wide these rolls may be 140 inches or
more long and perhaps three feet in diameter.
The rolls, of course, are kept flooded with water to keep them cool. At
first thought one would think that the water would cool the plates which
are being rolled. It does not materially do so, however, the extreme
heat apparently keeping the water from coming in actual contact with
them. Thus they are rolled down from the three-inch thick slab to ³⁄₁₆″,
¼″ or ⅜″, and from 6″, 8″, 10″, or 12″ slabs into ½″, ¾″ or possibly 1″
or 1¼″ plates.
[Illustration:
THE ROLLING OF PLATES
]
All plates must be rolled very accurately to gauge, allowance of
variation often being not over one or two hundredths of an inch. The
roller must be a man of experience and of very good judgment for slabs
for almost any plate may come of any one of several thicknesses and
lengths. He must know his temperatures, speeds of rolling and the amount
of reduction given with each pass, and, particularly in case of thin,
wide plates, the condition of his rolls, which after two or three days’
wear will produce plates thicker in the middle than at the edges. As the
“screw-down” man on top screws the rolls together a little with each
successive pass and the “hookers” under the roller’s direction keep the
plate entering the rolls properly, he must with his very accurate gauge
measure the thickness of the plate as it nears completion. Especially
when plates are ordered and paid for by average weight per square foot
must he judge accurately the thickness of the center of the plate where
he cannot measure, and pull down the edges enough that the finished
plate when sheared will average right.
It is fortunate for the steel mill men of this country which does not
know the advantages of the metric system that a steel plate one inch
thick weighs very close to 40.8 pounds per square foot. This is an easy
figure and the clerk, roller, hot bed foreman, weighers and all
concerned “think” in terms of a plate one foot square and one inch
thick. One-half inch plate, therefore, weighs 20.4 lbs.; ¼″, 10.2 lbs.;
and ³⁄₁₆″, 7.66 lbs. per square foot.
As will be seen when we consider wire drawing and cold-drawn seamless
tubes, the strength and other physical properties of steel depend first,
upon composition, and, secondly, upon temperature at which they are
hammered, rolled, or otherwise “worked.” Therefore, plates can be much
modified in physical properties by finishing at chosen temperatures. A
steel containing .19% of carbon and .45% of manganese, for instance,
which in one inch plate should give a tensile strength of around 55,000
pounds per square inch, 58,000 pounds in ½″ or 62,000 pounds in ¼″ when
finished at usual temperatures, by slightly “colder rolling” can be made
to show a considerably greater strength. Of course, the ductility is
somewhat reduced, but, with a moderate amount of cold rolling, it will
not be enough to do harm.
All of these and many other details must be not only kept in mind but
become second nature to the plate worker.
After the final pass the plates go upon the “hot bed” where they are
laid out side by side in the order in which they have been rolled. They
must now have marked out upon them the boundaries of the smaller plates
or pieces into which they are to be cut. From a duplicate of the
roller’s sheet the hot bed foreman marks upon the end of each plate what
is to be laid out and boys or men wearing shoes with thick soles of old
belting or other cheap non-conducting material go upon them with chalk
and “squares” which are somewhat similar to the carpenter’s square but
having “legs” six and twelve or fifteen feet long. Though the soles of
their shoes smoke from contact with the still hot plate, they very
quickly and accurately mark out upon its surface the design which the
hot bed foreman has signified.
Usually the plates laid out are rectangular and of standard size but
often the boys have to lay out pieces of odd sizes and shapes, and
sometimes, what are known as “sketches” have to be drawn using arcs,
chords, radii, etc., as the student in geometry draws his geometrical
figures. Round plates for boiler heads, tank ends, etc., in plate mill
parlance are termed “heads.” These are marked out with string and a
piece of chalk. A boy with a pot of white paint follows and paints on
the surface of each piece laid out its size, thickness, the customer’s
name, the order number and heat number. That the plate can always be
identified, even after exposure to severe service or weather conditions,
another boy with steel stamps follows and stamps into the steel the heat
number.
Cranes with magnets or hooks convey the long plates to the “goose necks”
over the small rollers of which they are pulled to the shears where the
powerful steam or hydraulically-operated square-edged knife with ease
trims the ends and irregular edges along the chalk lines into the sizes
marked. Accuracy is everywhere necessary as ¼″ over or under ordered
dimensions or a variation of two hundredths of an inch in thickness may
and probably will cause rejection of a plate.
After weighing and recording, the plates are conveyed to the shipping
yard, where they are loaded by electro-magnets into cars for shipping.
In the plate mill process above described plates anywhere between 30 and
120 inches in width, say, can be rolled. And as mentioned, the more or
less irregular edges on the sides are “sheared” off. This extra
allowance, which must be given, of course becomes “scrap.”
For plates which can best be rolled in long narrow lengths a “universal”
plate mill is often used. This has vertical rolls just back of the two
horizontal rolls, which are adjustable so that the plate can be
regulated, not only as to thickness, but also as to width as well. Such
mills give plates which have to be trimmed on the ends only, the sides
being quite smooth.
[Illustration:
LOADING PLATE FROM THE SHIPPING YARD
]
The rolling of plate has been described thus in detail that a slight
conception can be obtained of the refinement and the minutia which is a
necessary part of modern mill practice. American outputs which have
grown to as much as several hundred tons per twelve-hour turn, require
that every operation move along with “clock-like” precision. But with
this immense tonnage and with all of the handicaps of occasional broken
rolls and machine parts, electric crane delays, and illness of important
men, the work must be and usually is kept up without serious delay.
Modern metallurgical and rolling mill practice is a marvel.
Sheets
[Illustration:
THE ROLLING OF SHEETS
]
Most plates are rolled from slabs which are about 36 inches wide, but
“sheets,” which are plates less than ³⁄₁₆″ thick are rolled from much
smaller-sized slabs known as “sheet bars.” After “pulling out” into
sheets these may be folded once or even more times, so that from two to
eight thinner sheets are rolled at once. That they may not weld or stick
together under the heavy pressure, they must be rolled colder than are
single plates. They are later trimmed and pulled apart. Some mills start
sheet bars of smaller size, for each sheet a separate piece, which,
after drawing out somewhat are piled, two, three, four or five high.
With coal or charcoal dust—either dry or mixed with water—between them,
they are heated and rolled, the charcoal and coal dust keeping them from
sticking together. After annealing, pickling, etc., they may be cold
finished in polished rolls or otherwise treated according to the purpose
for which they are intended. After straightening some are galvanized,
others are tinned, blued or painted. Most of them are sold “black,”
i.e., with no coating at all. Terneplate has a coating of 75% of lead
and 25% of tin.
The Rolling of Rails and Structural Shapes
It will be readily understood after reading the above, that, instead of
using plain rolls, mills for rolling steel rails, I beams, channels,
angles, Z bars, rods, etc., must have grooved rolls. For these products
the first pass will be through a groove slightly smaller than the bloom
or billet. Successive passes will be through other grooves in the same
set of rolls which will gradually make smaller and bring more nearly to
the finished shape the piece being rolled.
[Illustration:
RAIL IN THE FINISHING ROLLS
]
Before our eyes the white-hot bloom enters the three-high mill, goes
backward and forward through the rolls and very shortly assumes the
general shape desired. Each pass thereafter brings it nearer to the
finished shape. Rails, for instance, are rolled out from the blooms into
one long rail perhaps 140 feet in length which glides along like a huge
snake to the swiftly revolving “hot” saws which are so spaced that four
33–foot rails are sawed from it at the same time. As the rails pass from
the saws to the cooling bed they are marked by a revolving stamp. When
cool they go to the straightening yard, are straightened, drilled,
inspected and later loaded into cars for shipment.
The production of all kinds of finished rolled iron and steel products
in the United States during the past twenty-eight years is given in the
following table which shows how extensive are our rolling mill
industries and the rapidity of their development.
────┬─────────┬─────────┬───────┬─────────┬──────────┬──────────┬──────────
Year│Iron and │ Plates │ Nail │Wire Rods│Structural│All Other │ Total
│ Steel │ and │ Plate │ │ Shapes │ Finished │Gross Tons
│ Rails │ Sheets │ │ │ │ Rolled │
│ │ │ │ │ │ Prod. │
────┼─────────┼─────────┼───────┼─────────┼──────────┼──────────┼──────────
1887│2,139,640│ 603,355│308,432│ │ │ 2,184,279│ 5,235,706
1890│1,885,307│ 809,981│251,828│ 457,099│ │ 2,618,660│ 6,022,875
1895│1,306,135│ 991,459│ 95,085│ 791,130│ 517,920│ 2,487,845│ 6,189,574
1899│2,272,700│1,903,505│ 85,015│1,036,398│ 850,376│ 4,146,425│10,294,419
1901│2,874,639│2,254,425│ 68,850│1,365,934│ 1,013,150│ 4,772,329│12,349,327
1903│2,992,477│2,599,665│ 64,102│1,503,455│ 1,095,813│ 4,952,185│13,207,697
1905│3,375,929│3,532,230│ 64,542│1,808,688│ 1,660,519│ 6,398,107│16,840,015
1907│3,633,654│4,248,832│ 52,027│2,017,583│ 1,940,352│ 7,972,374│19,864,822
1909│3,023,845│4,234,346│ 63,746│2,335,685│ 2,275,562│ 7,711,506│19,644,690
1911│2,822,790│4,488,049│ 48,522│2,450,453│ 1,912,367│ 7,316,990│19,039,171
1913│3,502,780│5,751,037│ 37,503│2,464,807│ 3,004,972│10,030,144│24,791,243
1915│2,204,203│6,077,694│ 31,929│3,095,907│ 2,437,003│10,546,188│24,392,924
────┴─────────┴─────────┴───────┴─────────┴──────────┴──────────┴──────────
Specifications and Inspection
Customers, of course, have a right to see that their specifications are
lived up to. Though years ago the mills perhaps intentionally sold to
customers products which did not fulfill his specifications to the
letter, it is not generally so to-day. Now the mills’ own inspectors are
commonly more severe in their rejections of products than are the
representatives which the customers themselves send. Not only does the
mill laboratory make careful and accurate analysis of each heat or batch
of steel made, but, after its application to orders, pieces of plate,
shapes or rails rolled from it are examined, gauged, and test bars of
the steel are pulled in the physical testing laboratory.
The mill rightly recognizes that it is for its own interest that the
standard of its product be kept high.
A trip through one of the large steel plants with its furnaces, its
blooming and slabbing mills, its rail, plate, structural and rod mills
is one of the most interesting that can be taken. If the visitor is not
afraid of smoke or dust or of what seems to him an uncomfortable heat on
a warm day he will discover new worlds. No particular attention is paid
to the casual visitor to the plant, but, for those who show real
interest, steel men have a warm welcome, from manager to the sample
boys.
CHAPTER XVIII
THE ROLLING OF RODS
Steel rods and what we will call large wires are rolled from billets
which are long, and approximately square blocks of steel. The size and
shape of the billets used vary, depending upon the process and the size
of the rod to be rolled. Much of the finished rod is sold as such for
various purposes for which round steel is desired and an immense tonnage
of one of the smaller sizes is used as the intermediate raw material
from which wire is drawn.
Rod mills have a very interesting history, which, by the way, is but one
of several histories of the development of the iron and steel processes
and products which should make us proud of the genius of man in the way
of metallurgical, mechanical and business development.
The Belgian Mill
The first bars and rods were rolled in the old two-high mill, where,
after each pass they were pulled back over the top roll and inserted
into the next groove by the roller as is usual with the two-high mill.
Then along came the three-high mill which probably resulted in greater
advantage to rod rolling than to other lines, important, even, as it was
to them.
But long, thin bars or rods are quite pliable when at white or rolling
heat and the “catcher” soon discovered a way by which he could save time
and push up his tonnage. He skillfully caught the forward end of the bar
as it came through, and, giving it a quarter twist, he inserted it in
the proper return groove without waiting for the whole bar to run
through to his side before starting it on its return journey. This, of
course, was easily possible with the slow speeds then used.
Naturally the rod coming through from one side and going back to the
other formed a loop.
The roller, at the front, was not slow in discovering that on long rods
he could do the same thing, so rods were soon regularly going through
three or more passes at the same time.
[Illustration:
WIRE DRAWING OF 350 YEARS AGO
]
It was soon found out that it was better to use a second set of rolls
placed end to end with the first for the third and subsequent passes as
the roller had hardly room or time to loop the rod on the return and
start his next one. This second set of rolls was connected on the same
shaft and therefore made to run at the same speed as the first. It
worked out that by use of such extra sets of rolls and an additional
helper or two the same long rods could be running through as many as six
or seven passes at once with a great saving in time.
As it came from the final pass the forward end of the finished rod was
seized in a pair of tongs by a boy who ran with it away from the rolls,
stretching it out along the floor to cool. As the various sets of rolls
were connected on the same driving shaft and revolved at the same speed,
the loops which were formed between passes continually grew longer.
Here, too, boys with iron hooks were useful in controlling the loops.
Later it was found advisable to have each succeeding mill speeded enough
to take the rod as the preceding pair of rolls delivered it; then the
loops remained of approximately constant length.
[Illustration:
PLAN OF A MODERN LOOPING MILL (THE GARRETT MILL)
]
To push production the mill was run faster and faster. As longer rods
were rolled, a hand-operated reel was devised to which the boy attached
the forward end of the rod while another turned the reel. But the speed
of the mill was limited mainly because of the slow reel and the awkward
method of getting the rod attached to it.
The Bedson Continuous Mill
About 1867 George Bedson of England, invented the first “continuous”
mill.
Instead of looping the rod around and back through the rolls he put
several sets of rolls each in front of the other with every other pair
vertical. This alternate horizontal and vertical roll arrangement was
necessary because the reduction of the billet or rod in any pass can be
only in a direction perpendicular to the axis of the rolls. The roller
and catcher had given it a quarter-turn twist each time they started it
into the two-high rolls. Bedson’s successive pairs of rolls were set
close together, each pair being speeded enough that it took the rod
exactly as fast as the preceding pair delivered it. The billet or rod,
therefore, traveled through them in a straight line.
This continuous process was, of course, of great advantage in that
considerable speed could be attained and there was not the rapid cooling
nor the opportunity for loss by scaling from exposure to the air which
occurred with the long loops of the Belgian Mill. It was a great
advance, but the speed was yet held down by the finishing pass and the
inability of the reel to coil the rod fast enough.
The greatest development came through the inventive genius of two men,
Charles Morgan and George Garrett, who developed the two separate types
of mill which have made the rod rolling industry what it is to-day. The
work of both was done in this country.
The Morgan Continuous Mill
Morgan’s also was a continuous process. The billet was put in at one end
of a new type of heating furnace which Morgan devised, and was gradually
pushed along to the other end. From this second or outgoing end, the
long, narrow, white-hot billet went through several pairs of two-high
rolls set close together, each successive pair having smaller grooves
than the one preceding it, so that, after traversing these several pairs
of rolls, the rods emerged from the last pair finished, having traveled
in a straight line through them.
Hand reeling was much too slow for Morgan who invented two different
types of high-speed automatic reel which, in the highly speeded mills of
to-day, receive and coil wire coming from the finishing passes at speeds
of over one-half mile per minute. It is stated that the billets and rods
therefrom traverse the rolls so fast and the pressure is applied so
quickly and so hard that the rods emerging from the finishing passes are
hotter than were the billets when they went into the rolls.
[Illustration:
THE BILLET TRAVERSES THE MORGAN CONTINUOUS MILL AT HIGH SPEED EMERGING
FROM THE LAST ROLL AS FINISHED ROD
]
As was explained, no reduction in thickness is brought about by the
sides of the grooves in the rolls. Therefore, a bar or rod must be
turned after each pass unless Bedson’s alternating horizontal and
vertical rolls are used. This turning Morgan did in spiral tubes
inserted between the successive sets of rolls, all of which were
horizontal. These tubes operate as does the “rifling” in a gun barrel,
in turning the rod.
In Bedson’s mill, with its alternating horizontal and vertical pairs of
rolls, it was possible to roll only one rod at a time. With Morgan’s
system, in which all rolls were horizontal, several rods could be
traversing the mill side by side.
With the high-speed reels and what are known as “flying shears” in which
billets or rods can be cut to any length while going at full speed,
Morgan’s mill had come to a high stage of development. It was
practically automatic.
The Garrett Mill
While Morgan was developing his continuous mill, William Garrett was
improving the old Belgian mill. Garrett believed and insisted that with
proper working, rods could be rolled from billets of much larger
diameter and greater weight than the long narrow billets which had been
used. He eventually did away with an intermediate rolling operation by
using larger billets.
You remember that in the Belgian mill the “catchers” looped the rods
back into the rolls. To do this work automatically Garrett inserted
between the sets of rolls looping troughs which guided the forward ends
of the rods around and into the next groove in the rolls. These troughs
are called “repeaters.” It was found that while they worked very well
for looping the cross-sections known as the squares, they were less
suitable for looping the oval sections which were produced with every
other pass. These were better and more safely done in the old way, i.e.,
by manual labor. They are generally so done to-day. With the Morgan
high-speed reel, sloping floors to better take care of the loops, and
with successive pairs of rolls each running at higher speed than the
preceding pair, the Garrett mill has apparently kept pace with the
Morgan.
Each has its advantages and each is used for certain classes of work.
For long continued runs on rod of one size the Morgan mill can produce
more cheaply, its product is more uniform in temper and the loss from
scaling is less as little of the rod is exposed while in the mill. As
the first pair of rolls in the Morgan mill is set close to the furnace,
less than one-quarter of the billet is in the mill at any one time and
the forward end of the billet is on the reel as finished rod before the
last of the billet leaves the furnace. With any process something
occasionally goes wrong so that the rod does not follow the path
intended. In such cases misshapen or tangled rod results. Such spoiled
billets or rods are called “cobbles.” With what is known as the “flying
shears,” which in the Morgan mill cuts the billet or rod while it is
traveling at high speed, the rear part of the piece can be cut off and
saved in case of cobbling. On this account the Morgan mill is said to
give less scrap than the Garrett.
The Garrett mill, on the other hand, gives rod which is more uniform all
along in shape and diameter and it has the considerable advantage that
it is quickly adaptable to change of product; it does not require such
complicated and nice adjustment as does the Morgan mill. So, despite the
greater danger to the rollers from the circling loops about them, which
occasionally become unmanageable, the Garrett mill is largely used.
CHAPTER XIX
WIRE AND WIRE DRAWING
It may be rather disconcerting to some enthusiastic ones who assume that
we Moderns have made all the progress that is worth while to learn that
so many of our supposedly new products were far antedated. In the case
of wire, again, we were antedated as much as 30 centuries. The wires
which were produced by the Ancients, however, were usually of the noble
metals, gold and silver. They were not drawn through dies as are the
wires that we know, but were hammered into shape from long, thin strips
of metal. The earliest use of our “drawplate” method of which we find
authentic mention was in the 14th century in Germany. The wire was
hand-drawn. Machine-drawn wire was being produced in England as early as
1565.
In the United States the wire drawing industry had become pretty well
established by the middle of the 17th century. As Cort had not at this
time invented the rolling process for bars and rods, very uneven strips
of metal only were available from which to draw the wires. But, even so,
with our highly developed rod mills and our present very satisfactory
No. 5 wire rod to begin with, our wire drawing methods are yet seemingly
crude and show small advance compared with the very great progress which
has been made in other lines of the iron and steel industry.
Unlike the processes of forging, rolling, etc., drawing of wire is done
cold. In this condition steel has its highest strength to stand the
strain. The rod or wire is pulled through very hard cast iron or steel
dies, the general process being well likened to pulling a rope through a
small knot hole.
As was seen in the rolling of plates, “cold working” raises the
strength, lowers the ductility and embrittles steel more than does the
regular “hot working” at the cherry-red or white heats which are usual
in the forging and rolling processes. If the so-called “cold finishing”
at very low red or black temperatures affects the physical properties of
plates, it can be readily understood that cold-drawing of wire and of
seamless tubes at ordinary temperatures of 70 to 100° F., must
considerably accentuate the effects noted. So much is this so that drawn
wires and tubes have to be frequently annealed, i.e., heated to a good
red heat for a time between successive trips through the dies, annealing
having the effect of off-setting the lowered ductility and increased
brittleness which would cause the wire to break. In some cases the wire
has to be annealed after each pass or draft, but oftener several passes
are possible before annealing is necessary. This depends largely upon
the quality of the steel used and the amount of “reduction” attempted in
each pass.
The raw material, No. 5 soft wire rod, which is about one-fifth of an
inch in thickness, comes to the wire-drawing plant from the rod mill.
This No. 5 rod is the thinnest which the mills have found it economical
to roll, so further pulling down in size can best be done by “drawing.”
[Illustration:
BEFORE DRAWING INTO WIRE THE SCALE MUST BE REMOVED FROM THE RODS BY
“PICKLING” IN ACID
]
As all iron and steel materials which have cooled in air from a red heat
are covered with a hard, brittle scale of iron oxide, the rod must first
be “pickled,” i.e., digested in hot, weak sulphuric acid, which, in 10
or 15 minutes so dissolves and loosens the hard surface that it can be
readily jarred loose and washed off. By immersion in a vat of boiling
milk of lime, the pickled rod is given a lime coating which neutralizes
any acid which remains, and, when dry materially aids in the lubrication
of the wire while it is going through the dies. Thorough drying, called
“baking,” is accomplished in the dry house at 300 or 400° F., from which
the rod with soft, scaleless, lime-coated surface goes to the “drawing”
benches. After rinsing free from the pickling acid, the wire is often
allowed to acquire a soft film of rust by spraying it with water and
keeping it wet in the air for a short time before going to the lime vat.
This rust or “sull” coat itself assists in the lubrication. However, the
color of the product is not as good as when the “sull” coat is not used,
and such wire usually goes into articles for which darker color is no
drawback.
[Illustration:
SINGLE WIRE-DRAWING BLOCK
]
[Illustration:
A WIRE-DRAWING DIE
]
The dies with more or less funnel-shaped holes of accurate diameter are
set vertically. They are of extremely hard material in order to stand as
long as possible the severe service without excessive wear, which sooner
or later so enlarges the holes that the dies become useless. Then they
must be removed and either discarded, or, in the case of steel dies, the
holes reduced by hammering and redrilled. Often the holes of worn cast
iron and steel dies are enlarged to the next larger size and so used.
[Illustration:
THE DRAWING OF WIRE
]
The loose end of each coil of rod of wire is hammered or otherwise made
smaller for a length long enough to be threaded through the hole in the
die and firmly grasped by tongs or “pliers.” The die being firmly
fastened, the draw head, of which the pliers are a part, is drawn away
until a sufficient length has come through that it can be attached to a
revolving block or drum which thereafter continuously pulls the rod
through the die and winds it into a coil about the block as it comes, at
speeds as high as 400 feet a minute.
As the rod passes through the box in which the die is fastened, besides
its lime or sull coating it is sometimes provided and takes up other
lubricants, such as soap, grease, or tallow.
The coil of wire is lifted off the drum and tied, or is redrawn to wire
of a smaller size.
After from one to several drafts, depending upon conditions, the wire
must be annealed for several hours, and again pickled to remove the
scale formed, washed, lime-coated and dried. For wire which must undergo
many reductions or passes, as must all small wires, several drawings,
annealings and cleanings must be gone through with before it is down to
the desired size.
In order to work properly and not cut the die too fast, which would
rapidly enlarge the hole and cause the rear end of the wire to be of
different thickness than the forward end, the rod or wire must be of
maximum softness. Either under-cleaning or over-cleaning will make the
metal harsh, as, also, will under-baking in the dry ovens. These they
endeavor to avoid in preparing the rod.
Other lubricants may be used and certain finishes or colors given to the
wires. Such are the various white to red coatings which come from using
tin and copper sulphate solutions instead of lime coatings. This is more
often done with fine wire than with thicker. Certain “patenting”
processes make use of various methods of tempering wires by cooling in
air after a last annealing.
Not only are wires ordered to size but many times also of certain temper
or grades of hardness and stiffness. These various tempers are
determined largely by the chemical analysis of the steel and the number
of drafts after the last annealing.
Piano wire is well known to have a very high strength, sometimes as high
as 400,000 pounds per square inch. Spring wire is rolled and drawn from
higher carbon steel than is ordinary wire, and, through heat treatment,
the best properties are imparted to it.
Besides being in this form used for miscellaneous purposes, thousands of
tons of wire are annually made into wire nails, staples, wire fence,
barbed wire, etc., all of which products are of American derivation,
with the possible exception of wire nails. Even this, however, if not
originally ours we have made ours through our great production. All are
very speedily turned out by automatic machines into which the wire or
wires feed. The product drops out into boxes below or is rolled into
coils as is the case of the wire fence and barbed wire. The speed of the
machines is so great that the eye cannot begin to follow the operation.
Wire being so well known it is not necessary to speak of its uses, but
it will be interesting to call attention to the very extensive
production of this steel product. Of the 32,000,000 tons of steel
produced in 1915, for instance, over 3,000,000 tons were rods and wire,
and when we think of the length of small or even average sized wire that
it takes to make a pound, a hundred weight, or a ton, some slight
conception of the amount of wire produced and used comes to us. As the
material for wire gauze and cloth, wire rope, cable, piano strings,
springs of all sorts, pins, needles, nails, fence, barbed wire, and the
myriad of other things, wire really is one of our great products.
It was mentioned that piano wire had been made which had a tensile
strength as high as 400,000 pounds per square inch, which is six or
seven times the strength of an equivalent cross-section of steel rail or
plate. Wire is undoubtedly the strongest product which is made from
steel. This is why wire rope or cable is so strong. It is made by
twisting together many small wires.
Though extremely recent from the standpoint of our world’s history, the
building of the famous Brooklyn Bridge by the Roeblings was far enough
back that we likely have forgotten that each of the four big 15¾–inch
cables is made up of 5,296 separate wires tied into a bundle.
Undoubtedly the best preservative against corrosion for iron and steel
is what is known as “hot galvanizing.” Much wire is hot galvanized. In
this process the wire is “pickled” in weak acid to remove scale, given a
soft red coating by dipping in weak muriatic acid and drying. The
strands of wire 20 or 30 abreast are run through a kettle of molten
zinc. The wires are wiped smooth and free from excess zinc by pulling
them through asbestos pads. A continuous coating of zinc is thus
permanently left upon the surfaces of the wires which very effectively
keeps them from rusting. Wires which have to stand severe weather
conditions may not have the excess zinc wiped from them. Telegraph and
telephone wires often have the thicker or unwiped coating.
Very naturally the drawing of wire requires use of much greater power
than the rolling of rod and the speed of drawing is nowhere near as
great. Therefore the cost of wire is comparatively much greater than the
cost of rolled products. In the case of watch springs it was once
computed that the product had to bring 50,000 times the cost of the
steel from which the wire was drawn.
“Continuous” wire drawing has not been so successful nor advantageous as
was the continuous rolling of rods. It has been possible in a small way
and with certain grades of the product to apply the continuous process,
but, mainly on account of mechanical difficulties, continuous drawing of
wire seems to be comparatively unimportant.
CHAPTER XX
THE MANUFACTURE OF PIPE AND TUBES
Tubes of some sort have been in use by man since very early times.
Nature provided the first ones in the way of hollow stems of shrubs,
such as alder and bamboo. Some of these we saw in use by the early
smiths conveying the blast of air from their goat-skin bellows into the
crude clay furnaces built in the hillside.
Tubes made of clay, stone, lead and bored logs were also used. Much
later pipes made of cast iron came into rather extended use for the
conveyance of water. The general use of gunpowder greatly accelerated
the manufacture of small tubes which the smiths produced for gun barrels
by hammering and welding together long, red-hot strips of wrought iron
about round rods or “mandrels.” About 1815 illuminating gas came to be
used in England for house lighting. This brought a demand for tubes of
considerably greater length, which were first made by screwing or
otherwise fastening together old gun barrels which were very plentiful
at that period.
The first patents for the making of welded pipe as we know it were taken
out in 1824 and 1825, the latter for the butt-welding method of pulling
a narrow iron plate, called “skelp,” through a bell-shaped orifice which
curled it and welded the edges together very much as is done to-day by
this process.
Our modern pipe of both butt-weld and lap-weld varieties is manufactured
either from wrought iron or from soft steel.
The Butt-Weld Process
[Illustration:
EARLY WATER PIPES
]
[Illustration:
HOW BUTT-WELD PIPES ARE MADE
]
From the double refined puddled iron, in the case of wrought iron, or
from billets of soft Bessemer or open-hearth steel, long narrow plates
are rolled. The width and thickness of these plates, which are known as
“skelps,” are exactly such as will give pipe of the desired diameter and
gauge. In order that the weld may be solid all along, the skelp as it
leaves the rolls has edges not exactly square but very slightly beveled,
so that the surface which is to form the interior of the pipe is
slightly narrower than the other.
After trimming the pieces of skelp so that each has one end with a sort
of point where the tongs are to take hold, they are laid side by side in
a heating furnace and left there until they have become white-hot.
Just in front of the furnace is the “bell,” with a second and slightly
smaller one in front. With strong tongs the workman reaches into the
furnace and fastens onto the pointed end of a piece of the white-hot
skelp. Hooking the handle of his tongs into the draw chain the skelp is
drawn through the first and second bell, the first bending it almost
into tube shape, the second completing the operation and pressing
together the edges of the plate in the top of the bell so tightly that
they weld.
[Illustration:
PLATES CALLED “SKELP” ARE FIRST ROLLED
]
The pipe now goes through what are known as cross rolls, the axes of
which are somewhere near parallel with the axis of the pipe. In these
the pipe is rapidly spun around, surface-cleaned and straightened. Going
up a cooling incline it goes to tables where the ends are cut off and
the product inspected.
A very important part of the inspection is the hydrostatic or water
test. One at a time the pieces are tightly fitted in between two
water-tight caps, water is turned into the pipe and gradually brought up
to the testing pressure of 600 pounds or more per square inch according
to specifications.
[Illustration:
CHARGING SKELP INTO THE HEATING FURNACE
]
[Illustration:
DRAWING BUTT-WELD PIPE
]
Pipes of diameters between ⅛″ and 3″ are usually made by the butt-weld
process.
Lap-Welded Pipes
Pipes larger than 3″, and boiler tubes or other particularly high grade
welded tubes of 2″ and over are usually “lap-welded.” This process gives
a considerably more reliable product than does the butt-welding process,
for reasons which are readily seen.
[Illustration:
HOW LAP-WELD PIPES ARE MADE
]
[Illustration:
BENT SKELP FOR LAP-WELDING BEING CHARGED INTO FURNACE
]
Skelp for lap-welding is rolled in just the same way as is skelp for
butt-welding except that the edges are “scarfed” or decidedly beveled so
that the two edges can make a considerable lap without increase of
thickness of that part of the wall. These pieces of skelp are charged
into the heating furnace just as occurred in the butt-weld process and,
after coming to a white heat, they are drawn through a sort of bell or
die which curves them so that one edge considerably overlaps the other.
Back they go into the furnace to regain any heat that has been lost,
for, to weld properly, the skelp must be hot enough that any scale which
had covered it drips off.
[Illustration:
THE LAP-WELDING ROLLS WITH MANDREL IN POSITION
]
The welding rolls are very short rolls, almost “sheaves” or wheels, with
concave edges of exactly the outside diameter of the pipe to be formed.
Between these two rolls, at the end of a long straight bar, is a mandrel
or projectile-shaped ball of high-speed steel over which the white-hot
tube must be pushed.
The reheated, curved skelp is pulled from the furnace and the forward
end forced into the rolls which shoot it through and over the mandrel at
high speed, forcing together and welding under heavy pressure the
overlapping edges of what was formerly the plate. Amid the noise and the
shooting sparks an unsuspecting bystander is quite startled by the
suddenness of it all.
While still hot the pipes pass to “sizing” rolls which correct any
variation in inside and outside diameters. The cross or straightening
rolls next smooth and clean their surfaces while straightening the pipes
or boiler tubes.
After the first trip through the welding rolls, boiler tubes and certain
other high grades of pipe go back into the furnace where they are
reheated. They are again put through the welding rolls to make
absolutely sure of a tight weld.
After cooling, the ends of each pipe are cut off. Because of the
“scarfing” of the edges and the great pressure of the rolls, it is
difficult to tell where the welds occur, the thickness of the walls
being practically uniform all around. Lap-welded pipe of as great as 36″
diameter has been made in this way.
[Illustration:
PIPES IN SIZING AND CROSS ROLLS
]
The water-pressure test is given to all lap-welded pipe as are certain
tests for tensile and torsional strengths, and for ability to flatten
without breaking. In the case of boiler tubes, a piece is cut from each
end of each tube, which must stand flanging or spreading “cold” and also
must crush down endwise under the heavy pressure applied in the testing
machine without fracture or opening of the welds.
The pipe may be “threaded” to order or shipped as it comes from the
testing bench.
[Illustration:
THE FINISHING END
]
As remarked, both butt and lap-welded pipe is regularly manufactured
from wrought iron and from steel.
[Illustration:
HYDROSTATIC TEST OF THE PIPES
]
It was suggested during the discussion of the manufacture of wrought
iron, that, owing mainly to high labor costs, wrought iron was with
difficulty competing with the soft steels. Wrought iron is noted for its
welding properties and it has always had its loyal admirers. Aside from
its application as “bar iron” which always has been and still is in
favor with many metal workers for miscellaneous purposes and its use as
Swedish bar iron or low phosphorus melting bar by makers of crucible
steel as a base for their product, wrought iron probably finds its next
most favored place as a material for pipe as is shown by the table given
in Chapter VI.
While not as strong as steel pipe under hydrostatic test, many pipe
users insist that, presumably on account of its slag enclosures and
cinder films which are supposed by some to surround and protect the
fibers, wrought iron pipe outlasts steel pipe when used under conditions
which induce corrosion. Others are as strenuous in their denial of this
assertion and this subject of comparative wrought iron pipe and
steel-pipe corrosion is still a very live issue. For many years this
matter has been under investigation. Hundreds of tests have been made
and discussed by learned societies and their committees. The
laboratories and testing departments, too, of the large pipe
manufacturers and their customers, have made extended investigations.
However, the conditions under which pipe is used are so varied and the
time required for any true and decisive test is so long that really
conclusive results have not been forthcoming. With other materials, each
condition and corrosive influence is largely a “law unto itself,” and
one wonders if such may not prove to be the case with these materials
also. As suggested, a great quantity of published information giving
comparative service tests is available for those who are particularly
interested in this subject. How much of the decline in tonnage and in
percentage of the total skelp produced, is due to the approximately 30%
greater cost of wrought iron pipe and how much to satisfactory
performance of its competitor must be left to you to judge.
Fortunately pipe of both kinds is available, meanwhile, and one can get
whichever he prefers.
The uses of pipe are almost innumerable. Great quantities are used for
conveyance of water, oils and gases, for ice-making and refrigeration,
the heating and draining of buildings, for dry kilns, hospital beds and
apparatus, electric light, railway and telegraph poles, pipe railings,
for conduit work, etc. For many of these applications, the seamless
variety is now utilized, however.
For many purposes coated pipe is highly desirable. This may be by hot
asphalt, or other liquid dip, by surface electro-galvanizing or by the
hot galvanizing method of dipping in molten zinc, by which method
probably the greater portion of coated pipe is treated. Certain other
protective coatings are used to a limited extent.
CHAPTER XXI
THE MANUFACTURE OF SEAMLESS STEEL TUBES
It is more than likely that the popularity of the bicycle, which created
the recent great demand for strong, light and perfect tubes, was largely
instrumental in developing the seamless tube industry, which may be said
to have “sprung up” within the last twenty-five years. Previously all of
the iron and steel pipes or tubes obtainable were either of the “butt”
or “lap-weld” variety with the exception of those which were made from
long pieces of metal by boring holes lengthwise through them. Tubes by
this boring method are, of course, quite difficult and expensive to
make.
Shorter and thicker billets of steel can more easily be bored. When the
holes have been enlarged by pushing larger and larger-nosed rams through
them in a hydraulic press, they can be rolled down to size over a
mandrel, just as lap-welded pipe is rolled, only in this case they are
put through several times and considerably reduced in diameter. In this
way some of the seamless tubes are made.
The important starting point in all processes for seamless steel tubes
is the piercing operation and it is mostly in the method of getting the
first hole through the billet that they differ, since the hot-rolling
and the cold-drawing processes by which they are finished have long been
known.
One of the most important modern processes for seamless tubes, the
Mannesmann, is based upon the principle that if a white-hot round steel
bar is rapidly rotated between “cross rolls,” a longitudinal rupture
which is almost a hole forms along its center. We may liken the motion
of the bar in the rolls to the whirling of a lead pencil between the
palms of our hands, except, of course, that the bar is kept rotating in
one direction only. Though something like this tendency of a steel bar
to open along the center through pressure applied at two opposite points
on the outside, seems to have been known to forgemen, the Brothers
Mannesmann came upon the similar tendency under action of the rolls, by
accident.
They were German tool steel manufacturers. A critical customer wanted
perfectly round and surface-polished bars of steel. They attempted to
give his bars this perfect shape and smoothness by finishing them
between cross rolls which spun the bars rapidly around while they were
slowly passing along through the machine. The pieces were perfect
outwardly, but, much to the steel makers’ chagrin, the customer reported
that the quality of the steel was not as satisfactory as that which he
previously had been receiving. Upon investigation it was found that this
cross-rolling under pressure tended to form a small hole along the
center of the bar with slight cracks in the metal all around it.
Upon this happenstance discovery is based the Mannesmann process for
piercing the bar, which consists in pushing over a piercing head such a
center-weakened piece as it comes through the cross rolls.
One or two modifications of the Mannesmann piercing method are also in
use.
The material generally used for seamless steel tubes is medium soft
open-hearth steel of .15% to .25% carbon. It is received as billets
which are rolled down and cut at the mill or they are purchased as 3″ to
6″ “rounds” and cut into such lengths as give proper amounts of steel
for the tubes which are to be formed. Usually the bars cut for tubes are
from three to five feet in length.
They are heated in a furnace and after the end has been dented at the
center, they go into the rolls.
[Illustration:
PIERCING A SOLID BILLET BY THE MANNESMANN PROCESS
]
[Illustration:
ROLLING DOWN THE PIERCED TUBE
]
The rolls seize the forward end of the bar and swiftly whirl it as it is
slowly pulled in. A piercing head of high-speed steel at the end of a
stiff mandrel extends between the rolls just as we saw it in the
pipe-rolling process. As the forward end of the rapidly whirling
white-hot steel bar pushes against this piercing head, the piece,
weakened along its center line is pierced. As neither the rolls nor the
piercing head can be resisted, it is forced through the rolls and grinds
its way over the piercing head with the supporting bar, the walls of the
white-hot tube being thinned down and the piece very materially
lengthened.
It comes out a rough tube with thick and irregular walls. After its
removal from the rolling mill bar upon which another and colder piercing
head is placed in readiness for the next tube, it goes to other rolls
through which it is passed, first without a mandrel inside, and later,
with one, until it has become somewhere near the desired size and the
walls have been pulled down to the proper thickness. The mandrel, of
course, determines the size of the interior of the tube, and the rolls,
its outside diameter.
Some are sold in this form as hot-finished tubes after having been
straightened and cut to length by removal of the ends.
A great deal of the seamless tubing made is given the cold finish, i.e.,
it is drawn through dies much as rods are drawn in the making of wire.
For cold-drawing, one end of each tube is reduced in size over a length
of a few inches, by forging or by other means. This is where the
“pliers” are to take hold.
Now we can never heat steel without forming upon it a brittle oxide or
scale which is much harder and harsher than the metal itself. During its
sojourn in the heating furnace and its journey through the rolls,
therefore, each of the tubes acquired a hard brittle surface which must
be removed before the tube can be “drawn.” The most practical way of
removing this scale is by “pickling” the tube in some weak acid, usually
sulphuric (oil of vitriol). The acid dissolves some of the scale and
loosens the remainder so that it can be washed off. To neutralize any
excess acid which clings to the tube and to aid lubrication, it is
dipped into lime-water and then dried.
The tube now goes to the drawing benches which are long steel frames
along which a heavy steel draw chain is continuously traveling from the
center toward one end. Anchored at the opposite end of the bench is a
long bar upon which is fastened the mandrel or ball which is to
determine the inside diameter of the tube in the drawing as did the
mandrel between the rolls in the rolling.
[Illustration:
TUBE-DRAWING BENCH
]
The tube to be drawn is threaded over the long rod which is anchored in
place, and the forged-down small end is pushed through the “die,” very
firmly fastened to the bench near its center. The pliers take hold of
the forged-down end of the tube with a vise-like grip, and are then
hooked into the draw chain. The tube is thus slowly drawn through the
hole in the die. As these dies are of very hard material, either hard
cast iron or hardened steel with hole a little smaller than outside of
the tube, they compress the tube upon the mandrel inside and the
thickness of wall is thus regulated, the excess metal being squeezed out
so that the tube is very materially lengthened. Tallow or grease with
the lime-coat lubricate the tube, a little being continually drawn into
and through the somewhat funnel-shaped die.
As was the case with the “cold finishing” of plate and the drawing of
wire, this cold working increases the elastic limit and tensile strength
of the steel. So cold-finished tubes are stronger than hot-finished. For
many purposes such increase in strength is highly desirable. The
exterior of the tube is also made very smooth and uniform in diameter by
the drawing.
[Illustration:
HOW TUBES ARE DRAWN
]
The cold-drawing has a disadvantage, however. It somewhat embrittles the
steel, as may be inferred from the increase in strength. This is not a
serious matter, however, unless the cold-drawing has been overdone.
But for smaller sizes of tubing many drawings have to be resorted to, to
reduce the steel to the size required. Sometimes ten or even fifteen
passes are required before the tubes reach their final size. In such
case the tubes have to be annealed and repickled, limed and dried after
each pass or two in order to restore to the steel its ductility. If this
were not done the tube would eventually break in the die.
The last pass is through an accurate “sizing” die which corrects any
variation in inside or outside diameter.
As the pulling strain which the steel will stand is limited, too much of
a reduction in size in any single pass must not be attempted.
As annealing, pickling, drying, etc., have to be done after every pass
or two, a considerable period of time elapses between the piercing of
the billet and its final pass as a small tube. For economy in handling,
tubes cannot be considered or handled singly, but must be treated in
quantity, so this period between billet piercing and the final pass may
be as much as two weeks, possibly more.
The tubes must next be straightened. This is done in cross rolls as has
been mentioned under the manufacture of lap-welded pipe, or in various
other types of machines.
[Illustration:
CUPPING AND DRAWING SEAMLESS TUBES FROM PLATES
]
Much seamless tubing goes into automobile, bicycle, and various other
products for which very high grade and perfect material is desirable.
One of the many interesting applications of seamless tubing is its use
in very fine sizes for hypodermic needles.
Seamless tubes are easily bent, swaged, upset, spun or otherwise changed
in form, as the material is ductile and there are no welds to open.
Very large tubes are not made in the way just described. They are rather
made by “cupping” flat, round steel plates through a die. A cup is then
in several successive drawings put through smaller dies, under which
treatment it grows longer each time and gets a thinner wall until it has
become a long tube with the one end still closed. For open ended tubes
this and the upper, open end are cut or trimmed off.
Cold-drawing here necessitates annealing to restore ductility just as it
does elsewhere and each annealing operation is necessarily followed by
pickling for removal of the scale formed.
By rapidly spinning large tubes in lathes or other machines and the
application of pressure with the proper tools and lubrication, the walls
of the tubes may be deformed. In this way the ends may be expanded, made
smaller, or completely closed. By such “spinning” operations large tubes
are made into articles of various shapes.
By this same “cupping” or hydraulic drawing of flat, well lubricated
sheets of soft steel, seamless high-pressure gas cylinders, steel drums,
barrels, and the like are made.
CHAPTER XXII
TRANSFORMATIONS AND STRUCTURES OF THE STEELS
It was “Ali Baba” who is quoted as saying, “Those who do not know how to
take the Philistine, better hadn’t!” or words to that effect.
Now through these chapters we have attempted to discuss in an entirely
non-technical manner the subjects presented. On this account we were
compelled to forego discussion of many things which are highly important
and interesting but which are more or less difficult of explanation
without the use of scientific terms and theories. One such has been the
“mechanism” of the hardening of steel and its opposite, its softening by
annealing. For those who may desire to get a glimpse into this
“wonderland” it is hardly fair to refrain from brief discussion of the
subject just because it is technical and difficult and so may prove to
be tedious to some who have little reason to be interested.
It seems desirable, therefore, to impose this more technical chapter or
two that the subject of the real metallurgy of iron and the steels may
at least be “hinted at.” We say “hinted at” advisedly for it is a long,
long story, and, even now, after a great many years of serious study no
one has yet read it to the end. We are not saying this in a discouraging
way, however, for there seems little reason to doubt that the multitude
of facts which have been disclosed through the tireless experiments and
the study of hundreds of investigators have put us well on our way to
the solution of this one of Nature’s great problems.
To those, however, who are not interested in the known details of “how”
and “why” hardening and softening of steel is possible and why hardening
of pure iron and mild steels does not and cannot take place, we must say
as would “Ali Baba”:—“Those who do not care to study it better hadn’t.”
Anyway, the study of this rather intricate subject is conducive of
“headaches,” and perhaps it is not extremely important when viewed from
the non-technical standpoint of these articles.
We have several times referred to the debt which civilization owes to
iron and steel structural materials, machinery and tools and
particularly to those tools which have hardened cutting edges. Almost
every one knows that hardened cutting edges are imparted to tools by
sudden cooling in water or oil from a good red heat. Probably most of
us, too, know that the blacksmith can again soften such tools by
reheating to the same red heat and allowing them to cool slowly. This he
calls annealing. In this softened or annealed condition a piece can
readily be sawed or filed, while in its hardened state a saw or file
produces no result upon it.
Now what are the facts, meaning and the cause of this dual life of the
alloy, steel, without which we would be so greatly handicapped.
To be better prepared to understand the answer, let us consider three or
four accompanying and closely allied phenomena which close observation
of the habits of steel has disclosed.
The “Point of Recalescence”
If we drill a hole in a small piece of carbon tool steel which we are
about to put into the heating furnace, and if into this hole we insert
the bare tip of an electric pyrometer, this heat-measuring instrument
will indicate at all times the rising temperature of the piece of steel
as it heats in the red-hot muffle or chamber of the furnace.
As we watch the piece grow red, the pyrometer registers 900°, 1000°,
1100°, 1200° F.,—gradually and uniformly indicating higher and higher
temperatures.
[Illustration:
APPARATUS FOR DETERMINING THE CRITICAL POINTS OF STEEL
]
But lo! Something must be wrong! The pyrometer needle does not now move
forward but is standing still. Though we know that in that hot furnace
the piece must be absorbing heat at the same rate as before, yet the
pyrometer needle does not budge!
But, as our wonderment grows and we are still undecided as to the
meaning, the needle again begins to advance and continues again
regularly and uniformly to higher and higher temperatures as though it
had never taken the vacation.
With the piece now at a white heat, we have proceeded far enough with
the heating.
Turning off the electric current from the furnace and allowing it to
cool we again watch the pyrometer needle as the temperature of the piece
in the cooling furnace gradually falls. Lower, lower, lower swings the
needle, always at a rate approximately uniform.
But again it suddenly stops and remains immovable, or perhaps even rises
slightly, for a period of several seconds, after which it resumes its
uniformly-timed downward course as though nothing had happened.
Yes, these pauses of the needle occurred at very nearly the same marking
on the pyrometer dial, but not at exactly the same ones. Going up it was
at 1350° F., and on the downward way it was at 1250° F. And you are
correct in surmising that these two points are closely related. They are
parts of the same, if we may so speak, and, in reality they represent
one point which is located about halfway between them, the divergence
resulting from what is known as “hysteresis” or “lag,” which means, of
course, a “being-behind-hand” or tardiness.
For the present we may say that all carbon steels have this “critical”
range as shown by such pauses of the pyrometer needle during heating or
cooling of the steel.
Now as the piece is most certainly continuing to absorb heat in the
furnace as it grows hotter and is losing it uniformly to the air as the
furnace cools, we have no alternative but to judge that the pause of the
needle on its upward way was caused by some internal affair of the piece
of steel itself, for which, at just that stage of its journey, it
required and used for its purpose (which was other than making itself
hotter), the heat furnished it by the furnace; and, that on the downward
journey, at just that same point, it gave out again that same heat. It
must have been the setting free of this imprisoned heat, if we may so
term it, which kept the piece for those few moments from cooling at the
usual rate. Indeed, had we conducted our experiment in a rather dark
room and observed the piece closely we would have noticed that during
the pause of the pyrometer the piece of steel did brighten or glow
somewhat, showing that it had extra heat from some hidden source.
Because of this “self-heating” of the steel as shown by the pyrometer
and the brightening, the temperature at which the phenomenon occurs has
been named the “point of recalescence,” which means the point at which
it spontaneously becomes hotter.
Loss of Magnetism
Now another curious thing took place had we but noticed it.
We all know that iron and steel are our most magnetic materials. From
childhood we have seen pins, needles, steel pens, and various other
steel or iron objects jump to a magnet held near them.
What, now, when we find that our piece of steel in the furnace when at a
red or higher heat is entirely unresponsive or dead to the attraction of
a strong magnet?
Strange! Do you suppose that our magnet has lost its power?
Let us see.
Suppose that every minute or so, while watching the pyrometer needle go
slowly down again after turning off the heat, we put the magnet to the
steel.
Continually lower comes the temperature of the piece—1500°, 1400°,
1375°, 1350°, 1325°, 1300°, 1275° F.,—and lo! the piece jumps, and from
this all the way down to cold it responds to the attraction of our
magnet. Just to make sure that we are not “seeing things,” we again
start our furnace, and, as the steel heats, we test it with the magnet.
So far there is no doubt about its being magnetic!
At 900° F., the pieces begins to show dark red, at 1000°, 1100°, 1150°,
1200°, 1250°, stronger and stronger red. At all of these temperatures
the steel is attracted. So it is at 1275° and at 1300° F.
But just as we are thinking that we must have been mistaken before, we
find that again the steel is suddenly “dead” to the pull of the magnet!
And at what temperature? The pyrometer indicates 1320° F. But was not
this the same or very nearly the same reading at which the pyrometer
needle paused on the way up, and do you not remember that it was only a
little below 1250° F. that it paused on the way down, and the
disagreement of the two temperatures we ascribed to “lag”?
No, we made no mistake. Steel loses all of its magnetic properties at
the “critical range” and has none above it.
Dilatation and Conductivity
Certain other great changes, too, occur here.
We know that most materials expand uniformly upon heating and contract
as they cool. Steel is no exception, but at the critical range on
heating it becomes fickle and for a short space contracts instead of
continuing its uniform expansion. Conversely, during cooling, it ceases
its uniform contraction and suddenly dilates or expands for a short
period when it reaches the critical range, after which aberration it
again resumes its old habit of uniform contraction as the temperature
falls.
Just so with its electrical conductivity. At the critical range the
electrical conductivity suddenly decreases abnormally as the piece gets
hotter and as abnormally increases as the steel cools through the
critical range on the return trip.
There are certain other happenings at or near this particular
temperature but we will not consider them here.
Manifestly all of this has a deep meaning.
Recalescence Indicates the Hardening Point
You remember that we said the divergence between the going up and the
coming down pauses of the pyrometer needle was due to tardiness or lag?
Among humans habitual tardiness is not considered a desirable trait, but
it is undoubtedly through this very lag or tardiness that steel becomes
so serviceable to us.
[Illustration:
HEATING AND COOLING CURVES OF STEEL WITH
.9 PER CENT OF CARBON
]
This lag is peculiar in that it grows less the more slowly we heat or
cool the steel, and, if the heating or cooling is done slowly enough,
the lag disappears almost entirely, i.e., the pause of the pyrometer
needle occurs at the same temperature on the upward as on the downward
way. Conversely, the disagreement or split grows or widens the faster
the temperature is raised or lowered.
Here is the vital point.
By extremely sudden cooling, such as quenching in water, the lag becomes
so great that it never catches up at all and any structure with its
consequent properties which was brought about in the steel by the higher
temperature is thus frozen or fixed and made to “persist” after the
steel has become cold.
It is just at this point, the “point of recalescence,” that steel
changes from its soft and malleable, to its extremely hard and brittle
condition. If it is quenched from temperatures above this point, it is
extremely hard, if from temperatures below it, even those only a little
below, it is soft and ductile. It is from just a little above this
point, then, usually between 1350° F., and 1500° F., that the blacksmith
hardens his tools by plunging them into cold water.
[Illustration:
HEATING AND COOLING CURVES OF STEEL WITH .46 PER CENT OF CARBON
]
Steels of Other Composition
Now it should be noted particularly that the specimen with which we have
been experimenting is a tool steel of .90% carbon or thereabouts. This
is important, for, while all of the carbon steels show this same
critical temperature, at which occurs the point of recalescence, those
containing from .45% to about .85% carbon have another point somewhat
higher on the temperature scale, and steels which contain from .10% to
.45% of carbon have two others, or three points in all. Further, steels
having less than .10% of carbon and iron with no carbon at all have the
two upper points but no point at 1290° F. This lower one has
disappeared.
All of this means that if instead of a piece of .90% carbon steel we had
used one having .60% of carbon, say, we would have found two different
critical ranges or points at which the pyrometer paused, the one at
1290° F., and another when we got to 1360° F. Had the steel been one
containing .30% carbon we would have discovered pauses at three
different points, viz., at 1290° F., at 1395° F., and at 1480° F. With
very low carbon steel or with wrought iron, the pyrometer would have
registered two pauses, one at about 1395° F., and the other at 1650° F.
When records are carefully kept of the time which is required for the
temperature to rise or lower over each and every twenty-five degree
period, say, on the upward and downward way, and these are “plotted,”
what are called “heating” and “cooling” curves can be drawn through the
stars and dots so set down and these form a record of the behavior of
the pyrometer needle at each temperature along the scale. Two
illustrations of such curves are shown.[9]
Footnote 9:
Special apparatus is now obtainable for determination of critical
points, heating and cooling curves.
Now if on properly spaced, dotted, vertical lines, which we will let
represent these various alloys, we mark points number three, two and one
as shown by our “cooling” curves, calling the topmost point three, it is
readily seen that the points are related. The lines and the alloys which
they represent are,
(a) The wrought iron,
(b) .15% carbon steel,
(c) .30% carbon steel,
(d) .45% carbon steel,
(e) .60% carbon steel,
(f) .75% carbon steel, and
(g) the steel with .90% carbon.
[Illustration:
CRITICAL POINT DIAGRAM OF PURE IRON AND THE STEELS
]
Of course many more cooling curves, especially of steels with other
percentages of carbon would be desirable, but we have enough that we are
safe in sketching the horizontal and oblique lines, Ar_{1}, Ar_{2},
Ar_{3}, Ar_{3·2} and Ar_{3·2·1}, through the points which we have
arranged.
For convenience, metallurgists everywhere mark these points Ar_{1},
Ar_{2} and Ar_{3}, the first being the lower, and Ar_{3} the upper one.
Ar_{cm} represents an upper point found in steels having more than .9%
of carbon. The letter “r” is derived from the French word,
“refroidissement,” meaning “cooling.” The corresponding points disclosed
during heating are marked Ac_{1}, Ac_{2}, Ac_{3·2·1}, etc., from the
word, “chauffage” meaning “heating.” The “A” apparently “just happened.”
Before the upper critical points were known it had been used by
Tschernoff to designate the temperature at which steels harden.
But it must not be supposed that the skeleton which we have constructed
can be fully accepted as true until it has been checked and rechecked
hundreds and hundreds of times by other investigators. A great many have
worked upon these critical points and upon the “freezing-point” curves
of the various alloys. It was of course impossible for all of them to
make their determinations in just the same manner and with exactly the
same materials. Examination of their work and consideration of the
results which they obtained show some discrepancies as might be
expected, largely probably because of difference in purity of the
materials tested, every impurity such as manganese, nickel, silicon,
sulphur, phosphorus, etc., modifying more or less the results obtained.
As we discovered, speed of heating and cooling also modify the results.
When we consider the difficulties which attend the making of
determinations on metals and alloys at high temperatures, the wonder is
that there is such close agreement. From these standpoints the
differences which exist in the published results seem quite small.
It was but twenty years ago that the first outline was drawn and the
whole “fusibility” or “equilibrium diagram” of the iron-carbon alloys
given in the next chapter has practically been developed within this
time. But over this period of twenty or so years the points upon which
these lines Ar_{1}, Ar_{2}, Ar_{3}, Ar_{3·2} and Ar_{3·2·1} are based
have been checked many times and they are now well substantiated. These
lines form but a small part of the complete “iron-carbon diagram.”
The Meaning of the Points
You remember that wrought iron and steels having less than .10% carbon
showed no point Ar_{1}, and that in all other steels this point becomes
stronger as they contain higher and higher carbon. There is little doubt
that the point Ar_{1} exists or results from and because of the carbon
of the alloy. In wrought iron there is no carbon, hence there is no
point Ar_{1}. If the extremely low carbon steels have an Ar_{1} it is so
weak that it cannot be detected.
Had we tested the .45% carbon steel for magnetic properties we would
have found that it lost magnetism at about 1395° F., instead of at 1290°
F., at which temperature the .90% steel became non-magnetic. The point
Ar_{2}, then, shows the temperature at which loss or gain of magnetism
occurs. The electrical conductivity change comes at neither of these
points, Ar_{1}, nor Ar_{2}, but at Ar_{3}.
However, with increase of carbon the line Ar_{3}, which was drawn
through the points, Ar_{3}, rapidly descends. At about .45% or .50%
carbon content, this line Ar_{3}, representing the changes in
conductivity, joins line Ar_{2}. Hence in steels having .45% carbon or
more, there is a common point, or one which in reality is made up of
both points. At this common point the phenomena peculiar to each of the
points occur.
This common line, now called Ar_{3·2}, itself lowers with further
increase of carbon until, in steels of around .90% carbon, there is but
the single point Ar_{3·2·1}, and the phenomena corresponding to all
three of the points occur at this one point at 1290° F., as we found in
our experiments.
As points Ar_{2} and Ar_{3} occur in carbonless iron, they cannot result
in any way from carbon but must have to do with the iron itself. From
their experiences with other materials, chemists and physicists are well
acquainted with such evolutions of heat as occur at Ar_{2} and at
Ar_{3}. These heat absorptions and evolutions, with the sudden
dilatation, gain in conductivity, etc., indicate that some internal
change or reorganization takes place in the iron itself.
Such changes seem to indicate what are known as “allotropic”
modifications. More familiar examples of allotropic forms of materials
may be mentioned. Phosphorus, for example, may exist, either as the
“yellow” variety which is poisonous and so inflammable that it must be
kept constantly under water, or as the “red” variety which is
non-poisonous and non-inflammable. Too, there is carbon, which may exist
in any one of several forms such as amorphous carbon (soot), graphite,
and the diamond. It is believed that iron, itself, exists in three
allotropic states. These have been named “alpha,” “beta” and “gamma”
iron. We do not need to go into this part of the great subject except to
state that at ordinary temperatures and up to Ar_{2}, we have alpha
iron, between Ar_{2} and Ar_{3}, beta iron, and above Ar_{3}, gamma
iron. Both beta and gamma iron are non-magnetic, while alpha iron is
strongly magnetic. In cooling through Ar_{3}, i.e., from gamma to beta
iron, some rearrangement of its molecules produces the dilatation or
expansion and the change in conductivity which was noted above.
From the fact that by chemical analysis any certain steel must have the
same composition in its hardened that it has in its unhardened
condition, it will readily be seen how futile it would be to expect
chemical analysis to give us complete information regarding it. Too,
tensile strength and the other usual physical tests can hardly tell us
all that we wish to know. Microscopic analysis or metallography,
however, shows us internal structure of properly prepared pieces of
either the hardened or unhardened alloy that we may see the actual
condition or grouping of the constituents. The view points given by all
three of these methods, chemical, physical and metallographical, are, of
course, much better than any one or two alone.
The Structures of Quenched and Unquenched Steel
We saw that the lag or tardiness is greater the more rapid the cooling.
Along with this very great lag which is brought about by very rapid
cooling comes increasing slowness, i.e., less ability to catch up, as
the temperature is lowered. Hence quenching produces such a wide lag and
so slows the changes which should take place that they do not take place
at all, i.e., the structure which the piece had at the higher
temperatures cannot change but is set or fastened by the quickness of
the cooling.
Though no degree of suddenness is sufficient to set completely the
structure existing at very high temperatures, for our present purposes
we can say that by quenching in cold water we can freeze or fix any
structure. Then after we have quenched a piece of steel, it will have
when cold, the structure which corresponded with or resulted from the
temperature which it had at the moment before the quenching.
If so, the microscope should give us aid.
[Illustration:
ILLUMINATION OF THE SAMPLE UNDER THE MICROSCOPE
]
By breaking off pieces of a quenched piece and very carefully and slowly
grinding and polishing without heating a surface which was an interior
part we find after etching that we can actually see the kind of
structure which corresponded with the temperature from which the piece
was quenched.
Photomicrograph No. 80 shows the appearance of a piece of hardened
carbon steel. Note the needle-like structure under the microscope at
magnification of 400 diameters. This structure is characteristic.
The constituent, having this needle-like appearance has been named
“martensite” in memory of a distinguished European metallurgist, A.
Martens. It is supposed to be “beta” iron, much the hardest allotropic
variety of iron, and to hold in solution the carbon of the alloy, either
as carbon alone or as the extremely hard chemical compound, iron
carbide, Fe_{3}C.
Martensite, then, is the extremely hard structure, necessarily
containing considerable carbon or iron carbide in solution which gives
to our carbon tool steels their hardness and great usefulness.
Unhardened steels never look like this. Their appearance is shown in
photomicrographs Nos. 3b, 5, 22 and 24a.
[Illustration:
No. 80. MARTENSITE, THE CONSTITUENT OF HARDENED STEEL
(_Magnification 400 Diameters_)
]
In unhardened steels having less than .90% of carbon we find two
constituents.
“Ferrite” is the name which has been given to one, the soft and ductile
constituent, pure iron. With ordinary etching the ferrite usually shows
as light-colored or white grains bounded by black lines, which, if the
patch is large enough, give a fish-net appearance. It is soft and
ductile like copper, for pure iron and pure copper are not so greatly
different in malleability and ductility as one might suppose.
The darker and more or less triangular patches at the corners of the
ferrite grains are “pearlite,” a name originating because of their
“pearly” appearance under the microscope. How this pearly appearance
comes about will be readily understood from photomicrograph No. 23e
which was taken at a magnification of 400 diameters. It is seen that it
results from alternate black and white layers.
Again we must give up the idea of any finality in the things we learn or
think we have learned. We just learned, for instance, that ferrite
usually was light or white in color. Well, in pearlite, as shown in
photomicrograph No. 23e, every other plate is of ferrite but they are
not the white but the black ones.
[Illustration:
No. 23e. PEARLITE AT MAGNIFICATION OF 400 DIAMETERS
]
You may not have understood before that color as shown under the
metallographic microscope depends not so much upon actual color of the
material itself as upon its ability to reflect light. For metallographic
observations it is necessary to have very strong illumination. Usually
the powerful beam from an electric arc is concentrated by means of
condensing lenses upon a thin disc of glass called an oblique reflector
which directs the beam upon the polished and etched specimen beneath the
objective of the microscope. Often a prism is used. The rays of light
returning from this highly illuminated “field” under observation return
up through the tube and eye piece of the microscope and can be focused
upon a small screen convenient for observation or upon the ground glass
of the attached camera by means of which the pictures are taken. Unless
the surface of the specimen being examined is perfectly plain and level,
not all of the vertical rays thrown down upon it will be reflected back
up through the tube and eye piece. Those portions of the field which are
absolutely at right angles to the vertical rays appear at the eye piece
or upon the screen as white or light-colored portions, while those
which, during the polishing or etching have been dug or eaten away
reflect the light imperfectly or in directions other than up the tube of
the microscope, wherefore such portions show as darker or black
sections.
The pearlite, then, is made up of little plates of soft ferrite
alternating with others of a very much harder constituent. The harder
plates are much less affected during polishing and etching than are
those of the softer ferrite, hence they stand out in relief and reflect
abundant rays of light, whereas the “dug-out” ferrite plates reflect the
light imperfectly or not at all and therefore appear as dark lines.
These white, hard plates of the pearlite contain all of the carbon of
the low carbon alloys. They are this other constituent, “cementite,” so
named because it was first discovered in steel made by the “cementation”
process. It is a very hard and brittle substance, hard enough to scratch
glass. It is the chemical compound (Fe_{3}C), unvarying in composition
as chemical compounds always are. It consists of just three atoms of
iron (93.4% by weight) and one of carbon (6.6%).
Pearlite, therefore, is a sort of mechanical mixture of two separate
constituents, ferrite or pure iron, and this chemical compound, carbide
of iron, which is called cementite. Pearlite is common to all unhardened
steels whether of low, medium or high carbon content and may be
considered characteristic.
That we may understand clearly the structures of the annealed steels,
let us start with pure iron and gradually change it into higher and
higher carbon steels by gradual addition of carbon. Pages 328 and 329
show such a series.
Photomicrograph No. 99b is open-hearth iron which is entirely made up of
free ferrite. In No. 3b there is considerable pearlite, here appearing
black, though the sample of steel yet contains but .10% of carbon. In
No. 5, which is of a steel containing .30% of carbon, we have more
pearlite and in No. 22c with .50% carbon we have yet more. Manifestly at
this rate the comparative pearlite areas are growing so that there will
soon be room for no ferrite at all. In No. 23g this has occurred. This,
the photomicrograph of a steel containing .86% of carbon is one of the
steels in which we found that the point of recalescence, loss of
magnetism, decrease in electrical conductivity and rate of expansion
take place all at the one point.
[Illustration:
No. 99b. CARBONLESS IRON
]
[Illustration:
No. 3b. STEEL WITH .1 PER CENT CARBON
]
[Illustration:
No. 5. STEEL WITH .3 PER CENT CARBON
]
[Illustration:
No. 22c. STEEL WITH .5 PER CENT CARBON
]
(_Magnification 60 Diameters._)
Now as we go still farther on up in percentage of carbon content, i.e.
(beyond .86%, we have a white constituent beginning to appear as cell
walls around the grains of the pearlite and this increases with increase
of carbon until, with alloys having carbon around 3%, we have a
proportionately small amount of pearlite while the white areas have so
increased that it appears that the more or less round patches of
pearlite float in a lake of white. This white which appears first as
cell walls, and later in greater and greater quantity is free
cementite.)
[Illustration:
No. 23g. STEEL WITH .9 PER CENT CARBON
]
[Illustration:
No. 24a. STEEL WITH 1.25 PER CENT CARBON
]
[Illustration:
No. 36b. STEEL WITH 2 PER CENT CARBON
]
[Illustration:
No. 109. WHITE CAST IRON WITH 3 PER CENT CARBON
]
(_Magnification 60 Diameters._)
Such are illustrated in photomicrographs Nos. 24a, 36b and 109 which
contain 1.25%, 1.98% and 3.00% of carbon respectively. While steels with
the typical white, free ferrite areas are so soft that a needle-point
will plow furrows across them, those with over 1.25% of carbon have such
excess of free cementite that they are very hard to scratch and too
brittle to use except for special purposes.
[Illustration:
AUSTENITE (WHITE) AND MARTENSITE (DARK) MAGNIFIED 1,000 TIMES THEIR
ACTUAL SIZE
]
So during ordinary cooling from the molten alloy or the slower cooling
of the steel during the annealing process, the martensitic structure
breaks down at the recalescent temperature into pearlite and ferrite
(soft iron) if the carbon content of the steel is lower than about .90%,
or pearlite and the other and very hard constituent, “cementite,” if the
steel has more than .90% of carbon. If the carbon content happens to be
just .90%, or thereabouts, there is exactly sufficient pearlite to make
up the total area of the field shown under the microscope.
Another constituent which is of great interest scientifically, though
not at all commercially, is “austenite.” By quenching _very_ high carbon
steels from a _very_ high temperature _very_ suddenly and completely, we
can fasten the “austenite” structure, which exists only at temperatures
higher than martensite, i.e., austenite is our gamma iron with the
carbon of the alloy in solid solution, perhaps as iron carbide, while
martensite is thought to be the beta iron solid solution, perhaps with
some gamma iron mixed with it.
While ordinary quenching fastens structures pretty well, it is not
usually quick enough to prevent the austenite from sliding along down
into martensite. However, carbon discourages such slipping, so, with
high carbon to act as a brake, we can fasten some of it by chilling very
suddenly and completely from a very high temperature. Steels with 1.5%
of carbon and temperatures of 2000° F., or over, are usually necessary
to accomplish it.
However, austenite, after we get it, is not as hard as martensite and we
have little use for it commercially. As was stated before, martensite is
the useful and proper structure for carbon steel tools.
[Illustration:
No. 73. ANNEALED STEEL HAS FINE GRAIN
(_Magnification 70 Diameters_)
]
Tempering or Drawing
“Tempering” is done to relieve the intense brittleness of steel after
quenching to martensite. While we dislike to sacrifice any of the
hardness, it pays to temper or “toughen” the steel, as the toolmaker
calls it, by reheating it to somewhere between 400° and 570° F.
The higher the temperature, the freer and quicker is the change from one
structure to another, as, for instance, the austenite to martensite. At
the low drawing temperatures the changes from martensite to the
pearlitic structure may be said to just creep along. A second quenching
then fastens it at the new structure which gives a trifle less hard but
a tougher steel. As you would guess, the microscope shows on these what
we may term a “transition” or “breaking-down” appearance and structures
not at all definite. These, of course, give to the steels the various
degrees of hardness and brittleness and other qualities which are so
desirable from the practical standpoint. The production of these fine
shades of temper by the practical tool maker or blacksmith may almost be
considered a fine art.
How and Why Do the Steels Harden?
Now, from all of these facts, what, shall we say, is the cause of the
hardening of steel?
The explanation most generally accepted seems to be, that, of the three
allotropic forms of iron the gamma and beta varieties are very much
harder than alpha iron, which is the one which we have in annealed steel
at ordinary temperatures, and, of the two, the beta is harder than the
gamma variety. It is thought also, that carbon, perhaps as carbide of
iron, is held in solution in gamma or beta iron after the quenching, and
increases the hardness proportionately with increase of the carbon
content of the steel. While this carbon or iron-carbide solid solution
may and probably does itself confer additional hardness to the steel,
its main function is to retard or slow down the change from gamma into
beta and alpha iron, which change in carbonless iron and low carbon
steels is so insistent and extremely rapid that not even the most severe
quenching, as in ice-water or liquid air, can prevent or stop it. Not
only do we fail to get austenite, which is gamma iron, until we get 1%
or more of carbon present and quench from very high to very low
temperatures, but we cannot even stop the transition at beta iron, the
next lower allotropic variety until we have at least .30% of carbon,
and, for serviceable hardening fully .60% of carbon in the steel.
Fortunately for us, this beta form of iron is the one we want, for it is
harder and more useful than the gamma form.
Our most serviceable constituent, martensite, then, is a solid solution
of carbon or iron carbide in beta iron. It is magnetic but this probably
results from its containing some alpha iron through incomplete stoppage
of the change by the quenching.
As has been stated, “tempering,” which means careful reheating to 400°
F., 500° F., or 600° F., allows the slight “slipping” of enough of the
beta solution, always eager at temperatures below the point of
recalescence to return to alpha condition, to relieve the excessive
brittleness of the hardened steel.
[Illustration:
No. 72. THE “SORBITE” GRAIN IS PRODUCED BY COOLING IN A BLAST OF AIR
AFTER ANNEALING. IT GIVES GOOD WEARING PROPERTIES
(_Magnification 70 Diameters_)
]
Annealing is the complete release of beta iron and the “trapped” carbon
which allows of their return to the normal condition of pearlite with
alpha iron. To accomplish this, the hardened steel has to be heated
above its point of recalescence and cooled more or less slowly.
Different speeds of cooling give different grain size, structures and
physical properties.
This explanation of hardening, which is known as the “allotropic”
theory, is not universally accepted, conclusive evidence being lacking
at more than one point.
It must be stated also that some who hold the “allotropic theory” of
hardening doubt the existence of beta iron. These contend that the
so-called beta iron and martensite are only decomposition or transition
forms of gamma iron and austenite.
Two or three other theories have been more or less strongly advocated
but these also suffer from lack of evidence. The one which perhaps ranks
next in number of advocates is the “carbon theory.” Its supporters
contend that by quenching, alpha iron is made to hold carbon or iron
carbide in solid solution and that it is this solution of carbon or
carbide which gives to the steel hardness in proportion to its carbon
content. Others hold that the great density or strain under which
quenched steel exists accounts for the hardness.
Like many other great problems of the universe this one is not yet
conclusively or satisfactorily solved, so reluctantly does Nature yield
her secrets. But while it may not have explained all of the “whys” and
“wherefores,” the work which investigators have done has brought about
great improvement in methods of manufacture and quality of the alloys
which are making our civilization greater and this Age more wonderful.
CHAPTER XXIII
THE EQUILIBRIUM DIAGRAM OF THE IRON-CARBON ALLOYS
It was a great day for metallurgy when it was discovered that molten
mixtures were governed by the same natural laws which govern our
ordinary liquid solutions. Probably it was because most metallic alloys
are solid at ordinary temperatures and liquid only at very high ones
that for so long a time we failed to suspect their similarity.
If a sensitive pyrometer is inserted into an ordinary solution or a
molten alloy which is being gradually cooled, it indicates the first
instant that solidification (freezing) begins as well as the termination
of the freezing period. Unlike the freezing of water or a pure metal,
complete solidification ordinarily does not take place at a definite
single temperature but over a greater or lesser range of temperature. By
taking such upper and lower freezing-point measurements for many
different percentage compositions of a binary (two metal) alloy, for
instance, curves can be plotted which show accurately the habits of any
and all of the possible combinations (i.e., alloys) of those two metals.
Such are called “freezing-point” curves, and, as we shall see later on,
a study of them will give us much valuable and interesting information.
Such curves have been constructed for a great many alloys since the
discovery of the analogy between their behavior and that of aqueous
solutions led us to study alloys after the manner which physical
chemists found so satisfactory for the study of ordinary solutions.
Since the study of binary or two metal alloys is often very difficult,
it can be readily understood why the determination and interpretation of
the curves of alloys which contain three, four or more metals is a very
much more serious matter. Much of it has to be done by methods which are
long and tedious, such as quenching and microscopic study of innumerable
specimens taken during the freezing and subsequent cooling of various
alloys of each series. The value of the results depends upon the skill,
devotion and clear sightedness of those who carry out the work.
[Illustration:
THE FREEZING-POINT CURVES OF THE IRON-CARBON ALLOYS
]
The “freezing-point” and “decomposition” curves of the iron-carbon
series of alloys have been brought to their present stage of development
after something like twenty years of labor by investigators in many
lands. If we look at the diagram on page 345, we note at once that the
curves are quite complicated. Even yet they are not complete for all
percentage combinations of iron and carbon, and those who have given the
most time and study to the subject have not yet been able to interpret
with entire satisfaction to all concerned all of the discoveries so far
made.
Without endeavoring to take up in detail the technique of the manner of
their production, which would be unprofitable for us without a great
deal more of preliminary study than we have time and space to give, we
will at once examine the freezing curves of the iron-carbon alloys as
now developed. The works named on page 354 as references for Chapters
XXII and XXIII may be consulted for the various types and methods of
construction and for explanation of freezing curves by those who desire
to study them.
Referring to the freezing-point diagram on page 336, the upper or broad
V-shaped line, ABC, indicates the temperatures at which the alloys of
various percentages of iron and carbon begin to freeze, and the lower
one, AED, the temperatures at which the freezing of these alloys ends.
From the diagram it is readily seen that pure iron (100%), has a very
high freezing-point and solidifies at once. Iron which contains about 2%
of carbon begins to freeze at a much lower temperature and has a long
period of solidification, while iron with 4.3% of carbon has the lowest
freezing-point of the series with an extremely short solidification
period or range.
Since we have been unable to go sufficiently into the methods and
technique of freezing-curve construction to be able to understand their
general classification, we must accept the statement that the curve of
the iron-carbon series is really a double one. The part of it that lies
to the left of the dividing line UV of the diagram on page 336, is of
the type exhibited by liquids which freeze from “_liquid solutions_”
into what are known as “_solid solutions_,” which by aid of the
microscope are found to be homogeneous mixtures of crystals. On the
other hand, alloys which lie to the right of UV, are of the type which
form “_eutectics_.” This will be described later. This dividing line UV,
which occurs at about 1.7% of carbon, divides the iron-carbon alloys
into these two natural divisions. It was the basis for calling those
having 2% of carbon or less, “steels,” and those with over this amount,
“cast irons.”
Molten iron is so greedy for carbon, that, when it can get it, it
readily holds in solution from 7% to 10% of this element. But solid
(frozen) iron cannot retain anything like this amount. As we learned in
the last chapter, gamma iron is the only variety which can exist above
our lines of loss of conductivity, magnetism and recalescence, i. e.,
Ar_{3}, Ar_{3·2}, etc. It is, too, the only variety of solid iron which
is able to retain carbon in solution, and it can retain only about 1.7%
of it.
So when molten steel containing 1.5% of carbon, say, cools until it
reaches the temperature represented by the line, AB, which, at its
intersection with the 1.5% carbon line would be at about 2582° F.,
particles or crystals begin to freeze out and float in the molten alloy.
As the temperature falls, more crystals separate until, when the
temperature determined by intersection of the 1.5% carbon line with the
lower freezing curve, AE, is reached, the last of the now mushy alloy
solidifies.
Alloys of all other compositions below 1.7% of carbon do just this way
except that the temperatures at which freezing begins and ends are
different and distinctive for each composition.[10] Upon freezing, every
one of them retains in “solid solution” in the “gamma” iron whatever
carbon it had in the liquid or molten solution. But, as stated above, it
can not be over the 1.7% limit.
Footnote 10:
Temperatures of beginning and end of freezing may always be
ascertained by locating on the freezing-point diagram the points at
which the vertical line representing the desired composition
intersects and crosses the lines of the freezing-point curves—in these
cases, AB and AE.
Of the iron-carbon alloys of compositions lying to the right of the line
UV, we find the case to be different, for each one of them has more than
the 1.7% of carbon which is the maximum amount which “gamma” iron can
retain. Now the lowest temperature at which any iron-carbon alloy can
exist without freezing is slightly above 2066° F., and there is but one
composition—95.7% of iron and 4.3% of carbon—which can survive until
_this_ low temperature is reached. A content of 4.3% of carbon then, is
the greatest and also the least concentration which Nature will allow to
remain molten down to this minimum temperature. This 4.3% carbon
composition which is the lowest melting, i.e., the easiest melted alloy,
is called the “eutectic” alloy from Greek words which mean “well
melting.” This eutectic composition may be said to divide or rather
subdivide this group of alloys into two groups, those containing between
1.7% and 4.3% of carbon, and those which have 4.3% and over.
As stated before, freezing is not an instantaneous but a progressive
process. During the freezing period of any of these alloys which have
over 1.7% of carbon the _still liquid portion_ which remains after
freezing begins to become smaller and smaller in quantity as freezing
progresses just as it did in alloys of the “_solid solution_” group. And
as Nature allows a concentration of 4.3% of carbon as the highest
concentration at the minimum temperature the very last of the remaining
liquid of every alloy eventually gets to this eutectic composition just
before the alloy freezes. Those to the left of the eutectic or exact
4.3% composition do so by the gradual freezing out of iron containing
the maximum or 1.7% of carbon, i.e., iron is taken out faster than
carbon, hence there is gradual concentration of carbon in the remaining
liquid. This goes on until 4.3% is reached. The compositions to the
right of the line WX throw out the chemical compound, Fe_{3}C, which
contains 6.6% of carbon, whereby carbon is eliminated faster than iron
and the desired 4.3% carbon alloy is arrived at from the other
direction.
To illustrate, take, say, the composition represented by the vertical
line at 3% carbon and 97% iron. As the molten alloy cools it reaches the
temperature 2330° F., at which temperature the vertical line
representing the 3% carbon composition cuts the line AB. Here the alloy
begins to freeze by the separation of small crystals of solidifying iron
containing definite amounts of carbon.[11] But as the carbon thus taken
along by the freezing crystals of iron is always less than 1.7%, a
proportionally greater amount of iron than carbon is removed from the
unfrozen part of the alloy and the remaining liquid or unfrozen part,
therefore, is left with slightly more than the 3% of carbon with which
it started.
Footnote 11:
The percentages of carbon carried by the particles of iron freezing at
any particular temperature of the solidification range may be
determined from the diagram but the works named in the reference list
should be consulted for method and explanation.
This we must now consider another alloy with a lower freezing-point, the
reason being, of course, its higher carbon content. At the next lower
temperature, more iron containing carbon is frozen out and the remaining
liquid is again left a little higher in carbon than before. In this way
the continually diminishing amount of remaining liquid keeps
concentrating, forming thereby a continuous succession of alloys of
higher and higher carbon content as the temperature continuously drops.
Eventually, of course, the concentration of this remaining liquor
becomes 4.3% of carbon just before completion of the freezing at 2066°
F.
[Illustration:
THE “EUTECTIC,” THE PART OF THE ALLOY WHICH SOLIDIFIES LAST
(_Magnification 700 Diameters_)
]
Now with alloys containing more than 4.3% of carbon, almost the opposite
occurs. Let us choose the one having 5% carbon and 95% of iron. This
molten alloy cools until at 2215° F., small crystals begin to freeze and
form in the molten mass. But, as the liquid already has more than the
favored 4.3% of carbon, it is not free iron which freezes out, but
instead, the chemical compound, Fe_{3}C, which contains 6.6% of carbon.
This, of course, takes out carbon proportionally faster than iron,
hence, at each very slightly lower temperature, the liquid which remains
unfrozen contains just a little less of carbon than did its predecessor.
So the constantly decreasing amount of remaining liquid progresses
through a succession of compositions each containing just a little less
of carbon than the previous one, and eventually, just before freezing we
get back to the mixture which contains 4.3% of carbon. Of course there
is left unfrozen by this time only a very small amount of the alloy and
it is this which has the composition stated.
The “Eutectic”
Now, having just the composition which she wants, whether arrived at
from alloys lower or higher than 4.3% in carbon, Nature lets this
composition freeze at once in thin alternating plates which lie side by
side about and among the earlier frozen crystals of the alloy. The
appearance of this typical eutectic formation under the microscope is
shown on page 341.
Had we chosen the 4.3% alloy itself, neither any of the solid solution
of carbon in iron nor the chemical compound, Fe_{3}C, would have frozen
out, but the whole mass would have remained liquid down to 2066° F.,
where the whole would have solidified at once in the plate-like eutectic
formation just described.
To sum up, iron-carbon alloys which contain less than 1.7% of carbon, in
other words, the steels, freeze as solid solutions of carbon in gamma
iron. This, of course, is the metallographic constituent which is called
austenite. It is not of a definite composition as it contains whatever
carbon is available up to 1.7%. Alloys containing between 1.7% and 4.3%
of carbon gradually freeze out this solid solution, austenite, more and
more being formed in the freezing alloy until, upon arriving at a
concentration of 4.3% of carbon for the remaining liquid, the latter,
too, freezes as a eutectic of alternating plates of more of this same
constituent, austenite, and the carbide of iron, Fe_{3}C, about and
among the crystals of the previously formed austenite. From alloys which
contain more than 4.3% of carbon, iron carbide, Fe_{3}C, gradually
freezes out as the temperature falls, until, at concentration of 4.3% of
carbon, the eutectic of remaining carbide and austenite forms about and
among the earlier frozen carbide crystals, always at the same
temperature, 2066° F., no matter what the original composition of the
alloy.
Upon reheating, the constituents melt in reverse order, the eutectic
liquifying first at 2066° F., the remainder of the alloy gradually
becoming liquid between this temperature and the temperature at which
the first freezing began during cooling.
Transformations and Decompositions
So far we have considered only the freezing of the iron-carbon alloys
from the molten to the solid condition. Now what happens to them at
temperatures below 2066° F.? Do they remain as we left them above, until
and after they are fully cold?
We must now combine the little sketch which we made on page 319, by
plotting the points, Ar_{1}, Ar_{2} and Ar_{3}, with the freezing-point
diagram which we have just now been considering. You remember that we
found all sorts of things happening to our 0% to 1.7% alloys—the
steels—at temperatures around 1290° F., 1395° F., and 1650° F.
Similarly, a great deal happens to these other alloys, as they cool from
their solidifying temperatures downward.
[Illustration:
THE FREEZING-POINT AND CRITICAL-POINT CURVES MAKE UP THE EQUILIBRIUM
DIAGRAM
]
But for the moment considering only the steels, i.e., the third of the
diagram to the left of the 1.7% carbon line, we remember that upon
completion of the solidification of any alloy, we had only a frozen
solution of all the carbon in iron. Now in the bottom part of this left
third of our diagram on page 344, the line GOSE does not look so very
much different than the freezing line, ABC, does it? It resembles it not
only in appearance but also in actual experience. But in this case it
represents not a freezing from liquid to solid but a decomposition, or
better perhaps, a transformation. The solid solutions or alloys which
contain less than .9% of carbon give up their excess of pure iron upon
getting down to temperatures lying along the line GOS, by gradual
decomposition of the austenite. In alloys lying to the right of this .9%
carbon vertical line the austenite rids itself of excess carbon by
throwing out of solid solution and freeing the chemical compound,
Fe_{3}C, as the line SE is reached and passed. That is, analogously to
what occurred during freezing, certain concentrations occur in the
_solid_, lower carbon alloys by gradual rejection of pure iron crystals
until the remainder of the mass has exactly .9% of carbon, or
_deconcentrate_ in the higher ones by rejection of Fe_{3}C until they
reduce the carbon to this .9% figure. In all cases, by the time the
temperature 1290° F., has been reached this has been accomplished and
the remaining undecomposed austenite, now with just .9% of carbon, in
some way splits into the alternating plate-like constituent which is
shown in the cut on page 341.
[Illustration:
THE EQUILIBRIUM DIAGRAM AND INTERPRETATION AS NOW TENTATIVELY ACCEPTED
]
This plate-like constituent we can hardly call eutectic or
“well-melting” alloy for it, as well as the rest of the alloy, has been
solid for a long time. But being so similar in derivation and appearance
to the eutectic which forms during freezing of a molten alloy, it is
proposed that it be called the next best thing, the “eutectoid.” It is
often called the eutectic, however.
By this time you doubtless have seen that the free iron which was thrown
out of the steel having less than .9% carbon is the ferrite which we
found in the soft steels, and that the chemical compound, Fe_{3}C, of
the higher carbon steels is the extremely hard constituent, cementite.
The eutectoid or plate-like structure is, of course, pearlite, which in
the last chapter we found to consist of just these alternating plates of
ferrite and cementite.
The Cast Irons
All of the alloys lying to the right of the line UV contain more than
1.7% of carbon, and, according to our classification, therefore, are
“cast irons.”
We have seen how they freeze either as eutectic alone, as crystals of
austenite with eutectic or as crystals of cementite (Fe_{3}C) and
eutectic, depending upon the original composition of the molten alloy.
At and just below the temperatures represented by the line ED, this
undoubtedly represents the situation.
What happens to the alloys from this temperature down to normal depends
upon conditions. Just what occurs and the mechanism of it is not
definitely known except in the practical way. Certain it is, the
“precipitation” of free carbon is necessary for cast irons which are to
be serviceable for usual purposes. This may occur with consequent
softening during the first cooling or they may be cast as “hard iron”
and softened afterward.
In Chapter XI we said that silicon was a “softener” as its presence
brought about precipitation of the carbon as graphite throughout the
cast iron, thereby softening it both by reason of the presence of the
soft flakes of graphite and because it leaves so little of the carbon in
the “combined” or hardening condition. So silicon is a ready means of
bringing about decomposition of the higher temperature structures as the
alloys cool.
The speed of the cooling also exerts a very powerful influence in
determining the amount of graphite which will separate. Other conditions
being equal, the slower the cooling, the greater the decomposition with
resulting graphite. Swift cooling, even such as results from the dumping
of castings from the molds while at nearly white or high red heat
results in insufficient graphite and otherwise harder metal. Cooling of
very hot castings on a cold floor or in a current of cool air has
considerable hardening effect even when the composition of the alloy
would otherwise give very soft and machinable metal. In an extreme and
very interesting case a few very hard cast iron flanges were each day
found among the many thousands of habitually soft castings regularly
produced. Each day two or three expensive “taps” were ruined by
attempting the impossible—the machining of these pieces of hardened
iron, which, on the outside, looked just like all the rest. It was soon
discovered that two or three mold dumpers each noon and evening were
warming water for “wash-up” by dropping into the pails a flange or two
which were still white-hot after dumping from the molds. The men were
innocent of any intention of harm but their warm water cost several
hundred dollars before their method of producing it was discovered.
Silicon and rate of cooling are the two most powerful influences but
presence of certain elements other than silicon also influences to some
extent the degree of hardness. However, while silicon has a strong
softening effect, manganese and sulphur have an opposite or hardening
tendency. On this account the amounts of these latter elements which can
be used or allowed must be strictly limited.
From the above it is seen that all sorts of cast iron can be produced
ranging from the extremely hard, high cementite, white irons with low
silicon content down to the very soft gray irons which result mainly
because of higher silicon content and slower cooling.
The white irons are more or less unstable as is shown by the
decomposition through which the hard, white iron castings become
“malleable” by annealing as was told in Chapter XII which discussed
Malleable Cast Iron.
[Illustration:
No. 31. GRAY CAST IRON WITH FERRITE, PEARLITE AND GRAPHITE FLAKES
(_Magnification 70 Diameters_)
]
The gray cast irons are much more stable. They consist of what, in an
early chapter, we referred to as “steels with an impurity, the graphite
flakes.” They consist, then, of free, soft iron or ferrite, certain
amounts of the characteristic steel constituent, pearlite, and the soft
graphite flakes.
They arrive at this composition, through breaking down of the austenite
and cementite structures during the cooling,—just how not having been
satisfactorily determined. Consistent study is being put upon this
subject and several unique and long-studied possible explanations of
this section of the full equilibrium diagram have been proposed and
debated, all based upon the data so far available. While much
information on this subject has been gained the matter is still so much
in dispute that it is best for us to venture nothing definite in regard
to just how the changes occur. The reference books for this chapter (see
page 354) give quite completely the data, theories and explanations so
far available.
Compared with the steels, the cast irons are vastly complicated. In them
we have elements which occur in practically negligible amounts in the
steels. Commercial cast irons, for instance, have silicon ranging
anywhere from ½% to 3%, phosphorus .10% to 2%, graphite 0% to 3.50%, and
carbon in the combined form (pearlite or cementite) from 3.50% to .10%.
If these represent the majority of cast irons what about our pig irons
which have 2, 5, 8, 10 or even 15% of silicon, others with 1 or 2 and
occasionally very much more of manganese, and still others which vary
widely in phosphorus content?—for from the metallographic and physical
chemistry standpoint pig irons are cast irons.
As we can never get perfectly pure iron-carbon alloys to experiment
with, their content of other elements, silicon, nickel, phosphorus,
etc., vitiate more or less the results obtained, but even could such
pure alloys be secured we are not greatly helped since our serviceable
alloys are never such. Each added element brings about greater
complication and one does not wonder that in the short twenty years
which have elapsed since study was seriously undertaken, metallurgical
science has not entirely solved the big problem.
REFERENCES
GENERAL & VARIOUS
“The A B C of Iron and Steel,” Penton Publishing Co., Cleveland.
“The Steel Foundry,” J. H. Hall, McGraw-Hill Book Co., New York.
“The Metallurgy of Steel,” H. M. Howe, The Scientific Publishing Co.,
New York.
“The Manufacture and Properties of Iron and Steel,” H. H. Campbell,
Hill Publishing Co., New York.
“The Metallurgy of Iron and Steel,” B. Stoughton, McGraw-Hill Book
Co., New York.
“The Metallurgy of Steel,” Harbord and Hall, Charles Griffin & Co.,
Ltd., London.
“Liquid Steel,” E. G. Carnegie, Longmans, Green & Co., London.
“Krupp’s Steel Works,” F. C. G. Muller, Wm. Heinemann, London.
“By Bread Alone” (a novel), I. K. Friedman, McClure, Phillips & Co.,
New York.
CHAPTER I
“The History of the Manufacture of Iron in All Ages,” James Swank, Am.
Iron and Steel Assoc., Philadelphia, Pa.
“The Materials of Engineering—Iron and Steel,” Robert Thurston, John
Wiley & Son, New York.
CHAPTER II
“The Honorable Peter White,” R. D. Williams, Penton Publishing Co.,
Cleveland, O.
“The Iron Ore Resources of the World,” International Geological
Society, Stockholm, 1910.
“The Story of Coal and Iron in Alabama,” Ethel Armes, Chamber of
Commerce, Birmingham, Ala.
CHAPTER III
“Modern Coking Practice,” T. H. Byrom, C. Lockwood & Son, London.
“Coal and Coke,” Frederick H. Wagner (1916), McGraw-Hill Book Co., New
York.
“Washing and Coking Tests of Coal,” A. W. Belden, G. R. Delamater, J.
W. Groves, Bulletin 368, U. S. Geological Survey, 1909.
“Washing and Coking Tests of Coal and Cupola Tests of Coke,” R.
Moldenke, A. W. Belden, and G. R. Delamater, U. S. Fuel Testing
Plant, St. Louis, Mo., U. S. Geological Survey, Bulletin 336, 1908.
“Manufacture of Coke and Other Prepared Fuels and the Saving of
By-Products,” John Fulton, 1905, International Text Book Co.,
Pittsburg, Pa.
CHAPTER IV
“The Blast Furnace and Manufacture of Pig Iron,” Robert Forsythe, D.
Williams & Co., New York.
“A Study of the Blast Furnace,” Harbison-Walker Refractories Co.,
Pittsburg, Pa.
“The Principles of Manufacture of Iron and Steel,” Sir I. L. Bell,
George Routledge & Son, London.
CHAPTER VI
“The Business Message of the Wrought Iron Bar,” Interstate Iron and
Steel Co., Chicago.
“The Control of Quality in Every Process,” A. M. Byers Co., Pittsburg,
Pa.
CHAPTER VII
“The Cementation of Iron and Steel,” Gioletti, McGraw-Hill Book Co.,
New York.
“The Fine Steel Industry,” J. A. Mathews, The Halcomb Steel Co.,
Syracuse, New York.
CHAPTER VIII
“Sir Henry Bessemer—An Autobiography,” Offices of Engineering, London.
“The Romance of Iron and Steel—The Story of a Thousand Millionaires,”
H. N. Casson, Munsey’s Magazine, April, 1906, and on; In book form,
A. S. Barnes & Co., New York.
CHAPTER IX
“The Manufacture of Open-Hearth Steel Castings,” W. M. Carr, Penton
Publishing Co., Cleveland.
“The Basic Open-Hearth Steel Process,” C. Dichmann, Constable & Co.,
Ltd., London.
“A Study of the Open-Hearth,” Harbison-Walker Refractories Co.,
Pittsburg, Pa.
CHAPTERS X & XI
“Cast Iron in the Light of Recent Research,” W. H. Hatfield, Charles
Griffin & Co., Ltd., London.
“Cast Iron,” W. J. Keep, John Wiley & Sons, New York.
“The Metallurgy of Cast Iron,” T. D. West, Cleveland Printing &
Publishing Co., Cleveland, O.
“American Foundry Practice,” T. D. West, John Wiley & Sons, New York.
CHAPTER XII
“The Production of Malleable Castings,” Richard Moldenke, The Penton
Publishing Co., Cleveland.
“Malleable Cast Iron,” S. J. Parsons, A. Constable & Co., Ltd.,
London.
CHAPTER XIII
“Electric Furnaces in the Iron and Steel Industry,” Rodenhauser &
Schoenawa (Translated by Vom Baur), John Wiley & Sons, New York.
“The Electric Furnace,” A. Stansfield, The McGraw-Hill Book Co., New
York.
“Electric Furnaces for Making Iron and Steel,” Lyon and Keeney, U. S.
Bureau of Mines, Bulletin No. 67 (1914).
CHAPTERS XIV & XV
“The Manufacture and Uses of Alloy Steels,” H. D. Hibbard, U. S.
Bureau of Mines, Bulletin No. 100 (1915).
“High Speed Steel,” O. M. Becker, McGraw-Hill Book Co., New York.
“Nickel Steel—Its Practical Development in the United States,” H. F.
J. Porter, Cassier’s Magazine, 1902, Page 480.
“Chrome-Vanadium Steel,” W. E. Gibbs, Cassier’s Magazine, June, 1910,
Page 174.
“Vanadium Steels,” J. Kent Smith, American Vanadium Co., Pittsburgh,
Pa.
“Alloy Steels in Motor Car Construction,” J. A. Mathews, Journal
Franklin Institute, May, 1909, Page 379.
“On the Art of Cutting Metals,” F. W. Taylor, Trans. Am. Soc. Mech.
Eng. 1906–28 No. 3.
“High Speed and Carbon Tool Steels,” Machinery Reference Series No.
117, Industrial Press, New York.
“Alloy Steels,” Machinery Reference Series No. 118.
CHAPTER XVI
“Drop Forging, Die Sinking and Machine Forging of Tools of Steel,” J.
V. Woodworth, N. W. Henley Publishing Co., New York.
“Metal Working,” Hasluck, David McKay, 1907.
“Drop Forging,” Machinery’s Reference Series No. 45.
“Press Working of Metals,” Oberlin Smith, John Wiley & Sons (1913).
CHAPTER XVII
“The Rolling Mill Industry,” F. Kindl, Penton Publishing Co.,
Cleveland, Ohio.
CHAPTERS XVIII & XIX
“Wire Manufacture and Uses,” J. B. Smith, John Wiley & Sons, New York.
CHAPTERS XX & XXI
“Modern Welded Pipe,” National Tube Co., Pittsburg, Pa.
“The Control of Quality in Every Process,” A. M. Byers Co., Pittsburg,
Pa.
“Seamless Tubing, Bulletin No. 17–A”, National Tube Co., Pittsburg,
Pa.
“The Manufacture of Iron and Steel Tubes,” E. C. R. Marks, Van Ostrand
& Co., New York.
CHAPTERS XXII & XXIII
“The Metallography of Steel and Cast Iron,” H. M. Howe, McGraw-Hill
Book Co., New York.
“The Metallography and Heat Treatment of Iron and Steel,” Albert
Sauveur, Sauveur and Boylston, Cambridge, Mass.
“Metallic Alloys: Their Structure and Constitution,” G. H. Gulliver,
Charles Griffin & Co., Ltd., London.
INDEX
Acid linings for furnaces, 139, 156
Allotropic modifications of iron, 322
Alloy steels, 232
Ammonia from coke ovens, 49
Analysis, chemical, 165
Annealing malleable cast iron, 203
cast steel, 220
tool steels, 109, 233, 333
tubes, 307
wire, 285
Bar, Merchant, 154
muck, 99
Basic linings for furnaces, 139, 156, 230
Beehive charcoal, 38
coke, 44
Benzol, 49
Bessemer, Sir Henry, 123
steel, =123=, 226
Black heart malleable iron, 209
Blast, 1, 11, 52, 58, 172
Blast furnace, 9, 10, 52
Blister steel, 117
Blowing of Bessemer steel, 128, 225
Boyden, Seth, 196
Busheled iron, 105
Butt-welded pipe, 293
By-products, 42, 47
Carbon, effect of, on iron, 8, 74, 83, 86, 109, 116, 180, 198, 233,
319, 337
effect of, on iron ore, 4, 57
Cast iron. See Iron, cast
Castings, 189
of cast iron, 178
Castings of malleable iron, 197
of pig iron (direct castings), 80
“rubber,” 192
of “semi-steel,” 188
of steel, 214
Cementation steel process, 106, =112=
Charcoal, 7, 37, 95
Chemical changes during refining, 7, 97, 119, 128, 149, 155, 160, 201,
230
Chemistry of gas producers, 158
Chromium in steel, 238, 243
Cinder in wrought iron, 96, 97, 101, 300
Classification according to chemical analysis, 83
of iron alloys, 69, 180
metallographic, 70
by physical test, 83, 188
of steels, 107, 155
volumetric, 88
Coal, 41, 49
Cold finishing of steel, 270, 306
Coke, 11, 41
Coke ovens, 44, 46
Conductivity of steels, 315
Converters, 126, 226
Cooling curves of steels, 316
Cort, Henry, 11, 95, 259
Critical temperature or range in steels, 315, 319
Crucibles, 118
Crucible steel process, 106, =118=, 223
Cupola melting process, 161, =171=, 198
Dilatation of steel, 315
Dolomite, 50
Drawing bench, 287, 306
Drawing, effect of cold-drawing, 248
of tubes, 305
of wire, 284
Duplex steel process, 155
Electric furnaces, 227
Electric steel, 227
Equilibrium diagram of iron-carbon alloys, 335, 345
Eutectic, 342
Eutectoid, 346
Fiber of wrought iron, 92, 93, 101
Fluor spar, 51
Flux, 50, 173
Forging, 248
Freezing-point curves of iron-carbon alloys, 337
Fuels, 6, 11, 15, 37, 94, 96, 119, 130, 157, 170
Furnaces, air, 198
Bessemer converters, =123=, 226
Catalan, 7
Cementation, 115
Cupola, 171
early types of, 6, 93
electric, 227
open-hearth, 142
puddling, 95
Siemens-Martin, 142
Talbot, 153
Gas, coke oven, 41, 47
natural, 157
producer, 157
Hadfield, Sir Robert, 235
Hammers for forging, 249
Hardening, air or self, 241
Hardening of steels, 109, 233, 240, 316, 333
Holley, Alexander L., 127
Huntsman crucible melting process, 10, 118
Ingots, steel, 245
Irons, “black heart” malleable cast, 209
Busheled, 105
cast (See also Semi-steel), 7, 79, =160=, =178=, 346
classification of, 69, 89
early history of, 1, 4, 91
ingot, 75
malleable, 4, 123
malleable cast, 81, =195=
pig, 63
puddled, 75, =91=
strengths of the iron alloys, 83, 188
Swedish, 93
white, 81, 184, 198
“white heart” malleable cast, 21
wrought, 75, 77, =91=
wrought, early processes for, 4, 93
Kelly, William, 124
Lap-welded pipe, 296
Lime, use of, 139, 149, 230
Limestone as flux, 50, 173
Linings, acid and basic, 139, 156, 230
of Bessemer converters, 139
of electric furnaces, 230
of open-hearth furnaces, 148
of puddling furnaces, 97
Magnetism, loss of, in steels, 314
Manganese, 135, 151
Hadfield’s manganese steel, 235
Mannesmann process for billet piercing, 302
Melting points of iron-carbon alloys, 4, 6, =83=, 94, 142
Metallography, =70=, 180, 324
Metalloids, 86, =89=, 128
Mills. See Rolling mills
Mixer, 131, 153
Molds and Molding, 13, =189=, 218
Muck bar, 99
Mushet, Robert F., 135, 240
Nasmyth, James, 251
Nickel in steel, 237
Open-hearth steel process, =142=, 223
Ore, iron, 3, 17, 53, 144
Mesaba, 26
unloaders, 31
use of (iron oxide), in refining processes, 97, 149, 230
Phosphorus in iron and steel, 97, 139, 144, 152, 230
Pickling of steel, 285, 305
Piercing of steel billets for tubes, 302
Pig bed, 61
Pig iron, 63
castings (“direct castings”), 80
Pipes and tubes, early, 292
Pipes, butt-welded, 293
lap-welded, 296
in steel ingots, 245
Plates, making of steel, 264
Press, forging by hydraulic, 256
Puddling of iron, 97
Rails, rolling of, 138, 273
Reaumur, 115, 195
Recalescence, 311, 316
Recarburization, 135, 151
References, 350
Regenerative system of heating, 146
Risers on steel castings, 219
Rods, rolling of, 277
Rolling mills, 259, 277, 294, 304
Rolling of muck bar, 99
of rails, 138, 273
of rods, 277
Rolling of skelp for pipe, 293
of steel plates, 264, 268
Rolling vs. forging of steel, 258
Segregation in steel ingots, 246
“Semi-steel,” 78, 185
Sheets, rolling of, 273
Siemens-Martin steel process. See Open-hearth
Silicon in iron-carbon alloys, =83=, 120, 133, 168, =182=, 346
Skelp for pipe, 293
Slag and cinder, 56, 58, 96, 97, 151
Spiegeleisen addition in Bessemer process, 135
Squeezer for puddled iron, 99
Steam hammer for forging, 251
Steels, acid, 139, 156
alloy, 107, 232
annealing of, 109, 220, 233, 285, 307, 333
basic, 139, 156, 230
Bessemer process, 123, 154, 226
blister, 117
carbon, 107
castings. See Castings of steel
cementation, 106, =112=
chrome, 238
classification of, 107, 155
crucible, 106, =118=, 223
duplex, 155
electric, 227
hardening of, 109, 233, 240, 241, 316, 333
high-speed, 107, 240
manganese, 235
nickel, 237
open-hearth, 107, =142=, 223
place in the iron family, 78
“semi-steel,” 79, 185
shear, 118
structures, decompositions and transformations of, 310, 337, 343
tool, 107, 240
tungsten, 236, 243
Wootz, Damascus, Toledo, etc., 8, 121
Strengths of iron-carbon alloys, 83, 188
of steel plate, 270
of wire, 290
Structures of the irons and steels, 310, 323, 337, 343
See also Metallography
Sulphur in fuels and alloys, 40, 43, 95, 97, 155, 168, 203, 230
Tar, coal, 48
Taylor, Frederick W., 241
Tempering of steels, 109, 240, 331
Titanium in steel, 234
Tools, 3, 107, 212
Tool steels, carbon, 107
high-speed, 107, =240=
Tubes and tube making, 302
Tungsten in steel, 236, 241
Uses of irons and steels, 71, 83, 184, 212, 213, 215, 249
Vanadium in steel, 235, 243
Welding rolls, 297
Welding of steel, 83, 294, 296
“White heart” malleable cast iron, 210
White iron, 81, 184, 198
White, Maunsel, 241
Wire, drawing of, 284
Wootz steel process, 8, 121
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TRANSCRIBER’S NOTES
1. Silently corrected palpable typographical errors; retained
non-standard spellings and dialect.
2. Reindexed footnotes using numbers.
3. Enclosed italics font in _underscores_.
4. Enclosed bold or blackletter font in =equals=.
5. Denoted superscripts by a caret before a single superscript
character or a series of superscripted characters enclosed in
curly braces, e.g. M^r. or M^{ister}.
*** End of this LibraryBlog Digital Book "Non-technical chats on iron and steel and their application to modern industry" ***
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