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.