CAST IRON, WROUGHT IRON, AND STEEL
IT USED to be that wars were fought for gold, but nowadays the
possession of rich iron mines is enough to arouse the cupidity of
neighboring nations less favored by nature. In fact, even though an
ounce of gold is worth twice as much as a ton of iron, the value of
the iron we dig out of the earth each year is far greater than that
of gold. When that rough ore is converted into iron and steel and
then into thousands of useful articles, its value mounts so high that
it cannot be estimated. The qualities of iron and its alloys are so
excellent and so varied under different treatment that this metal may
truly be said to form the foundation of all our mechanical progress.
On the one hand it spans our wide rivers, carries the burden of heavy
freight trains, or, in the form of armor plate, resists the terrific
impact of high-powered shells; on the other hand the same metal, spun
into a hair spring, governs the ship’s chronometer, or, in the compass,
points a trembling finger to guide the navigator on his course.
IRON IN ANCIENT DAYS
The first use of iron in the service of man dates far back into the
ages. An iron tool was found in the pyramid of Kephron which must
have been used 3,500 years before Christ. However, because of the
difficulty of working it, iron was not extensively employed except for
swords and cutlery. The conversion of iron into steel and the tempering
of steel blades grew to be an art which gave Damascus and Toledo a
world-wide reputation that dates back over a thousand years.
The ancients used to smelt their iron ore in what was known as a
Catalan forge because of its extensive use in Catalonia, Spain. Whether
the forge was invented there or not we cannot say. Similar forges have
been found in India and other widely remote places. They comprised an
inclined tray leading to a pot which formed the furnace and in which
a charcoal fire was kindled. The ore and charcoal were placed on the
tray and from time to time were raked down into the furnace and air
was forced into the bottom of the furnace by means of bellows. In an
improved form of the Catalan forge air was furnished by means of an
air compressor operated by a stream of water. This has already been
referred to and illustrated on page 90. Limestone served as a flux to
melt the earthy matter. The iron obtained from these primitive furnaces
was not heated sufficiently to flow as a stream, but was merely reduced
to a pasty mass which was then hammered into shape by the blacksmith.
Ten or twelve pounds of metal per day was considered a fair output for
one of these forges.
DISCOVERY OF COKE
It was not until the middle of the 14th Century that a blast furnace,
crudely similar to those we have to-day, was first built and with it a
temperature was obtained that was high enough to turn the metal into a
liquid which could be cast in molds. Charcoal continued to be the fuel
used until about four centuries later, when Abraham Darby discovered
that by baking coal to remove its free gases, he could produce a new
fuel known as coke which was a good substitute for charcoal. This gave
a wonderful impetus to the iron industry in England where there were
ample deposits of coal adjacent to the iron mines. Shortly after that,
Mr. Henry Cort of Gasport, England, invented the processes of puddling
and rolling the product of the blast furnace, thus converting the iron
into a tough, malleable metal.
ALLOYS OF CARBON AND IRON
We must pause here to learn the difference between cast iron, wrought
iron and steel. Iron, as we know, has a high affinity for oxygen. When
exposed to air and moisture it oxidizes, rusts very quickly. The iron
we find in nature is largely oxidized. In other words, it is rusty. It
is also found in combination with other elements as well. The object of
putting iron ore through a furnace is to rid it of oxygen and this is
most readily accomplished by melting it in a carbon fire. The highly
heated carbon combines with the oxygen and passes off as carbon dioxide
and carbon monoxide gas. But a certain amount of carbon unites with the
iron and it is this alloy of carbon and iron that makes cast iron so
stiff and brittle. The less carbon present the softer is the metal and
pure iron is very ductile.
It was to rid cast iron of its carbon content that Cort invented the
puddling process. As the metal came out of the blast furnace it ran
into a “reverberatory” furnace where, without coming in contact with
coke or other carbon fuel, it was exposed to flames from an adjoining
furnace which burned out the carbon, and then the carbon-free iron was
cast into large pieces known as blooms which were hammered to rid them
of slag. The final product was known as wrought iron. Wrought iron then
differs from cast iron in having no carbon. Steel, on the other hand,
stands half way between wrought iron and cast iron in having a small
percentage of carbon. How steel is made will be described later.
MECHANICAL HANDLING OF ORE
Of course machinery plays a large part in the modern iron industry. It
would be an endless task even to load one of the big blast furnaces
by hand and then the enormous output of molten metal--40 tons for
every pound produced by the old Catalan furnaces--could not be
handled without ponderous machines whose huge arms and fingers are
not scorched and blistered by the intense heat. Along the Great Lakes
vast loading machines fill the holds of ore vessels and at the plant
there are enormous unloading machines that travel on rails. These have
long bridgelike arms that reach out over the ore boat and drop huge
clam-shell buckets into their holds. The buckets quickly unload the
boats and dump the ore on shore where other buckets pick up the ore,
carry it back and pile it up in big heaps that look like mounds of
reddish earth.
THE MODERN BLAST FURNACE
Blast furnaces are towering cylindrical structures of steel lined with
fire brick. They are loaded from the top with alternate layers of
coke and ore. Limestone also is added to act as a flux for the earthy
matter, as explained above. Running around the furnace near the base
there is a large pipe known as the “bustle” pipe. Through this air is
fed to a set of “tuyeres” which lead to the base of the furnace and
admit blasts of air requisite to maintain combustion at an intense
heat. The gases of combustion are not allowed to escape freely into
the atmosphere. The top of the furnace through which the ore and fuel
is admitted is closed by an air lock and the flaming hot gases are led
into “stoves” where they give up a large part of their heat to preheat
the air which is pumped to the blast furnace. The gases being mainly
composed of carbon monoxide are further combustible and may be used for
heat, light, and power purposes. In fact, they are commonly used to
drive the air compressors which feed the blast furnaces.
The blast furnace has two openings, one above the other. Through the
upper one slag is drawn off while the molten iron which trickles down
and collects at the bottom of the furnace is tapped off through a hole
near the base of the furnace. The fiery stream pours out into a lot of
small trough-shaped molds and is thus formed into “pigs.” These pigs
are all connected to the main body of the metal stream and must be
broken off. To save the time of cooling and of breaking off the pigs a
machine is used which consists of a series of molds connected to form
an endless belt. The molten iron is poured into these molds which in
their course dip into a trough of water. Here the iron is cooled and
solidified. The molds then run up an incline and finally dump the pigs
directly into railway cars which haul them away.
BURNING OUT THE CARBON
The production of steel economically and on a large scale dates back
to the inventions of Henry Bessemer. While searching for an improved
method of making big guns, Bessemer hit upon the idea of forcing a
blast of air through the molten iron and thus burning away carbon,
silicon, and manganese in the cast iron. No fuel was supplied except
the carbon and silicon in the iron itself. In burning out this carbon
sufficient heat was generated to keep the metal fluid.
When Bessemer made the announcement of his new process before the
British Association in 1856, his paper met with skepticism, but he
was able to demonstrate by actual experiment that cast iron could be
converted into malleable iron in this way. However, when several firms
operating under licenses from the inventor endeavored to reproduce his
experiment on a commercial scale they were unsuccessful, and after
costly experiments the process was given up as a failure. Bessemer,
however, persisted in his efforts and succeeded eventually in producing
malleable iron of a quality equal if not superior to that on the
market. But iron makers after the failure of the first experiments
would have nothing to do with the new process until Bessemer began to
turn out quantities of iron at $100 a ton below the prevailing market
price. Then iron makers woke up and Bessemer had no difficulty in
placing his process with numbers of firms on a very profitable royalty
basis.
This process pertained to the making of iron and not steel. When
Bessemer tried to produce steel he was confronted with serious
difficulties. The steel he obtained was very brittle. He tried purer
ores with little better success. Then a solution of his problem was
offered by Robert F. Mushet, who discovered a compound which would be
added to the molten metal to purify it. This compound which is known as
“spiegeleisen” is composed of iron, carbon, and manganese. It removes
the oxide of iron and the sulphur and regulates the amount of carbon in
the steel.
A Bessemer converter furnishes by far the most spectacular operation
in steel manufacture. The converter consists of a large bottle-shaped
vessel lined with refractory material. In the bottom of the vessel
there are openings through which the air blast is admitted. The molten
metal is poured into the flask and then the air blast is turned on. The
metal begins to boil violently. A dazzlingly brilliant blast of flame
and sparks comes roaring out of the mouth of the converter. Bubbles
of metal are thrown high into the air where they burst into showers
of sparks. The effect is similar to that of a volcanic eruption. In
from ten to twenty minutes the eruption subsides and then a quantity
of spiegeleisen is added. The converter is mounted on trunnions so
that when the operation is completed the vessel is tilted over and the
charge of molten metal now converted into steel is poured out.
OPEN HEARTH FURNACES
[Illustration: MOLTEN METAL FROM A BLAST FURNACE]
[Illustration: BESSEMER CONVERTER BLOWING AIR THROUGH A MASS OF MOLTEN
IRON]
[Illustration: A STEEL BEAM PASSING THROUGH THE FINISHING ROLLS OF A
STEEL MILL]
[Illustration: FIG. 74.--SECTIONAL VIEW OF A REGENERATIVE OPEN-HEARTH
FURNACE]
While the Bessemer converter provides a very economical and expeditious
method of converting cast iron into steel, it is difficult to regulate
the carbon content with great accuracy and hence the use of the
open-hearth furnaces which furnish a slower method of burning out the
carbon. Figure 74 is a diagrammatic representation of such a furnace.
Below the hearth of the furnace there are two pairs of chambers, _A_,
_B_ and _C_, _D_, filled with a checkerwork of bricks. Gas is passed
through one chamber _A_, and air through the other _B_, and they
combine to form a very intense flame above the hearth _E_ in which
the metal is placed. The burnt gases pass over and through the other
pair of chambers, _C_, _D_, on their way to the stack. By this means
the bricks in the latter chambers are raised to a white heat. Then the
process is reversed; air flows through the hot checkerwork of bricks
in the chamber _C_ and gas through the hot checkerwork in chamber _D_,
and after combustion in the furnace the burnt gases are drawn through
the bricks of the first pair of chambers. By alternating the direction
of flow the air and gas fed to the furnace are always preheated by the
stored-up heat of the previously burned gases. While it takes but a few
minutes to convert cast iron into steel in the Bessemer converter, the
open-hearth process occupies from eight to twelve hours.
The steel produced in the open-hearth furnaces is poured into ingot
molds. These are approximately rectangular in section and slightly
larger at the bottom than at the top. They are open at the top and
bottom, but at the bottom rest upon a base plate. As soon as the steel
has hardened the plunger of a stripping machine holds down the glowing
ingot while a pair of hooks lift off the mold, leaving the ingot
resting on the base plate.
ROLLING INGOTS INTO RAILS
In the manufacture of railroad rails the ingots are placed on a
traveling “table” consisting of a series of rapidly turning rollers.
These carry the ingot to a pair of large steel rolls between which it
passes. The rolls compress the ingot slightly and it is automatically
turned over and passed through a second pair of rolls. After passing
through four “stands” of rolls, turning over between each stand, it
is considerably reduced in cross-sectional area and correspondingly
lengthened. It is now termed a “bloom.” The bloom goes through a series
of rollers which gradually reduce its section until it is some forty
feet long. Then it is cut in two and each section passes through other
rolls, until finally it is reduced to the required rail section. Each
section is then about a hundred and twenty feet long and the glowing
writhing rail passes on to the saws where it is cut into ten-yard
lengths. A similar process is employed in rolling other forms of rails
and in making steel plates and sheets.
STEEL FOR BIG GUNS
The largest machines employed in the steel industry are those used for
the manufacture of armor and big guns. A modern large high-powered gun
is not a single solid casting or forging, but is made up of a series
of steel tubes that are shrunk one upon another so that the inner tube
is compressed. The reason for this is that the explosives used are so
powerful that they would expand the inner tube or lining of the gun
beyond its elastic limit and in that way enlarge the bore. By having it
compressed to start with it can expand farther without exceeding the
elastic limit. This expansion takes place so suddenly that the lining
rebounds or returns to its original dimensions before the outer tubes
have felt the full pressure and they too are thus prevented from being
expanded too far. In some cases the compression is effected by winding
the gun with a heavy wire of rectangular cross section.
SQUEEZING OUT THE “PIPES”
Steel for guns and armor is made in the open-hearth furnace where the
quality of the metal may be regulated to a nicety. Gun tubes are cast
in vertical molds and during the cooling of the ingot it is subjected
to pressure so as to prevent segregation and the forming of “pipes.”
Pipes are cavities that are liable to form in the center of the ingot
due to contraction during cooling. Steel, as we have learned, is not
pure iron, but an alloy, and the various constituents have different
temperatures of solidifying, consequently they exhibit a tendency to
segregate. It is to overcome such tendencies that a so-called “fluid”
compressor is used. This is virtually a hydraulic press with a plunger
that bears down on the fluid metal as it is solidifying. Modern big
guns are enormously large. A sixteen-inch 50-caliber gun, for instance,
is nearly seventy feet long, consequently the ingot must be even
longer than this and the fluid compressor for so large a piece must be
correspondingly powerful. After the ingot has been cast and cooled,
the ends are cut off and it is bored to form a tube. Then it is placed
in a furnace and raised to a white heat, after which a bar or mandrel
is inserted in the bore and the tube is placed under the hydraulic
forge press. This is a very powerful machine with an immense hammer
that is actuated by hydraulic pressure. The stroke of the hammer is
carefully regulated so that the forging as it is turned in the forge
is subjected to equal blows. In the forge the tube is roughly formed
to the dimensions it is eventually to have when finished. The process
of forging subjects the metal to strains which must be relieved and
so the tube has to go to the annealing oven where it is raised to a
temperature which destroys crystallization. In this oven it is allowed
to cool very slowly, letting the molecules of the metal adjust and
rearrange themselves. When the temperature of the tube has been lowered
to a certain point it is taken out and plunged into a bath of oil. This
sudden cooling tempers the metal, giving it a high degree of elasticity
and tensile strength. Again the tube must be annealed to relieve it of
any strains occasioned by the tempering, and then it goes to the shop
to receive its finish boring and turning.
The process as here briefly described seems simple enough, but we must
not forget the enormous size of these pieces and their tremendous
weight. They would be difficult enough to handle when cold, but much
of the work is done while the pieces are at a white heat so that the
men who control and operate the machinery that handles the big forgings
must keep their distance. The casting, annealing, and tempering
operations are performed with the piece in vertical position, and lofty
machines and cranes are required to deal with these tall castings. A
visit to a plant which manufactures big guns is bound to impress the
visitor with awe and give him increased respect for the men who are
able to handle such huge masses of metal and also for the men who have
conceived and developed such gigantic operations.