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Iron and Steel Manufacture

Technology related to the production of iron and its alloys,
particularly those containing a small percentage of carbon.
The differences between the various types of iron and steel
are sometimes confusing because of the nomenclature used.
Steel in general is an alloy of iron and carbon, often with an
admixture of other elements. Some alloys that are
commercially called irons contain more carbon than
commercial steels. Open-hearth iron and wrought iron
contain only a few hundredths of 1 percent of carbon. Steels
of various types contain from 0.04 percent to 2.25 percent of
carbon. Cast iron, malleable cast iron, and pig iron contain
amounts of carbon varying from 2 to 4 percent. A special
form of malleable iron, containing virtually no carbon, is
known as white-heart malleable iron. A special group of iron
alloys, known as ferroalloys, is used in the manufacture of
iron and steel alloys; they contain from 20 to 80 percent of an
alloying element, such as manganese, silicon, or chromium.

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History

The exact date at which people discovered the technique of smelting iron
ore to produce usable metal is not known. The earliest iron implements
discovered by archaeologists in Egypt date from about 3000 BC, and iron
ornaments were used even earlier; the comparatively advanced technique
of hardening iron weapons by heat treatment was known to the Greeks
about 1000 BC.
The alloys produced by early iron workers, and, indeed, all the iron alloys
made until about the 14th century AD, would be classified today as wrought
iron. They were made by heating a mass of iron ore and charcoal in a forge
or furnace having a forced draft. Under this treatment the ore was reduced
to the sponge of metallic iron filled with a slag composed of metallic
impurities and charcoal ash. This sponge of iron was removed from the
furnace while still incandescent and beaten with heavy sledges to drive out
the slag and to weld and consolidate the iron. The iron produced under
these conditions usually contained about 3 percent of slag particles and 0.1
percent of other impurities. Occasionally this technique of ironmaking
produced, by accident, a true steel rather than wrought iron. Ironworkers
learned to make steel by heating wrought iron and charcoal in clay boxes
for a period of several days. By this process the iron absorbed enough
carbon to become a true steel.

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After the 14th century the furnaces used in smelting were increased in size, and
increased draft was used to force the combustion gases through the ”charge,” the
mixture of raw materials. In these larger furnaces, the iron ore in the upper part of the
furnace was first reduced to metallic iron and then took on more carbon as a result of
the gases forced through it by the blast. The product of these furnaces was pig iron, an
alloy that melts at a lower temperature than steel or wrought iron. Pig iron (so called
because it was usually cast in stubby, round ingots known as pigs) was then further
refined to make steel.
Modern steelmaking employs blast furnaces that are merely refinements of the
furnaces used by the old ironworkers. The process of refining molten iron with blasts of
air was accomplished by the British inventor Sir Henry Bessemer who developed the
Bessemer furnace, or converter, in 1855. Since the 1960s, several so-called minimills
have been producing steel from scrap metal in electric furnaces. Such mills are an
important component of total U.S. steel production. The giant steel mills remain
essential for the production of steel from iron ore.
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The basic materials used for the manufacture of pig iron are iron ore, coke, and
limestone. The coke is burned as a fuel to heat the furnace; as it burns, the coke gives
off carbon monoxide, which combines with the iron oxides in the ore, reducing them to
metallic iron. This is the basic chemical reaction in the blast furnace; it has the
equation: Fe
2
O
3
+ 3CO = 3CO
2
+ 2Fe. The limestone in the furnace charge is used as
an additional source of carbon monoxide and as a “flux” to combine with the infusible
silica present in the ore to form fusible calcium silicate. Without the limestone, iron
silicate would be formed, with a resulting loss of metallic iron. Calcium silicate plus
other impurities form a slag that floats on top of the molten metal at the bottom of the
furnace. Ordinary pig iron as produced by blast furnaces contains iron, about 92
percent; carbon, 3 or 4 percent; silicon, 0.5 to 3 percent; manganese, 0.25 to 2.5
percent; phosphorus, 0.04 to 2 percent; and a trace of sulfur.
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The basic materials used for the manufacture of pig iron are iron ore, coke, and
limestone. The coke is burned as a fuel to heat the furnace; as it burns, the coke gives
off carbon monoxide, which combines with the iron oxides in the ore, reducing them to
metallic iron. This is the basic chemical reaction in the blast furnace; it has the
equation: Fe
2
O
3
+ 3CO = 3CO
2
+ 2Fe. The limestone in the furnace charge is used as
an additional source of carbon monoxide and as a “flux” to combine with the infusible
silica present in the ore to form fusible calcium silicate. Without the limestone, iron
silicate would be formed, with a resulting loss of metallic iron. Calcium silicate plus
other impurities form a slag that floats on top of the molten metal at the bottom of the
furnace. Ordinary pig iron as produced by blast furnaces contains iron, about 92
percent; carbon, 3 or 4 percent; silicon, 0.5 to 3 percent; manganese, 0.25 to 2.5
percent; phosphorus, 0.04 to 2 percent; and a trace of sulfur.
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The air used to supply the blast in a blast furnace is preheated to temperatures
between approximately 540° and 870° C (approximately 1000° and 1600° F). The
heating is performed in stoves, cylinders containing networks of firebrick. The bricks in
the stoves are heated for several hours by burning blast-furnace gas, the waste gases
from the top of the furnace. Then the flame is turned off and the air for the blast is
blown through the stove. The weight of air used in the operation of a blast furnace
exceeds the total weight of the other raw materials employed.
An important development in blast furnace technology, the pressurizing of furnaces,
was introduced after World War II. By “throttling” the flow of gas from the furnace
vents, the pressure within the furnace may be built up to 1.7 atm or more. The
pressurizing technique makes possible better combustion of the coke and higher
output of pig iron. The output of many blast furnaces can be increased 25 percent by
pressurizing. Experimental installations have also shown that the output of blast
furnaces can be increased by enriching the air blast with oxygen.
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The process of tapping consists of knocking out a clay plug from
the iron hole near the bottom of the bosh and allowing the molten
metal to flow into a clay-lined runner and then into a large, brick-
lined metal container, which may be either a ladle or a rail car
capable of holding as much as 100 tons of metal. Any slag that
may flow from the furnace with the metal is skimmed off before it
reaches the container. The container of molten pig iron is then
transported to the steelmaking shop.
Modern-day blast furnaces are operated in conjunction with basic
oxygen furnaces and sometimes the older open-hearth furnaces as
part of a single steel-producing plant. In such plants the molten pig
iron is used to charge the steel furnaces. The molten metal from
several blast furnaces may be mixed in a large ladle before it is
converted to steel, to minimize any irregularities in the composition
of the individual melts.

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Other Methods of Iron Refining
Although almost all the iron and steel manufactured in the world is
made from pig iron produced by the blast-furnace process, other
methods of iron refining are possible and have been practiced to a
limited extent. One such method is the so-called direct method of
making iron and steel from ore, without making pig iron. In this
process iron ore and coke are mixed in a revolving kiln and heated
to a temperature of about 950° C (about 1740° F). Carbon
monoxide is given off from the heated coke just as in the blast
furnace and reduces the oxides of the ore to metallic iron. The
secondary reactions that occur in a blast furnace, however, do not
occur, and the kiln produces so-called sponge iron of much higher
purity than pig iron. Virtually pure iron is also produced by means of
electrolysis (see Electrochemistry), by passing an electric current
through a solution of ferrous chloride. Neither the direct nor the
electrolytic processes has yet achieved any great commercial
significance.
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Open-Hearth Process
Essentially the production of steel from pig iron by any process
consists of burning out the excess carbon and other impurities
present in the iron. One difficulty in the manufacture of steel is its
high melting point, about 1370° C (about 2500° F), which prevents
the use of ordinary fuels and furnaces. To overcome this difficulty
the open-hearth furnace was developed; this furnace can be
operated at a high temperature by regenerative preheating of the
fuel gas and air used for combustion in the furnace. In regenerative
preheating, the exhaust gases from the furnace are drawn through
one of a series of chambers containing a mass of brickwork and
give up most of their heat to the bricks. Then the flow through the
furnace is reversed and the fuel and air pass through the heated
chambers and are warmed by the bricks. Through this method
open-hearth furnaces can reach temperatures as high as 1650° C
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The furnace itself consists typically of a flat, rectangular brick hearth about 6 m by 10
m (about 20 ft by 33 ft), which is roofed over at a height of about 2.5 m (about 8 ft). In
front of the hearth a series of doors opens out onto a working floor in front of the
hearth. The entire hearth and working floor are one story above ground level, and the
space under the hearth is taken up by the heat-regenerating chambers of the furnace.
A furnace of this size produces about 100 metric tons of steel every 11 hr.
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The furnace is charged with a mixture of pig iron (either
molten or cold), scrap steel, and iron ore that provides
additional oxygen. Limestone is added for flux and
fluorspar to make the slag more fluid. The proportions of
the charge vary within wide limits, but a typical charge
might consist of 56,750 kg (125,000 lb) of scrap steel,
11,350 kg (25,000 lb) of cold pig iron, 45,400 kg
(100,000 lb) of molten pig iron, 11,800 kg (26,000 lb) of
limestone, 900 kg (2000 lb) of iron ore, and 230 kg (500
lb) of fluorspar. After the furnace has been charged, the
furnace is lighted and the flames play back and forth
over the hearth as their direction is reversed by the
operator to provide heat regeneration.
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Chemically the action of the open-hearth furnace consists of lowering the
carbon content of the charge by oxidization and of removing such impurities
as silicon, phosphorus, manganese, and sulphur, which combine with the
limestone to form slag. These reactions take place while the metal in the
furnace is at melting heat, and the furnace is held between 1540° and
1650° C (2800° and 3000° F) for many hours until the molten metal has the
desired carbon content. Experienced open-hearth operators can often
judge the carbon content of the metal by its appearance, but the melt is
usually tested by withdrawing a small amount of metal from the furnace,
cooling it, and subjecting it to physical examination or chemical analysis.
When the carbon content of the melt reaches the desired level, the furnace
is tapped through a hole at the rear. The molten steel then flows through a
short trough to a large ladle set below the furnace at ground level. From the
ladle the steel is poured into cast-iron molds that form ingots usually about
1.5 m (about 5 ft) long and 48 cm (19 in) square. These ingots, the raw
material for all forms of fabricated steel, weigh approximately 2.25 metric
tons in this size. Recently, methods have been put into practice for the
continuous processing of steel without first having to go through the
process of casting ingots.

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Basic Oxygen Process
The oldest process for making steel in large quantities, the Bessemer
process, made use of a tall, pear-shaped furnace, called a Bessemer
converter, that could be tilted sideways for charging and pouring. Great
quantities of air were blown through the molten metal; its oxygen united
chemically with the impurities and carried them off.
In the basic oxygen process, steel is also refined in a pear-shaped furnace
that tilts sideways for charging and pouring. Air, however, has been
replaced by a high-pressure stream of nearly pure oxygen. After the
furnace has been charged and turned upright, an oxygen lance is lowered
into it. The water-cooled tip of the lance is usually about 2 m (about 6 ft)
above the charge although this distance can be varied according to
requirements. Thousands of cubic meters of oxygen are blown into the
furnace at supersonic speed. The oxygen combines with carbon and other
unwanted elements and starts a high-temperature churning reaction that
rapidly burns out impurities from the pig iron and converts it into steel. The
refining process takes 50 min or less; approximately 275 metric tons of
steel can be made in an hour.

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Electric-Furnace Steel
In some furnaces, electricity instead of fire supplies the heat for the
melting and refining of steel. Because refining conditions in such a
furnace can be regulated more strictly than in open-hearth or basic
oxygen furnaces, electric furnaces are particularly valuable for
producing stainless steels and other highly alloyed steels that must
be made to exacting specifications. Refining takes place in a tightly
closed chamber, where temperatures and other conditions are kept
under rigid control by automatic devices. During the early stages of
this refining process, high-purity oxygen is injected through a lance,
raising the temperature of the furnace and decreasing the time
needed to produce the finished steel. The quantity of oxygen
entering the furnace can always be closely controlled, thus keeping
down undesirable oxidizing reactions.

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Most often the charge consists almost entirely of scrap. Before it is
ready to be used, the scrap must first be analyzed and sorted,
because its alloy content will affect the composition of the refined
metal. Other materials, such as small quantities of iron ore and dry
lime, are added in order to help remove carbon and other impurities
that are present. The additional alloying elements go either into the
charge or, later, into the refined steel as it is poured into the ladle.
After the furnace is charged, electrodes are lowered close to the
surface of the metal. The current enters through one of the
electrodes, arcs to the metallic charge, flows through the metal, and
then arcs back to the next electrode. Heat is generated by the
overcoming of resistance to the flow of current through the charge.
This heat, together with that coming from the intensely hot arc itself,
quickly melts the metal. In another type of electric furnace, heat is
generated in a coil.
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Steel is marketed in a wide variety of sizes and
shapes, such as rods, pipes, railroad rails, tees,
channels, and I-beams. These shapes are produced at
steel mills by rolling and otherwise forming heated
ingots to the required shape. The working of steel also
improves the quality of the steel by refining its
crystalline structure and making the metal tougher.
The basic process of working steel is known as hot
rolling. In hot rolling the cast ingot is first heated to
bright-red heat in a furnace called a soaking pit and is
then passed between a series of pairs of metal rollers
that squeeze it to the desired size and shape. The
distance between the rollers diminishes for each
successive pair as the steel is elongated and reduced
in thickness.

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The first pair of rollers through which the ingot passes is
commonly called the blooming mill, and the square billets of steel
that the ingot produces are known as blooms. From the blooming
mill, the steel is passed on to roughing mills and finally to finishing
mills that reduce it to the correct cross section. The rollers of mills
used to produce railroad rails and such structural shapes as I-
beams, H-beams, and angles are grooved to give the required
shape.
Modern manufacturing requires a large amount of thin sheet steel.
Continuous mills roll steel strips and sheets in widths of up to 2.4
m (8 ft). Such mills process thin sheet steel so rapidly, before it
cools and becomes unworkable. A slab of hot steel over 11 cm
(about 4.5 in) thick is fed through a series of rollers which reduce
it progressively in thickness to 0.127 cm (0.05 inc) and increase
its length from 4 m (13 ft) to 370 m (1210 ft).
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Continuous mills are equipped with a number of
accessory devices including edging rollers, descaling
devices, and devices for coiling the sheet automatically
when it reaches the end of the mill. The edging rollers
are sets of vertical rolls set opposite each other at
either side of the sheet to ensure that the width of the
sheet is maintained. Descaling apparatus removes the
scale that forms on the surface of the sheet by
knocking it off mechanically, loosening it by means of
an air blast, or bending the sheet sharply at some point
in its travel. The completed coils of sheet are dropped
on a conveyor and carried away to be annealed and cut
into individual sheets.

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. A more efficient way to produce thin sheet steel is to
feed thinner slabs through the rollers. Using
conventional casting methods, ingots must still be
passed through a blooming mill in order to produce
slabs thin enough to enter a continuous mill.
By devising a continuous casting system that produces
an endless steel slab less than 5 cm (2 in) thick,
German engineers have eliminated any need for
blooming and roughing mills. In 1989, a steel mill in
Indiana became the first outside Europe to adopt this
new system.

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Pipe
Cheaper grades of pipe are shaped by bending a flat
strip, or skelp, of hot steel into cylindrical form and
welding the edges to complete the pipe. For the smaller
sizes of pipe, the edges of the skelp are usually
overlapped and passed between a pair of rollers curved
to correspond with the outside diameter of the pipe. The
pressure on the rollers is great enough to weld the
edges together. Seamless pipe or tubing is made from
solid rods by passing them between a pair of inclined
rollers that have a pointed metal bar, or mandrel, set
between them in such a way that it pierces the rods and
forms the inside diameter of the pipe at the same time
that the rollers are forming the outside diameter.

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Tin Plate
By far the most important coated product of the steel
mill is tin plate for the manufacture of containers. The
“tin” can is actually more than 99 percent steel. In some
mills steel sheets that have been hot-rolled and then
cold-rolled are coated by passing them through a bath
of molten tin. The most common method of coating is by
the electrolytic process. Sheet steel is slowly unrolled
from its coil and passed through a chemical solution.
Meanwhile, a current of electricity is passing through a
piece of pure tin into the same solution, causing the tin
to dissolve slowly and to be deposited on the steel. In
electrolytic processing, less than half a kilogram of tin
will coat more than 18.6 sq m (more than 200 sq ft) of
steel.
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For the product known as thin tin, sheet and strip
are given a second cold rolling before being coated
with tin, a treatment that makes the steel plate extra
tough as well as extra thin. Cans made of thin tin
are about as strong as ordinary tin cans, yet they
contain less steel, with a resultant saving in weight
and cost. Lightweight packaging containers are also
being made of tin-plated steel foil that has been
laminated to paper or cardboard.
Other processes of steel fabrication include forging,
founding, and drawing the steel through dies.

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Wrought Iron
The process of making the tough, malleable alloy
known as wrought iron differs markedly from other
forms of steel making. Because this process, known as
puddling, required a great deal of hand labour,
production of wrought iron in tonnage quantities was
impossible. The development of new processes using
Bessemer converters and open-hearth furnaces
allowed the production of larger quantities of wrought
iron.
Wrought iron is no longer produced commercially,
however, because it can be effectively replaced in
nearly all applications by low-carbon steel, which is
less expensive to produce and is typically of more
uniform quality than wrought iron.

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The puddling furnace used in the older process has a
low, arched roof and a depressed hearth on which the
crude metal lies, separated by a wall from the
combustion chamber in which bituminous coal is
burned. The flame in the combustion chamber
surmounts the wall, strikes the arched roof, and
“reverberates” upon the contents of the hearth. After the
furnace is lit and has become moderately heated, the
puddler, or furnace operator, “fettles” it by plastering the
hearth and walls with a paste of iron oxide, usually
hematite ore. The furnace is then charged with about
270 kg (about 600 lb) of pig iron and the door is closed.
After about 30 min the iron is melted and the puddler
adds more iron oxide or mill scale to the charge,
working the oxide into the iron with a bent iron bar
called a raddle.
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The silicon and most of the manganese in the iron are
oxidized and some sulfur and phosphorus are
eliminated. The temperature of the furnace is then
raised slightly, and the carbon starts to burn out as
carbon-oxide gases. As the gas is evolved the slag
puffs up and the level of the charge rises. As the
carbon is burned away the melting temperature of the
alloy increases and the charge becomes more and
more pasty, and finally the bath drops to its former
level. As the iron increases in purity, the puddler stirs
the charge with the raddle to ensure uniform
composition and proper cohesion of the particles. The
resulting pasty, spongelike mass is separated into
lumps, called balls, of about 80 to 90 kg (about 180 to
200 lb) each.
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The balls are withdrawn from the furnace with tongs
and are placed directly in a squeezer, a machine in
which the greater part of the intermingled siliceous
slag is expelled from the ball and the grains of pure
iron are thoroughly welded together. The iron is then
cut into flat pieces that are piled on one another,
heated to welding temperature, and then rolled into a
single piece. This rolling process is sometimes
repeated to improve the quality of the product.

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The modern technique of making wrought iron uses
molten iron from a Bessemer converter and molten
slag, which is usually prepared by melting iron ore, mill
scale, and sand in an open-hearth furnace. The molten
slag is maintained in a ladle at a temperature several
hundred degrees below the temperature of the molten
iron. When the molten iron, which carries a large
amount of gas in solution, is poured into the ladle
containing the molten slag, the metal solidifies almost
instantly, releasing the dissolved gas.
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The force exerted by the gas shatters the metal into
minute particles that are heavier than the slag and
that accumulate in the bottom of the ladle,
agglomerating into a spongy mass similar to the
balls produced in a puddling furnace. After the slag
has been poured off the top of the ladle, the ball of
iron is removed and squeezed and rolled like the
product of the puddling furnace.

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Classifications of Steel
Steels are grouped into five main classifications.
Carbon Steels
More than 90 percent of all steels are carbon
steels. They contain varying amounts of carbon
and not more than 1.65 percent manganese, 0.60
percent silicon, and 0.60 percent copper.
Machines, automobile bodies, most structural steel
for buildings, ship hulls, bedsprings, and bobby
pins are among the products made of carbon
steels.

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Alloy Steels
These steels have a specified composition, containing
certain percentages of vanadium, molybdenum, or
other elements, as well as larger amounts of
manganese, silicon, and copper than do the regular
carbon steels. Automobile gears and axles, roller
skates, and carving knives are some of the many things
that are made of alloy steels.

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High-Strength Low-Alloy Steels
Called HSLA steels, they are the newest of the five chief
families of steels. They cost less than the regular alloy steels
because they contain only small amounts of the expensive
alloying elements. They have been specially processed,
however, to have much more strength than carbon steels of
the same weight. For example, freight cars made of HSLA
steels can carry larger loads because their walls are thinner
than would be necessary with carbon steel of equal strength;
also, because an HSLA freight car is lighter in weight than the
ordinary car, it is less of a load for the locomotive to pull.
Numerous buildings are now being constructed with
frameworks of HSLA steels. Girders can be made thinner
without sacrificing their strength, and additional space is left
for offices and apartments.

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Stainless Steels
Stainless steels contain chromium, nickel, and other alloying
elements that keep them bright and rust resistant in spite of
moisture or the action of corrosive acids and gases. Some
stainless steels are very hard; some have unusual strength
and will retain that strength for long periods at extremely high
and low temperatures. Because of their shining surfaces
architects often use them for decorative purposes. Stainless
steels are used for the pipes and tanks of petroleum
refineries and chemical plants, for jet planes, and for space
capsules. Surgical instruments and equipment are made from
these steels, and they are also used to patch or replace
broken bones because the steels can withstand the action of
body fluids. In kitchens and in plants where food is prepared,
handling equipment is often made of stainless steel because
it does not taint the food and can be easily cleaned.

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Tool Steels
These steels are fabricated into many types of tools or into
the cutting and shaping parts of power-driven machinery for
various manufacturing operations. They contain tungsten,
molybdenum, and other alloying elements that give them
extra strength, hardness, and resistance to wear.

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Structure of Steel
The physical properties of various types of steel and of
any given steel alloy at varying temperatures depend
primarily on the amount of carbon present and on how it
is distributed in the iron. Before heat treatment most
steels are a mixture of three substances: ferrite, pearlite,
and cementite. Ferrite is iron containing small amounts of
carbon and other elements in solution and is soft and
ductile. Cementite, a compound of iron containing about 7
percent carbon, is extremely brittle and hard. Pearlite is
an intimate mixture of ferrite and cementite having a
specific composition and characteristic structure, and
physical characteristics intermediate between its two
constituents.
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The toughness and hardness of a steel that is not heat
treated depend on the proportions of these three
ingredients. As the carbon content of a steel increases,
the amount of ferrite present decreases and the amount
of pearlite increases until, when the steel has 0.8 percent
of carbon, it is entirely composed of pearlite. Steel with
still more carbon is a mixture of pearlite and cementite.
Raising the temperature of steel changes ferrite and
pearlite to an allotropic form of iron-carbon alloy known as
austenite, which has the property of dissolving all the free
carbon present in the metal. If the steel is cooled slowly
the austenite reverts to ferrite and pearlite, but if cooling
is sudden, the austenite is “frozen” or changes to
martensite, which is an extremely hard allotropic
modification that resembles ferrite but contains carbon in
solid solution.

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Heat Treatment of Steel
The basic process of hardening steel by heat treatment
consists of heating the metal to a temperature at which
austenite is formed, usually about 760° to 870° C (about
1400°) and then cooling, or quenching, it rapidly in water
or oil. Such hardening treatments, which form martensite,
set up large internal strains in the metal, and these are
relieved by tempering, or annealing, which consists of
reheating the steel to a lower temperature. Tempering
results in a decrease in hardness and strength and an
increase in ductility and toughness.

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The primary purpose of the heat-treating process is to
control the amount, size, shape, and distribution of the
cementite particles in the ferrite, which in turn determines
the physical properties of the steel.

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Many variations of the basic process are practiced.
Metallurgists have discovered that the change from austenite
to martensite occurs during the latter part of the cooling
period and that this change is accompanied by a change in
volume that may crack the metal if the cooling is too swift.
Three comparatively new processes have been developed to
avoid cracking. In time-quenching the steel is withdrawn from
the quenching bath when it has reached the temperature at
which the martensite begins to form, and is then cooled
slowly in air. In martempering the steel is withdrawn from the
quench at the same point, and is then placed in a constant-
temperature bath until it attains a uniform temperature
throughout its cross section.
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The steel is then allowed to cool in air through the
temperature range of martensite formation, which for
most steels is the range from about 288° C (about
550° F) to room temperature. In austempering the
steel is quenched in a bath of metal or salt maintained
at the constant temperature at which the desired
structural change occurs and is held in this bath until
the change is complete before being subjected to the
final cooling.

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Other methods of heat treating steel to harden it are used.
In case hardening, a finished piece of steel is given an
extremely hard surface by heating it with carbon or
nitrogen compounds. These compounds react with the
steel, either raising the carbon content or forming nitrides
in its surface layer. In carburizing, the piece is heated in
charcoal or coke, or in carbonaceous gases such as
methane or carbon monoxide. Cyaniding consists of
hardening in a bath of molten cyanide salt to form both
carbides and nitrides. In nitriding, steels of special
composition are hardened by heating them in ammonia
gas to form alloy nitrides.