Category: HEAT TREATMENT OF STEEL
One of the most important means of investigating the properties of
pure metals and their alloys is by an examination of their heating
and cooling curves. Such curves are constructed by taking a small
piece and observing and recording the temperature of the mass at
uniform intervals of time during a uniform heating or cooling.
These observations, when plotted in the form of a curve will show
whether the temperature of the mass rises or falls uniformly.
The heat which a body absorbs serves either to raise the temperature
of the mass or change its physical condition. That portion of the
heat which results in an increase in temperature of the body is
called sensible heat, inasmuch as such a gain in heat is apparent
to the physical senses of the observer. If heat were supplied to the
body at a uniform rate, the temperature would rise continuously,
and if the temperature were plotted against time, a smooth rising
curve would result. Or, if sensible heat were abstracted from the
body at a uniform rate, a time-temperature curve would again be a
smooth falling curve. Such a curve is called a cooling curve.
However, we find that when a body is melting, vaporizing, or otherwise
suffering an abrupt change in physical properties, a quantity of
heat is absorbed which disappears without changing the temperature
of the body. This heat absorbed during a change of state is called
latent heat, because it is transformed into the work necessary to
change the configuration and disposition of the molecules in the
body; but it is again liberated in equal amount when the reverse
change takes place.
From these considerations it would seem that should the cooling
curve be continuous and smooth, following closely a regular course,
all the heat abstracted during cooling is furnished at the expense
of a fall in temperature of the body; that is to say, it disappears
as sensible heat. These curves, however, frequently show horizontal
portions or arrests which denote that at that temperature all
of the heat constantly radiating is being supplied by internal
changes in the alloy itself; that is, it is being supplied by the
evolution of a certain amount of latent heat.
In addition to the large amount of heat liberated when a metal
solidifies, there are other changes indicated by the thermal analysis
of many alloys which occur after the body has become entirely
solidified. These so-called transformation points or ranges may
be caused by chemical reactions taking place within the solid,
substances being precipitated from a solid solution, or a sudden
change in some physical property of the components, such as in
magnetism, hardness, or specific gravity.
It may be difficult to comprehend that such changes can occur in
a body after it has become entirely solidified, owing to the usual
conception that the particles are then rigidly fixed. However, this
rigidity is only comparative. The molecules in the solid state
have not the large mobility they possess as a liquid, but even so,
they are still moving in circumscribed orbits, and have the power,
under proper conditions, to rearrange their position or internal
configuration. In general, such rearrangement is accompanied by a
sudden change in some physical property and in the total energy
of the molecule, which is evidenced by a spontaneous evolution or
absorption of latent heat.
Cooling curves of the purest iron show at least two well-defined
discontinuities at temperatures more than 1,000 deg.F., below its
freezing-point. It seems that the soft, magnetic metal so familiar
as wrought iron, and called alpha iron or ferrite by the
metallurgist, becomes unstable at about 1,400 deg.F. and changes into
the so-called beta modification, becoming suddenly harder, and
losing its magnetism. This state in turn persists no higher than
1,706 deg.C., when a softer, non-magnetic gamma iron is the stable
modification up to the actual melting-point of the metal. These
various changes occur in electrolytic iron, and therefore cannot be
attributed to any chemical reaction or solution; they are entirely
due to the existence of allotropic modifications of the iron in
its solid state.
Steels, or iron containing a certain amount of carbon, develop
somewhat different cooling curves from those produced by pure iron.
Figure 45 shows, for instance, some data observed on a cooling
piece of 0.38 per cent carbon steel, and the curve constructed
therefrom. It will be noted that the time was noted when the needle
on the pyrometer passed each dial marking. If the metal were not
changing in its physical condition, the time between each reading
would be nearly constant; in fact for a time it required about 50
sec. to cool each unit. When the dial read about 32.5 (corresponding
in this instrument to a temperature of 775 deg.C. or 1,427 deg.F.) the
cooling rate shortened materially, 55 sec. then 65, then 100, then
100; showing that some change inside the metal was furnishing some
of the steadily radiating heat. This temperature is the so-called
upper critical for this steel. Further down, the lower critical
is shown by a large heat evolution at 695 deg.C. or 1,283 deg.F.
Just the reverse effects take place upon heating, except that the
temperatures shown are somewhat higher--there seems to be a lag
in the reactions taking place in the steel. This is an important
point to remember, because if it was desired to anneal a piece of
0.38 carbon steel, it is necessary to heat it up to and beyond
1,476 deg. F. (1,427 deg.F. plus this lag, which may be as much as 50 deg.).
It may be said immediately that above the upper critical the carbon
exists in the iron as a solid solution, called austenite by
metallographers. That is to say, it is uniformly distributed as atoms
throughout the iron; the atoms of carbon are not present in any fixed
combination, in fact any amount of carbon from zero to 1.7 per cent
can enter into solid solution above the upper critical. However,
upon cooling this steel, the carbon again enters into combination
with a definite proportion of iron (the carbide cementite, Fe3C),
and accumulates into small crystals which can be seen under a good
microscope. Formation of all the cementite has been completed by
the time the temperature has fallen to the lower critical, and
below that temperature the steel exists as a complex substance
of pure iron and the iron carbide.
It is important to note that the critical points or critical range
of a plain steel varies with its carbon content. The following
table gives some average figures:
Carbon Content. Upper Critical. Lower Critical.
0.00 1,706 deg.F. 1,330 deg.F.
0.20 1,600 deg.F. 1,330 deg.F.
0.40 1,480 deg.F. 1,330 deg.F.
0.60 1,400 deg.F. 1,330 deg.F.
0.80 1,350 deg.F. 1,330 deg.F.
0.90 1,330 deg.F. 1,330 deg.F.
1.00 1,470 deg.F. 1,330 deg.F.
1.20 1,650 deg.F. 1,330 deg.F.
1.40 1,830 deg.F. 1,330 deg.F.
1.60 2,000 deg.F. 1,330 deg.F.
It is immediately noted that the critical range narrows with increasing
carbon content until all the heat seems to be liberated at one
temperature in a steel of 0.90 per cent carbon. Beyond that composition
the critical range widens rapidly. Note also that the lower critical
is constant in plain carbon steels containing no alloying elements.
This steel of 0.90 carbon content is an important one. It is called
eutectoid steel. Under the microscope a properly polished and
etched sample shows the structure to consist of thin sheets of
two different substances (Fig. 46). One of these is pure iron,
and the other is pure cementite. This structure of thin sheets
has received the name pearlite, because of its pearly appearance
under sunlight. Pearlite is a constituent found in all annealed
carbon steels. Pure iron, having no carbon, naturally would show no
pearlite when examined under a microscope; only abutting granules
of iron are delicately traced. The metallographist calls this pure
iron ferrite. As soon as a little carbon enters the alloy and a
soft steel is formed, small angular areas of pearlite appear at the
boundaries of the ferrite crystals (Fig. 47). With increasing carbon
in the steel the volume of iron crystals becomes less and less, and
the relative amount of pearlite increases, until arriving at 0.90
per cent carbon, the large ferrite crystals have been suppressed and
the structure is all pearlite. Higher carbon steels show films of
cementite outlining grains of pearlite (Fig. 48).
This represents the structure of annealed, slowly cooled steels.
It is possible to change the relative sizes of the ferrite and
cementite crystals by heat treatment. Large grains are associated
with brittleness. Consequently one must avoid heat treatments which
produce coarse grains.
In general it may be said that the previous crystalline structure
of a steel is entirely obliterated when it passes just through the
critical range. At that moment, in fact, the ferrite, cementite or
pearlite which previously existed has lost its identity by everything
going into the solid solution called austenite. If sufficient time
is given, the chemical elements comprising a good steel distribute
themselves uniformly through the mass. If the steel be then cooled,
the austenite breaks up into new crystals of ferrite, cementite
and pearlite; and in general if the temperature has not gone far
above the critical, and cooling is not excessively slow, a very
fine texture will result. This is called refining the grain;
or in shop parlance closing the grain. However, if the heating
has gone above the critical very far, the austenite crystals start
to grow; a very short time at an extreme temperature will cause
a large grain growth. Subsequent cooling gives a coarse texture,
or an arrangement of ferrite, cementite and pearlite grains which
is greatly coarsened, reflecting the condition of the austenite
crystals from which they were born.
It maybe noted in passing that the coarse crystals of cast metal
cannot generally be refined by heat treatment unless some forging
or rolling has been done in the meantime. Heat treatment alone does
not seem to be able to break up the crystals of an ingot structure.
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