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July, 1925 Forging- Stamping - Heat Treating 247
The dark areas represent austenite, and the lined
areas, pearlite. In steels having less than 0.90 per
cent carbon, the white areas represent ferrite, and in
steels having more than 0.90 per cent carbon, they
represent cementite. On cooling from the austenite
state there is a gradual separation of either ferrite or
cementite from the grains of austenite, beginning at
the grain boundaries. In the higher carbon steels, some
cementite may also separate within the grains. The
only sudden change in structure occurs at the Al
point, where the austenite is converted into pearlite.
If specimens having the structure shown at the base
of the diagram, are slowly reheated, the structure will
pass through the successive stages, from bottom to
top.
(The changes of structure, except recrystallization,
which take place in the different steels, on heating and
cooling, may be strikingly illustrated by cutting a
horizontal slit about y% in. wide in a piece of cardboard,
placing it over the diagram, Fig. 113, and moving
it up and down. The structures corresponding to
the various temperatures for each steel will appear in
the slit. The slit should extend across the full width
of the picture, including the temperature column.)
Time a Factor.
The changes in the microstructure of steel, which
have been described above, do not take place instantaneously.
The processes of solution and of atomic
rearrangement, require time for their completion, just
as it requires time for particles of sugar to dissolve
in a cup of coffee. Time is therefore a highly important
factor in determining the structural changes which
take place in the heating and cooling of steel and
consequently in determining the changes in physical
properties which are produced by heat treatment.
If heating is too rapid, the changes will not take
place until temperatures higher than those of equilibrium
have been reached. If cooling is too rapid,
the changes will be greatly modified, or may even be
prevented or suppressed.
Recrystallization.
When Alpha iron or ferrite changes to Gamma iron
or austenite, complete recrystallization takes place. A
new set of crystalline grains is produced. Tiny Gamma
iron grains start to form at many points, but chiefly
at the boundaries of the former Alpha grains, and follow
the ordinary laws of grain growth. The Alpha
or ferrite grains are completely obliterated, and are replaced
by new and (at first) very small grains of
Gamma iron or austenite.
When a grain of pearlite is heated through Al and
changes into austenite, the little layers of ferrite and
cementite dissolve in each other, while, at the same
time, the Alpha iron (body centered lattice) changes
to Gamma iron (face centered lattice). Simultaneously
a new set of crystalline grains comes into existence
in the little mass of austenite, which has replaced
the pearlite grain. Tiny crystals start to form at many
points and grow until they meet each other.
On cooling through the critical range, there is
also recrystallization. At Ar3 many small Alpha
iron grains form from each Gamma iron grain. On
cooling through Arl, each austenite grain is replaced
by one or more grains of pearlite. The number and
size of the new grains of Alpha iron or of pearlite, are
influenced by the rate of cooling through the transformation
points.
Grain Growth.
The fact that the crystalline grains of metals are
able to grow while in the solid state was mentioned in
Chapter III. The larger grains rob their smaller
neighbors, taking atoms from the adjoining surfaces
of the smaller grains, and fitting them to their own
orientation. In this way the large grains get larger
and the smaller grains finally disappear. If this process
were continued indefinitely, a given piece of metal
would finally be converted into a single huge crystalline
grain. But as the grains get larger their tendency
to grow decreases, so that the process finally comes
to a stop.
Grain growth does not take place in iron or its
alloys at ordinary temperatures. A certain amount of
heat is necessary to give the atoms the power (energy
and mobility), to move from one grain to another,
across the grain boundary. There is, in general, a
minimum temperature at which grain growth will take
place in any metal or alloy. (This is influenced by
certain other conditions, especially the presence of
strains in the metal, discussed in Part 2 of this chapter.)
As the temperature is raised above the minimum
value, grain growth becomes more rapid. No grain
growth occurs in slowly cooled (normal) steel, when
it is reheated to temperatures below the critical range.
The new austenite grains which are formed on
heating through the Al point grow very slowly at
this temperature, but the speed of growth increases
as the temperature is raised, and may be fairly rapid
at A3 or Acm (in low or high carbon steels), and
quite rapid at still higher temperatures.
As the grains of any free ferrite or cementite which
is present in hypo- or hyper-eutectoid steel are completely
obliterated upon being absorbed in the austenite,
any grain growth which may take place in either of
these constituents during heating to A3 or Acm, is
not important.
In order to obtain the best results in the hardening
of steel it is essential to first get all of the ferrite or
cementite into solid solution, and to produce the finest
(smallest) possible grain structure. The reasons for
this will appear in the following section. Evidently,
therefore, the steel must be heated above the A3 or
the Acm point, so that all of the free ferrite or cementite
may be absorbed, and it must be held at this temperature
long enough to permit complete solution to
take place. But the temperature must not be raised
much above the critical range, or grain growth will be
rapid, and it must not be held above the critical range,
too long, or even a slow rate of grain growth will produce
a coarse structure. For hyper-eutectoid steels
a double treatment may be necessary, in order first
to get the cementite into solution, and then refine the
grain.
Pearlite Grains.
It is customary to speak of "grains" of pearlite. It
may be well to point out the distinction between pearlite
grains and crystalline grains of a pure metal or
solid solution. Pearlite grains are not crystalline bodies,
in the same sense that grains of ferrite are crystalline
bodies. We have seen that a crystalline grain of
metal is a body made up of atoms all arranged along
definite straight lines and planes. A grain of pearlite
is not made up in this way. It is composed of alternate
thin layers or plates of two different materials, ferrite