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24(. Forging- Stamping - Heat Treating
of austenite will be converted into grains of pearlite.
The free cementite will be unaffected during cooling
through Al.
Separation of Excess Ferrite or Cementite.
It is evident, from these examples, that, whether
we start with a low carbon steel (hypo-eutectoid) or
a high carbon steel (hyper-eutectoid), the action during
slow cooling from above the critical range is much
the same. First, all of the carbon is held in a solid
solution of austenite. Then, on reaching A3 or Acm,
the excess constituent, either ferrite or cementite, as
the case may be, begins to precipitate out. This brings
the carbon content of the remaining austenite nearer
and nearer to 0.90 per cent as the temperature falls,
until, at the Al point the remaining austenite has a
carbon content of 0.90 per cent, whereupon, it is converted
bodily into pearlite. The excess ferrite in
hypo-eutectoid steel and cementite in hyper-eutectoid
steel usually separates to the grain boundaries of the
Austenite
0.70% 0.25% O.50°/o 0.65%
Hypo-Eutectoid -
July, I92S
is converted into a great number of extremely small
austenite grains, which fill the space formerly occupied
bv the grain of pearlite. This "re-crystallization"
has very important effects, and will be discussed more
fullv in following pages.
The free ferrite which surrounded the former
pearlite grains will not be affected at the Al point. As
the temperature rises above Al, the austenite masses
will begin to take up the free ferrite from the surrounding
grains. The absorbtion of free ferrite by
austenite will continue with rising temperature, until
upon reaching A3 no free ferrite will remain, and the
mass will consist entirely of austenite, holding the
carbon in solution. (On reaching Al, any ferrite which
has not yet been absorbed will become non-magnetic
and take the Beta form.)
In a eutectoid steel (0.90 per cent carbon) the entire
mass will change from pearlite to austenite, with
recrystallization, upon heating through Al, 2, 3. There
0.75% 0.90% 7.70% 7.40% 7.60%\
Eutectoid - Hyper-Eutectoid—4
|«-Cementite
Austenite
Pearlite
mentite
FIG. 113—Changes in constituents of steels of various carbon content, on slow heating and cooling. (Stead)
cooling austenite, and will appear as a network surrounding
the grains of pearlite.
Reversal of Changes.
In allowing our specimens to cool slowly to room
temperature, from above the critical range, we have
put them in what might be called a normal condition.
Reheating them slowly to a temperature above the
critical range will cause a reversal of the changes
which took place on cooling. Considering first a low
carbon steel, containing say 0.30 per cent carbon, no
change will take place on heating, until a temperature
of 725 deg. C. is reached. Here the pearlite grains will
be converted into austenite. The alternate thin plates
(layers) of cementite and ferrite, composing the pearlite
will dissolve in each other, forming a mass of austenite
having the same size and shape as the original
pearlite grain.
Here, however the change is not simply a reversal
of the action on cooling. Instead of being converted
into a single grain of austenite, each grain of pearlite
will be no changes of state below or above that temperature.
If the steel is hyper-eutectoid (over 0.90 per cent),
all of the pearlite present will be converted into austenite
at Al, 2, 3, but the free cementite will be unaffected.
As the temperature rises above this point, a
gradual absorption of the free cementite will take
place, until, at Acm, it will have been completely
absorbed, and the mass will consist entirely of austenite,
holding all of the carbon in solid solution.
Change Pictured.
The structural changes (except recrystallization)
which take place in specimens of steel of various carbon
content on slow cooling from above the critical
range, is illustrated diagrammatically in Fig. 113. Each
vertical strip represents a specimen of steel having the
carbon content indicated below it. The structure it
will have at any temperature is shown opposite that
temperature, on the vertical scale.