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September, 1925
EB there is some residual liquid, which is of eutectic
composition. This liquid then solidifies at constant
temperature, forming the eutectic mixture of austenite
and cementite.
In alloys containing more than 4.3 per cent carbon,
cementite freezes out on cooling from BD to BC. At
BC the residual liquid is of eutectic composition and
solidifies at constant'temperature.
The Solidus.
The line AEBC is called the solidus. Below this
line all alloys are completely solid."
Phase Changes During Slow Cooling.
The iron carbon diagram will be clarified by* following
the changes of phase which take place in some
typical examples of steel and cast iron during slow
cooling from the molten state. See the reproduction
of Iron-Carbon diagram, Fig. 121. Consider
first pure iron. Above the point A, about 1530
deg. C, it will be completely molten. Upon cooling
to this temperature it will solidify completely, chang-
/60cx
,3S .30 A3 1-7 4-3
Percent Carbon, by k/e/ght
FIG. 121—Phase changes of various iron-carbon alloys.
(Archer.)
ing into crystals of Gamma iron. This will change
to Alpha iron at the point G, (900 deg. C), as we have
seen in the critical point diagram. Pure iron may
therefore exist in any of three phases, liquid, solid
Gamma or solid Alpha. The magnetic change of
Alpha iron, which occurs at the A2 point, is not regarded
as a change of phase, and is therefore not included
on the diagram.
Consider next a hypo-eutectoid steel containing,
say 0.35 per cent carbon, such as is used to a large
extent in forgings, and the like. The addition of carbon
to iron (up to 4.3 per cent carbon) lowers the
freezing or melting point. This steel will not begin
to solidify until it has cooled to the point 1, Fig. 121.
Here crystals of austenite will begin to form. These,
at first, will contain less than 0.35 per cent carbon.
Their carbon content may be found by drawing a horizontal
line from the point 1 to the solidus line AE,
Forging- Sf amping - Heaf Treating
327
and dropping a vertical line from this point 2 to the
base line. The vertical line through 2 represents the
alloy of iron and carbon which would be completely
solid at the temperature 1-2, and according to the diagram,
contains about 0.15 to 0.20 per cent carbon. This,
therefore, is the carbon content of the first austenite
crystals formed at 1. Since these crystals have less
than the average carbon content (0.35 per cent) of the
alloy under consideration, the liquid portion of the alloy
at 1, must contain more than 0.35 per cent carbon.
The freezing point of the remaining liquid is therefore
lowered. Evidently, as cooling progresses, the solidifying
austenite and the remaining liquid both become
richer in carbon. Final solidification will take place
on reaching the point 3, on the solidus line. Just before
reaching this point, the little liquid remaining, and
the austenite crystals which are formed from it, will
have a carbon content equal to that of the alloy 2a,
whose solidification begins at the temperature opposite
3-2a. This carbon content is about 0.90 per cent.
It is evident, therefore, that, during the freezing of
the alloy, there is a tendency to produce an uneven
distribution of carbon, the beginnings or centers of the
crystalline grains having the lowest carbon content
and the portions near the grain boundaries, the highest.
This tendency is more or less counterbalanced by
the fact that the carbon in solid solution in austenite
tends to distribute itself uniformly, by the process of
diffusion. Diffusion of carbon begins in the austenite
grains as soon as they are formed, and continues until
the mass has cooled down to the transformation point,
Al, indicated by the line PSK. Heating in the soaking
pit, and hot working, also favor the even distribution
of carbon. A^fter cooling through the point 3, no further
change of phase takes place until the alloy has
cooled to the point 4, on the line GS. This is the
critical point, A3, and, as we have already seen, free
ferrite or Alpha iron begins to separate from the austenite
grains. Precipitation of ferrite continues, with
falling temperature, down to the point 5. This ferrite
is nearly free from carbon, therefore the remaining
austenite must become richer in carbon as cooling progresses
from 4 to 5. At the point 5. which is the Al
critical point, the austenite grains will have attained
a carbon content of 0.90 per cent, and will then be
converted into pearlite. The latter as we know, consists
of alternate layers of ferrite and cementite. Below
the line PSK, therefore, it will be found that two,
and only two, phases exist, namely ferrite, and cementite.
Part of the ferrite will be in the form of individual
grains (called free ferrite or proeutectoid ferrite)
and the remainder will be mixed with the cementite
(called eutectoid- or pearlitic-ferrite). The alloy
under consideration will now have a structure con­
sisting of 0.35
0.90
X 100 = 39 per cent pearlite and 61
per cent free ferrite. The pearlite itself is made up of
100 X —•— = 13.5 per cent cementite and 86.5 per
6.67
cent ferrite.
Let us next follow the phase changes, during cooling,
of a eutectoid (0.90 per cent carbon) steel, such
as is largely used for springs and tools. Solidification
begins at the point 6 and is complete at the point 7.