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July, 1925
Solubility.
Gamma iron (austenite), is able to hold moderate
quantities of carbon in solid solution. The presence
of dissolved carbon lowers the temperature at which
Gamma begins to change to Alpha iron on cooling.
This causes a downward slope of the line GS, representing
A3 (and A2, 3), in the diagram, from 900 deg.
C. for pure iron, to 725 deg. C. for iron containing 0.90
per cent carbon*.
Alpha iron (ferrite), is able to hold in solid solution
considerable quantities of certain elements, such as
nickel, silicon and phosphorus, but very little carbon.
The maximum amount of carbon it can take up, is
probably about 0.05 per cent to 0.10 per cent. This
fact has an important bearing on the hardening of steel,
as will be shown.
Hypo-Eutectoid Steel.
Let us now consider a hypo-eutectoid steel, containing,
say, 0.30 per cent carbon, which is heated to a
temperature above A3. It will consist entirely of austenite—that
is, grains of Gara_ma iron holding the carbon
in solid solution. If the piece is allowed to cool
slowly, to the Ar3 point, some of the austenite (Gamma
iron) will change to ferrite, which will separate as
a distinct constituent which we may call "free ferrite."
Since this ferrite is unable to hold appreciable amounts
of carbon in solid solution, the carbon which was in
it will go into solution in the remaining austenite.
This will increase the carbon content of the austenite
and will therefore lower the temperature at which
ferrite separates out. As cooling continues, more ferrite
will be liberated, and the remaining austenite will
become richer in carbon. The diminishing austenite
must hold all of the carbon which is present in the
steel. When a temperature of 725 deg. C. has been
reached, the specimen will consist of two-thirds ferrite,
containing practically no carbon, and one-third
austenite, containing 0.90 per cent carbon. At this
temperature, which is the point Al, austenite will
take no more carbon into solution—it is saturated.
Further slow cooling will now cause the austenite
to break down into ferrite and cementite (Fe3C).
These two constituents will separate in alternate thin
plates or lamellae, producing what we know as pearlite.
Each grain of austenite which remained at 825
deg. C, is converted bodily into one or more grains of
pearlite. These pearlite grains will be imbedded in
or>surrounded by, free ferrite.
No change takes place in the free ferrite on cooling
thru the Al point. This point is caused by the conversion
of the austenite into pearlite, and its intensity
therefore increases from zero, in carbonless iron, up
to a maximum in steel containing 0.90 per cent carbon.
The intensity of the point depends upon the
amount of austenite present at the Al temperature.
Forging- Stamping - Heat Treating
*There is some question whether the line GOS should run
straight from the point G to the point S, on the iron-carbon diagram,
or whether it should be curved or bend downward to the
Point O. Critical point diagrams, used in connection with heattreating
practice, almost always show the latter construction.
This seems to be justified by actual experience. Archer, in ref.
12, makes the line straight, because he considers that there is insufficient
evidence upon which to base the construction of a
curved or broken line. In Fig. 110 the usual practice in the construction
of critical point diagrams has been followed, and the
line has been shown broken at O. The curves of Carpenter and
Keeling, Sauveur, Fig. 175, ref. 8, are substantially in agreement
with this construction.
245
The ferrite which separa-ted from the solid solution
above the A2 point (768 deg. C), was in the nonmagnetic
or Beta state. Upon cooling through Ar2,
this ferrite changed to magnetic or Alpha ferrite (without
any change in its crystalline structure). Further
separations of ferrite with falling temperature, were
in the Alpha state.
Suppose that we now have a specimen of hypoeutectoid
steel containing 0.60 per cent carbon, heated
to the austenitic state, that is, above A2, 3. This
specimen will cool to about 750 deg. C. before separation
of ferrite begins. Separation of carbonless ferrite
in the Alpha state, will continue down to the Al point
(725 deg. C). The steel will then consist of one-third
ferrite and two-thirds austenite, the latter containing
all of the carbon and having a carbon content of 0.90
per cent. Upon cooling through Arl, the grains of
austenite will be converted into grains of pearlite.
It may be that in separating from austenite at temperatures
below 768 deg. C, the ferrite first takes
the Beta form, and then immediately changes to the
Alpha form. On the other hand, Gamma iron may
change directly into Alpha iron when the A3 point is
below 768 deg. C. In either case, there will be a
change from a non-magnetic to a magnetic state on
cooling, and we may regard the A2 point as following
the line OSK in the diagram.
A
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*+
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FIG. Ill (left)—Alpha iron, body centered cubic arrangement.
FIG. 112 (right)—Gamma iron, face centered cubic arrangement.
(Archer.)
Eutectoid Steel.
Let us now consider a specimen containing 0.90
per cent carbon (the eutectoid composition). When
heated above the critical temperature, this piece will
consist of austenite, holding 0.90 per cent carbon in
solid solution. No change will take place on cooling,
until the Al point (725 deg. C), is reached. Here
the entire mass will be converted into pearlite. At
the same-time, the steel will change from the non-magnetic
to the magnetic state.
Hyper-Eutectoid.
Suppose now that we have a hyper-eutectoid steel
containing say, 1.20 per cent carbon, heated above the
Acm point (for example to 900 or 950 deg. C). All
of the carbon will be held in solid solution in the
austenite. But the solubility of carbon in austenite
decreases as the temperature is lowered, and upon cooling
to the Acm point (the line Se), excess carbon in
the form of cementite will begin to precipitate from
the solid solution. More cementite will-be precipitated
as cooling continues. This will lower the carbon content
of the austenite, until, when a temperature of 725
deg. C. is reached, the austenite will have only 0.90
per cent carbon in solution. The remaining carbon
will be present in the form of free cementite, Fe3C.
The Al change will now take place, that is, the grains
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