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334 forging- Sf amping - Heaf Treating
Some time-temperature heating and cooling curves
were also obtained for this steel. The bars used for
these determinations were pieces of spring bar 8 in. x
2 in. x .28 in. A 3/16 in. hole was drilled in each specimen
in the direction of its width a little over one-half
way through and symmetrical with respect to the
other two dimensions. A small chromel-alumel thermo-couple
was inserted and asbestos was packed
around it at the opening of the hole. The leads were
connected to a Leeds & Northrup potentiometer.
The furnace used for these determinations was an
electric resistance furnace. The door was slightly
raised and the opening closed by asbestos blocks leaving
space for the passage of the couple wires. Another
couple was kept in the furnace connected to the
same potentiometer by leads and a couple throw
switch.
In running this test the furnace was first heated
to a certain temperature, the specimen with couple
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at 1000 deg. F. for the three cases:—
Heating
Curve
Fig. 1
Fig. 2
Fig. 3
September, 1925
Rate of Heating
at 1000 deg. F.
118 deg. per min.
192 deg. per min.
229 deg. per min.
The steel used in both the hardness tests and the
heating and cooling curve determinations is eutectoid
steel. A polished and etched section under the microscope
will reveal a very fine lammelar structure
that requires high magnification for its resolution.
When a lammelar structure is visible it is called pearlite,
Fig. 4. It consists of alternating plates of ferrite
and cementite, which are the light and dark bands
of the micrograph.
When, as in the case of the iron-cementite series,
the system is a two component one and two phases
are in equilibrium with each other the composition of
each phase will be a definite function of the temperature.
When the phases in equilibrium are ferrite
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FIGS. 1, 2 and 3—Heating curves, obtained when furnace was raised to 1580, 1680 and 1700 degs. F., respectively.
attached was placed therein and raised about one inch
from the floor by asbestos blocks at each end. Readings
were taken every 15 seconds. Data for the cooling
curves was obtained while the test pieces were in position
in still air and protected against contact with any
other material by asbestos blocks at the ends.
In order to determine the character of the heating
curves for different rates of heating of the specimens
the furnace was raised to different temperatures,
before the introduction of the specimens. Heating
curves Figs. 1, 2, and 3, are those obtained when the
furnace was raised to 1580 deg., 1680 deg., and 1700
deg. F. respectively. The part of the curve where
the lag occurs is the point of change of alpha to gamma
iron. It will be noticed that the time required
for the transformation, at the abnormal part of the
curve, decreases as the temperature of the furnace
increases.
The higher the temperature of the furnace the
more rapid is the heating of the specimens at any
temperature. The following are the rates of heating
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and austenite the carbon content of the latter will decrease
as the temperature rises, and when cementite
and austenite, the carbon content ofthe latter will increase
with rising temperature. When three phases
are present and in equilibrium the system is non-variant.
When the three .phases in equilibrium are
ferrite, cementite and austenite, the carbon percentage
of the latter is .875 and the temperature 1330 deg. F.
which is the lowest temperature at which austenite
can exist as a stable substance. These are the figures
as given on the iron cementite equilibrium diagram
for the eutectoid.
In the ordinary treatment of steel equilibrium
hardly ever exists between the phases. This is due
in the first place to the rigidity of the particles, thus
requiring a considerable attractive impulse to be exerted
on them to bring about change of atomic arrangement,
and in the second place the change usually
does take place with considerable velocity, being in
most cases of a spontaneous nature. Carbon steel when
slowly cooled from above the critical range is composed
almost entirely of pearlite if the carbon per-