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June, 1925
The leading manufacturers of pyrometric equipment
are always ready to give advice and assistance
in the installation and care of their apparatus — a
courtesy which, because of the specialized nature of
their product, is well worth accepting.
CHAPTER V — THERMAL ANALYSIS*
The critical points of steel may be studied, and the
temperatures at which they occur determined, by observing
any of the physical changes which take place
at these points. Methods of observation which have
been used with success include accurate measurement
of changes of length and volume, change of electrical
resistance, changes of thermo-electric effect with reference
to another metal, changes of magnetic properties,
etc.
By far the most useful method has been by means
of heating and cooling curves. This method, called
"thermal analysis", is based on the fact that energy,
in the form of heat, is either liberated or absorbed as
a result of the atomic or molecular changes which take
place at the critical points. The test piece or specimen,
usually in the form of a small cylinder, is placed in a
furnace whose temperature may be raised or lowered
at a uniform rate. The temperature of the specimen
will rise or fall uniformly with that of the furnace,
except when passing through a critical point. The
absorbtion or liberation of energy which occurs at
such a pqint due to the change in physical state, tends
to produce a rise or fall in the temperature of the piece,
independent of that caused by the heating or cooling
of the furnace. This results in a change in the rate
of heating or cooling of the specimen. The change
in rate, and the temperature at which it occurs, is observed
by means of one or more thermocouples, by one
of the methods described below.
Time — Temperature Curves.
The simplest method, but not the most accurate.
consists in placing a thermocouple in contact with the
specimen, during heating and cooling, and taking readings
of temperature at regular intervals of time, such
as every minute. Or, readings may be taken at regular
temperature intervals, such as every 10 deg. C,
and the time for successive readings after the first
reading recorded. The intervals chosen will depend
upon the circumstances of the test, but should be
close enough to insure that no change is overlooked.
The test is started at a temperature well below the
lower critical point, the furnace is heated at a slow
and uniform rate to somewhat above the upper critical
point, and is then allowed to cool at about the same
rate at which it was heated, until the lower critical
point has been passed. The readings are plotted on
cross section paper, with temperature on a vertical
scale and time horizontally. The curve will resemble
those in Figs. 67 and 68 of Chapter III. The thermocouple
should be of small wire, and should have its
hot junction inserted in a hole drilled in the specimen,
so as to obtain the maximum sensitivity. The arrangement'is
illustrated in Fig. 104.
Inverse Rate Curve.
Some of the critical points in the curve described
above may not produce a very pronounced hump, or
may show only a gentle change in the slope of the
•References (8), (9) and (10).
Forging-Stamping-Heat Treating
199
line. This is especially the case with low carbon
steels. The points may be brought out much more
clearly by recording the actual intervals of time, in
minutes or seconds, between each even temperature
interval, and plotting these values on a horizontal
scale, against temperature. This is called an "inverse
rate" curve. Such a curve might also be constructed
by plotting the successive changes in temperature in
degrees, for each interval of time. Inverse rate curves
should always be plotted when studying heating and
cooling curves by the foregoing method.
Difference Curves.
Any change in the rate of heating or of cooling of
the furnace in the time-temperature method (such as
would result from a change in the voltage of the line
supplying the furnace, the presence of drafts of air
FIG. 103—Temperature-e.m.f. curve.
around or through the furnace, etc.), would cause an
irregularity in the rate of heating or cooling of the
specimen, which might be mistaken for a critical point.
Errors from this cause may, for the most part, be
avoided by the "difference method". A piece having
about the same size, shape and heat capacity' as the
specimen under test, but consisting of a metal such
as nickel, which has no critical point within the range
of the test, is placed in the furnace close to the test
piece. It is called the "neutral body" During heating
and cooling, the temperature of the test piece and
of the neutral body would rise and fall simultaneously,
and would therefore be alike or nearly so, were it not
for the critical points. When the test piece passes
through a critical point, its rate of heating or cooling
will differ from that of the neutral body, and this will
cause a difference in temperature between them. The
critical points may be studied by observing this difference.
A difference in temperature between the neutral
body and the test piece is readily detected by means
of a thermocouple having its "hot" junction in contact
with one and its "cold" junction in contact with the
other. Which is which does not matter. This is
called a "differential thermocouple", and its e.m.f. will
be proportional to the difference in temperature between
the two pieces. An ordinary thermocouple is
used to measure the actual temperature of the test
piece. Irregularities in the rate of heating or cooling
of the furnace will affect both pieces alike, and will
therefore not affect the difference in temperature be-