two well-defined
discontinuities at temperatures more than 1,000 deg.F., below its
freezing-point. It seems that the soft, magnetic metal so familiar
as wrought iron, and called "alpha iron" or "ferrite" by the
metallurgist, becomes unstable at about 1,400 deg.F. and changes into
the so-called "beta" modification, becoming suddenly harder, and
losing its magnetism. This state in turn persists no higher than
1,706 deg.C., when a softer, non-magnetic "gamma" iron is the stable
modification up to the actual melting-point of the metal. These
various changes occur in electrolytic iron, and therefore cannot be
attributed to any chemical reaction or solution; they are entirely
due to the existence of "allotropic modifications" of the iron in
its solid state.

[Illustration: FIG. 45.--Inverse Rate Cooling Curve of 0.38 C Steel.]

Steels, or iron containing a certain amount of carbon, develop
somewhat different cooling curves from those produced by pure iron.
Figure 45 shows, for instance, some data observed on a cooling
piece of 0.38 per cent carbon steel, and the curve constructed
therefrom. It will be noted that the time was noted when the needle
on the pyrometer passed each dial marking. If the metal were not
changing in its physical condition, the time between each reading
would be nearly constant; in fact for a time it required about 50
sec. to cool each unit. When the dial read about 32.5 (corresponding
in this instrument to a temperature of 775 deg.C. or 1,427 deg.F.) the
cooling rate shortened materially, 55 sec. then 65, then 100, then
100; showing that some change inside the metal was furnishing some
of the steadily radiating heat. This temperature is the so-called
"upper critical" for this steel. Further down, the "lower critical"
is shown by a large heat evolution at 695 deg.C. or 1,283 deg.F.

Just the reverse effects take place upon heating, except that the
temperatures shown are somewhat higher--there seems to be a lag
in the reactions taking place in the steel. This is an important
point to remember, because if it was desired to anneal a piece of
0.38 carbon steel, it is necessary to heat it up to and beyond
1,476 deg. F. (1,427 deg.F. _plus_ this lag, which may be as much as 50 deg.).

It may be said immediately that above the upper critical the carbon
exists in the iron as a "solid solution," called "austenite" by
metallographers. That is to say, it is uniformly distributed as atoms
throughout the iron;