a microscope; only abutting granules
of iron are delicately traced. The metallographist calls this pure
iron "ferrite." As soon as a little carbon enters the alloy and a
soft steel is formed, small angular areas of pearlite appear at the
boundaries of the ferrite crystals (Fig. 47). With increasing carbon
in the steel the volume of iron crystals becomes less and less, and
the relative amount of pearlite increases, until arriving at 0.90
per cent carbon, the large ferrite crystals have been suppressed and
the structure is all pearlite. Higher carbon steels show films of
cementite outlining grains of pearlite (Fig. 48).

This represents the structure of annealed, slowly cooled steels.
It is possible to change the relative sizes of the ferrite and
cementite crystals by heat treatment. Large grains are associated
with brittleness. Consequently one must avoid heat treatments which
produce coarse grains.

[Illustration: FIG. 47.--Structure of low carbon steel, polished,
etched and viewed under 100 magnifications. Tiny white granules
of pure iron (ferrite) have small accumulations of dark-etching
pearlite interspersed between them. Photograph by H. S. Rawdon.]

[Illustration: FIG. 48.--Slowly cooled high-carbon steel, polished,
etched and viewed at 100 magnifications. The dark grains are pearlite,
separated by white films of iron carbide (cementite). Photograph
by H. S. Rawdon.]

In general it may be said that the previous crystalline structure
of a steel is entirely obliterated when it passes just through the
critical range. At that moment, in fact, the ferrite, cementite or
pearlite which previously existed has lost its identity by everything
going into the solid solution called austenite. If sufficient time
is given, the chemical elements comprising a good steel distribute
themselves uniformly through the mass. If the steel be then cooled,
the austenite breaks up into new crystals of ferrite, cementite
and pearlite; and in general if the temperature has not gone far
above the critical, and cooling is not excessively slow, a very
fine texture will result. This is called "refining" the grain;
or in shop parlance "closing" the grain. However, if the heating
has gone above the critical very far, the austenite crystals start
to grow; a very short time at an extreme temperature will cause
a large grain growth. Subsequent cooling gives a coarse texture,
or an arrangement of ferrite, cementite and pearlite grains which
is gre