the atoms of carbon are not present in any fixed
combination, in fact any amount of carbon from zero to 1.7 per cent
can enter into solid solution above the upper critical. However,
upon cooling this steel, the carbon again enters into combination
with a definite proportion of iron (the carbide "cementite," Fe3C),
and accumulates into small crystals which can be seen under a good
microscope. Formation of all the cementite has been completed by
the time the temperature has fallen to the lower critical, and
below that temperature the steel exists as a complex substance
of pure iron and the iron carbide.

It is important to note that the critical points or critical range
of a plain steel varies with its carbon content. The following
table gives some average figures:

Carbon Content. Upper Critical. Lower Critical.
0.00 1,706 deg.F. 1,330 deg.F.
0.20 1,600 deg.F. 1,330 deg.F.
0.40 1,480 deg.F. 1,330 deg.F.
0.60 1,400 deg.F. 1,330 deg.F.
0.80 1,350 deg.F. 1,330 deg.F.
0.90 1,330 deg.F. 1,330 deg.F.
1.00 1,470 deg.F. 1,330 deg.F.
1.20 1,650 deg.F. 1,330 deg.F.
1.40 1,830 deg.F. 1,330 deg.F.
1.60 2,000 deg.F. 1,330 deg.F.

It is immediately noted that the critical range narrows with increasing
carbon content until all the heat seems to be liberated at one
temperature in a steel of 0.90 per cent carbon. Beyond that composition
the critical range widens rapidly. Note also that the lower critical
is constant in plain carbon steels containing no alloying elements.

[Illustration: FIG. 46.--Microphotograph of steel used in S. K.
F. bearings, polished and etched with nitric acid and magnified
1,000 times. Made by H. O. Walp.]

This steel of 0.90 carbon content is an important one. It is called
"eutectoid" steel. Under the microscope a properly polished and
etched sample shows the structure to consist of thin sheets of
two different substances (Fig. 46). One of these is pure iron,
and the other is pure cementite. This structure of thin sheets
has received the name "pearlite," because of its pearly appearance
under sunlight. Pearlite is a constituent found in all annealed
carbon steels. Pure iron, having no carbon, naturally would show no
pearlite when examined under