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58 Fbrging-Stamping - Heat Treating
than about 0.05 per cent of carbon in iron is not evident
in its microstructure, for this amount is apparently
held in solid solution by ferrite at room temperature.
It should be mentioned here that in the present discussion
we are considering specimens which have been
allowed to cool slowly from above the critical temperature,
so as to eliminate such effects on the structure
as would be caused by quenching the metal or by cold
working.
FIG. 39.—Steel containing about 0.20 per cent carbon, annealed.
(500x.) FIG. 40.—Steel containing about 0.40 per
cent carbon, annealed. (500x.) (By Downes, in author's
laboratory). FIG. 41.—Steel containing about 60 per cent
carbon, annealed. (500x.)
A slowly cooled steel specimen which contains
about 0.20 per cent carbon will show grains of ferrite,
interspersed with small dark areas, as in Fig. 39. If
the carbon content is about .40 per cent it will be
noted that the dark areas represent about y2 of the
total area (Fig. 40) and if 0.60 per cent they will cover
about 2/3 of the entire area, the ferrite grains now
having the appearance of a net-work between the darker
areas I Fig. 41). If the carbon content is .85 to .90
per cent the entire area will be composed of the dark
constituent. See Fig. 42. Close examination of the
February, 1925
dark constituent will show that it is made up of many
thin layers or laminations of two different materials,
so that it resembles mother-of-pearl. This constituent
is called "pearlite". An exceptionally large and clear
example is shown in Fig. 43. The smooth light areas
are free ferrite.
Pearlite is composed of alternate layers of ferrite
and cementite. Above certain temperatures, all the
carbide or cementite in steel goes into solid solution in
FIG. 42 Steel containing about .90 per cent carbon, annealed.
(500x.) FIG. 43.—Exceptionally large formation of pearlnc.
'H. (500x.) >,juux.,» Smooth omootn white wmte islands are _ free ferrite grains. ,
FIG 44.—Steel containing about 1.20 per cent carbon, annealed.
(500x.)
the ferrite, as will be described later. At lower temperatures
its solubility decreases, and it finally separates
out completely, in thin curved plates, combining
mechanically during its separation with alternate
layers or plates of ferrite, and in such a proportion
that the amount of carbon in the pearlite is about 0.85
per cent to .90 per cent. This fact affords a convenient
means of estimating the carbon content in a
piece of plain carbon steel. The specimen is first heated
to a fairly high temperature, above the critical
range, say to 1650 deg. F. and slowly cooled, as by