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March, 1925 Forging- Stamping - Heat Treating
H E A T T R E A T M E N T and M E T A L L O G R A P H Y of STEEL
A P r a c t i c a l C o u r s e in t h e E l e m e n t s o f
CHAPTER III.
METALLOGRAPHY — Continued
PART 4
MICRO-CONSTITUENTS OF STEEL
IN the preceding part of this chapter the crystalline
I nature of metals and alloys and the microstructure
of iron-carbon alloys in their "normal" state (that
is, slowly cooled from a temperature of about 1,650
deg. F.), was briefly considered.
It was shown that pure iron consists of minute
crystalline1 grains which look very much like the
grains of any other pure metal, and it was explained
that these grains of iron may hold in solid solution
certain other elements such as nickel, without a noticeable
change in their appearance. Iron, in its normal
state, with or without one or more elements in solid
solution is called "ferrite" (from the latin name of iron,
ferrum). See photomicrograph, Fig. 32.
Cementite and Pearlite.
It was explained that when small quantities of carbon
are added to iron, a chemical compound, Fe3C,
iron carbide, is formed. This carbide goes into solid
solution in the iron grains above certain temperatures*,
but upon slow cooling to room temperature, it
separates out as a hard brittle constituent known as
"cementite". Cementite is an extremely hard substance,
being about as hard as glass, and nearly as
brittle. When the cementite separates from solid solution
in the manner just described, it forms small thin
curved plates, and combines mechanically with a cer-
*Note: There is some question whether the carbide goes
into solution as Fe3C, or whether it dissociates into atoms of Fe
and C on dissolving. For present purposes we may consider that
the Fe3C goes into solution as such.
The author is Chief Metallurgist, Naval Aircraft Factorv,
United States Navy Yard, Philadelphia, Pa.
Copyright, 1925, by H. C. Knerr.
P h y s i c a l M e t a l l u r g y
tain amount of ferrite, in alternate plates, in such a
way as to resemble mother-of-pearl. This duplex constituent
is called "pearlite" and is illustrated in Fig.
43.
It should be borne in mind that pearlite is not a
chemical compound, but a mechanical mixture of
peculiar characteristics. One of these peculiarities is
that its chemical composition is practically always the
same, that is, in a plain carbon steel the proportion
of carbide to ferrite in normal pearlite is such that
the pearlite contains approximately 0.90 per cent of
carbon, by weight.
If the carbon content of the steel is less than about
.90 per cent all of the carbon will be present as carbide
in grains of pearlite and these pearlite grains will
be surrounded by grains or a network of free ferrite.
See Figs. 39 to 41 inclusive. But if the carbon content
is greater than 0.90 per cent the excess carbide
will be rejected as a network around the pearlite
grains, as shown in Fig. 44. Pearlite is softer and
more ductile than cementite, but not so soft and ductile
as ferrite. This brief review is given for the
sake of clearness.
Austenite.
It was mentioned in Part 3 of this chapter that
the arrangement of the atoms (atomic pattern) in the
crystalline grains of iron undergoes a change when the
metal is heated above a certain temperature, and that
this brings about important changes in the properties
and microstructure of the metal. This is called a
"transformation", or an "allotropic change". To distinguish
between these two forms or conditions of
iron, the one below the transformation point is called
"alpha" iron (from the Greek letter a) and the one
above the transformation "gamma" iron (from the
third letter of the Greek alphabet).
Gamma iron will take carbide (or carbon) into
solid solution, whereas alpha iron will not. (It is
for this reason that the carbide separates out as