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386 Forging - Stamping - Heat Treating November, 1925
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
KNERR
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 VII—PART III
HARDENING AND TEMPERING
T l I E heat treating operation known as "hardening"
may be described a^ heating slightly above the
critical range, holding at this temperature until
recystallization, solution and diffusion have taken
place and then cooling rapidly enough to retain a
martensitic, troostitic or sorbitic structure.
The distinction between hardening and full annealing
or normalizing, lies chiefly in the rate of cooling
from above the critical range. If this is veryslow,
as in full annealing, the result is a pearlitic
structure of moderately large grain size, and nearly
maximum softness. If cooling is more rapid, as in
normalizing, a finer pearlitic structure, or a sorbitic
structure will result, except in certain alloy steels,
where troostite. martensite or even austenite may be
retained. Here, there evidently will be a hardening
effect, and it follows that no sharp line may be drawn
between hardening and annealing where the definition
of the latter includes normalizing. In the low anneals,
in which the temperature is not carried above the A,
point, no hardening can. of course, take place. Compare
Figs. 122 A. B. C, 125 and 126.
The microstructure, and consequentlv the degree
of hardening brought about in a piece of steel, will
depend upon its composition and the rate at which it
is cooled through the critical range. As the speed of
cooling increases, sorbite, troostite, martensite or even
austenite will be produced, with corresponding physical
properties. There may of course be a mixture
of two or more of these constituents. The cooling
speeds required to produce the high constituents will
be less as the amount of carbon increases. Most alloying
elements have a similar influence. The rate of
cooling will depend upon the dimensions of the piece
The author is Consulting Metallurgist. Philadelphia, Pa
Copyright, 1925, H. C. Knerr.
P h y s i c a l M e t a l l u r g y
and the medium or method by which it is cooled. This
will be discussed further on.
In hardening operations, it is usually desired to
cool as rapidly as possible without subjecting the piece
to excessive cooling stresses. Cooling is ordinarily
done by "quenching" the piece in water, oil, or other
liquid medium.
Rapid cooling through the critical range causes
maximum refinement of the ferrite grains (which may
be submicroscopic in size) and maximum dispersion
of the carbide particles, as explained in Chapter VI.
Slower cooling permits the formation of larger ferrite
grains and carbide particles. Maximum hardness is
obtained by the formation of martensite, and less hardness
but more ductility, by the formation of troostite
or sorbite. The speed of cooling might be controlled
so as to retain the desired one of these structures at
room temperature, with its corresponding combination
of hardness and ductility. Such control however,
is seldom practicable, and even if it were, might not
produce entirely satisfactory results. The rapid cooling
of steel unavoidably sets up internal stresses in
the mass, because of the unequal contraction of the
outer and inner layers, or of unequal sections. These
stresses have a weakening effect and may cause failure
of the piece. They may be removed by reheating
the piece to moderate temperatures.
It is, therefore the usual practice to cool the piece
rapidly, producing a greater hardness than is actually
required, and then to re-heat to some temperature
below the critical range. This is called tempering.
Tempering.*
1 he effects of tempering are to reduce or remove
internal stresses, reduce hardness and increase
*This operation has also been called "drawing" or "drawing
back", but because of the many meanings of the word drawing,
its use in this sense is hardly desirable, and is being abandoned.
^Tempering" is much more definite. The use of the word
'temper" in place of "harden," as is sometimes done, is misleading
and incorrect.