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November, 1925
herited, so that the piece is likely to be brittle and
have a lowered resistance to shock. This is true no
matter what the carbon content of the steel, although
the effect is probably most pronounced in high carbon
steels. Overheating must therefore be avoided.
Underheating, that is, not heating completely
through the critical range, is equally unsatisfactory,
for in this case, the hardening constituent, carbide,
or carbon, is not uniformly distributed and therefore
cannot exert maximum hardening effect. If a hypoeutectoid
steel in heated above Aj but not tip to As, and
Fbrging-Stamping - Heat Treating
FIG. 135a—Quenched at 1500 deg. F. Brinell 390.
FIG. 135b.—Same, tempered at 800 deg. F. Yield stress, 68,-
000; ultimate stress, 113,000; elongation, 19 per cent; reduction
of area, 60 per cent; brinell, 230.
quenched, the areas which were originally pearlitic,
together with that portion of the surrounding ferrite
which was taken into solid solution to form austenite,
will be hardened, but the remaining ferrite will be
unaffected. This is illustrated in accompanying series of
photomicrographs, showing a piece of .30 per cent carbon
steel which was first annealed, Fig. 132, and specimens
of which were then heated to various temperatures
from the A1 point to the A3 point and quenched.
389
Fig. 133a, representing the specimen quenched just
above A1( is particularly interesting. The light gray
areas are martensitic. These areas replace the original
pearlite grains, which were changed into austenite
containing .90 per cent carbon, on heating through
Av Bordering these martensitic areas are dark
patches of troostite. These represent the austenite
whose carbon content was lower than .90 per cent,
which was formed by the absorbtion of some of the
ferrite by the areas which were originally pearlitic (or
by diffusion of carbon from those areas into the adjacent
ferrite). The remaining, white, areas are ferrite,
which was unchanged by the treatment. The specimen
shown in Fig. 134a, was heated nearly to the A3 point.
The dark areas are troostite, or a martensite of lower
carbon content than .90 per cent, since the carbon from
the pearlite areas had greater opportunity to diffuse.
Unaffected ferrite areas are smaller than in Fig. 133a.
Fig. 135a shows the structure produced by heating
through the critical range, and in which no free ferrite
is visible. Figs. 133b, 134b, and 135b, show the structure
produced by tempering the same specimens at 800
deg. F.
Eutectoid.
A eutectoid steel evidently must be heated slightly
above the Aj 2 3 point, and held long enough to permit
the completion of the change from pearlite into austenite,
and uniform diffusion of the carbon. To avoid
grain growth, the temperature must not be higher nor
held longer than necessary. The speed of cooling
will depend upon the results desired. If great hardness
is called for, as in metal cutting tools, water
quenching, to produce a martensitic structure, will
be required. For many purposes the more moderate
hardness and greater toughness of a troostitic or sorbitic
structure will suffice, and the work may be cooled
by quenching in a milder medium, such as oil.
Hyper-Eutectoid.
For hyper-eutectoid steels, the problem is a little
less simple. The treatment depends to some extent
upon the original condition of the steel. If the steel
has been fully annealed, its structure will consist of
grains of pearlite, surrounded by a network of free
cementite. (See Fig. 123.) On heating through the
A: „ 3 point, the pearlite grains will be transformed
into fine grained austenite, just as in a eutectoid steel,
but the free cementite will remain practically unaffected.
Quenching in this condition would transform
the austenitic areas into areas of martensite,
troostite or sorbite, but these areas would still be
surrounded by the original cementite net-work. Such
steel would be hard, but the network of cementite
would render it brittle. It would, in fact, be likely
to crack in quenching.
On the other hand, heating the piece to a temperature
above the Acm point, so as to absorb all of the
cementite, would cause coarsening of the austenite
grains, and this steel, if quenched, would therefore
also be brittle and inferior. A double treatment avoids
both of these difficulties. The steel is first heated
above Acm, held long enough to absorb and diffuse
the excess cementite and then cooled at a moderate
speed, as by air cooling, if the piece is small, or
quenching in hot water or oil, if large, so as to avoid
danger of cracking. This moderately rapid cooling
prevents the cementite from gathering into large particles,
or separating at the grain boundaries, and re-