transforms into pearlite at the high temperature and the
quickly cooled to and held at a somewhat lower
remainder transforms into martensite at the low
temperature, say 1,200F, decomposition begins in
temperature (Ms-Mf temperature range). The resulting
about 3 seconds and is completed after about 30
material will then transform to a nonuniform structure
seconds, the resultant pearlite being finer and harder
that is usually not desirable.
than that formed at 1,300F. At a temperature of about
1,050F, the austenite decomposes extremely rapidly,
Continuous Cooling
with less than 1 second elapsing before the trans-
formation starts and 5 seconds before it is completed.
which are superimposed five theoretical cooling curves.
The resultant pearlite is extremely fine and its hardness
Curves A to E represent successively slower rates of
is relatively high. This region of the S-curve where
cooling that would be obtained, for instance, by cooling
decomposition of austenite to fine pearlite proceeds so
in (A) iced brine, (B) water, (C) oil, (D) air, and(E) the
rapidly is called the "nose" of the curve.
furnace.
BAINITE: If the austenite is cooled unchanged to
Steel cooled according to curve E begins to trans-
temperatures below the nose of the S-curve (1,050F),
form at temperature t1 and completes transformation at
the time required for its decomposition begins to
t2; the final product is coarse pearlite with relatively low
increase. The final product of decomposition now is not
hardness. When the steel is cooled according to
pearlite, but a new structure called bainite, possessing
curve D, transformation begins at t3 and is complete at
unusual toughness with hardness even greater than that
t4; the final product is fine pearlite and its hardness is
of very fine pearlite. Depending on the temperature, a
greater than that of the steel cooled according to curve
certain interval of time is necessary before austenite
E. When the steel is cooled according to curve C,
starts to transform into either pearlite or bainite.
transformation begins at t5 and is only partially
Additional time is necessary before the transformations
complete when temperature t6 is reached. The product
arc completed.
of this partial transformation is very fine pearlite. The
MARTENSITE: If the austenite is cooled
remainder of the austenite does not decompose until the
unchanged to relatively low temperatures (below about
Ms temperature is reached. Then it begins to transform
430F for the eutectoid carbon steel), partial
to martensite, and completes this transformation at the
transformation takes place instantaneously; the product
Mf temperature. The final structure is then a mixture of
of transformation is martensite. Austenite transforms
fine perlite and martensite (typical of an improperly
into martensite over a temperature range with the
quenched steel) with a higher hardness than was
obtained with the steel cooled according to curve D.
amount that transforms being a function of the
The rate of cooling represented by curve B is just
temperature. Only minute amounts will transform at
sufficient to intersect the nose of the S-curve. Con-
about 430F; practically all of the austenite will be
sequently, only a minute amount of austenite
transformed at about 175F. The beginning of this
transformation range is termed the M, (martensite start)
decomposes into fine pearlite at temperature t7; the
temperature and the end of the range is termed the Mf
remainder of the austenite is unchanged until the
martensite transformation range is reached. If the steel
(martensite finish) temperature, as shown in figure
is cooled at a slightly faster rate, so no transformation
15-15. As long as the temperature is held constant
takes place at the nose of the S-curve, the steel is
within the MS - Mf range, the portion of the austenite that
completely hardened. This particular rate is termed the
does not transform instantaneously to martensite
critical cooling rate and is defined as the slowest rate at
remains untransformed for a considerable length of
which the steel can be cooled and yet be completely
time, eventually transforming to bainite.
hardened. Since this rate cannot be directly determined,
In ordinary heat treatment of the plain-carbon
the rate indicated by curve B, producing only a trace of
steels, austenite does not transform into bainite.
fine pearlite, is frequently used as the critical cooling
Transformation of the austenite takes place either above
rate. The hardness of the resultant martensite is
or at the nose of the S-curve, forming pearlite, or in
equivalent to the maximum that can be obtained.
passing through the Ms - Mf range, forming martensite or
Samples cooled at a faster rate, such as that indicated
both. For austenite to be transformed entirely into
by curve A, are also completely martensitic but arc no
martensite, it must be cooled rapidly enough so the
harder than the sample cooled according to the critical
temperature of the center of the steel, which cools more
cooling rate.
slowly, is lowered past the nose of the S-curve in less
The rate at which a steel cools through the
time than is necessary for transformation to start at this
temperature in the vicinity of the nose of the S-curve is
temperature. If this is not done, part of the steel
15-18