Isothermal Transformation Diagram.

This diagram is formed by quenching very thin specimens
of steel in salt baths set at various temperatures. For example, thin specimens of
0.79% C steel can be quenched into seven different liquid salt baths set at 650, 600, 550,
500, 450, 400, and 200 C. The specimens are held for various times at each temperature
then pulled from the bath and quickly quenched in cold water. The result will be a diagram
called an isothermal transformation (IT) diagram, as shown in Fig. 7. The diagram is essentially
a map showing where various constituents form. For example, at 650 C, austenite (A)
begins to transform to pearlite if held in the bath for 10 s. The curve drawn through this
point is the pearlite transformation start temperature and is labeled beginning of transformation
in Fig. 7. At about 100 s the pearlite transformation is finished. The second curve
represents the pearlite transformation finish temperature and is labeled the end of transfor
mation in Fig. 7. In this steel, pearlite forms at all temperatures along the start of the
transformation curve from 727 C (the equilibrium temperature of the iron–carbon diagram)
to 540 C, the ‘‘nose’’ of the curve. At the higher transformation temperatures, the pearlite
interlamellar spacing (the spacing between cementite plates) is very coarse and decreases in
spacing as the temperature is decreased, i.e., nose of the IT diagram is approached. This is
an important concept since a steel with a coarse pearlite interlamellar spacing is softer and
of lower strength than a steel with a fine pearlite interlamellar spacing. Commercially, rail
steels are produced with a pearlitic microstructure, and it has been found that the finer the
interlamellar spacing, the harder the rail and the better the wear resistance. This means that
rails will last longer in track if produced with the finest spacing allowable. Most rail producers
employ an accelerated cooling process called head hardening to obtain the necessary
conditions to achieve the finest pearlite spacing in the rail head (the point of wheel contact).
If the specimens are quenched to 450 C and held for various times, pearlite does not
form. In fact, pearlite does not isothermally transform at transformation temperatures (in this
case, salt pot temperatures) below the nose of the diagram in Fig. 7. The new constituent is
called bainite, which consists of ferrite laths with small cementite particles (also called
precipitates). An example of the microstructure of bainite is shown in Fig. 8. This form of
bainite is called upper bainite because it is formed in the upper portion below the nose of
the IT diagram (between about 540 and 400 C). Lower bainite, a finer ferrite–carbide microstructure,
forms at lower temperatures (between 400 and about 250 C). Bainite is an
important constituent in tough, high-strength, low-alloy steel.
If specimens are quenched into a salt bath at 200 C, a new constituent called martensite
will form. The start of the martensitic transformation is shown in Fig. 7 as Ms (at 220 C).
Martensite is a form of ferrite that is supersaturated with carbon. In other words, because of
the very fast cooling rate, the carbon atoms do not have time to diffuse from their interstitial
positions in the bcc lattice to form cementite particles. An example of martensite is shown
in Fig. 9. Steel products produced with an as-quenched martensitic microstructure are very
hard and brittle, e.g., a razor blade. Most martensitic products are tempered by heating to
temperatures between about 350 and 650 C. The tempering process allows some of the
carbon to diffuse and form as a carbide phase from the supersaturated iron lattice. This
softens the steel and provides some ductility. The degree of softening is determined by the
tempering temperature and the time at the tempering temperature. The higher the temperature
and the longer the time, the softer the steel. Most steels with martensite are used in the
quenched and tempered condition.

Mechanical Engineers’
Handbook: Materials and Mechanical Design,
Volume 1, Third Edition.
Edited by Myer Kutz


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