Steels_ Metallurgy and Applications, Third Edition

204 Steels: Metallurgy and Applications

9)

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o. F:

I"- 4)

Surface

Centre

Temper

Transformation

Log time'

Figure 3.2 Martempering treatment

q•p,/.Surface Centre

ion

i, M~ ,|

Log time

Figure 3.3 Austempering treatment

In austempering, Figure 3.3, a component is quenched into a salt bath, again at
a temperature above Ms, but the material is held at this temperature long enough
to allow complete isothermal transformation to bainite. The component is then
quenched or air cooled to room temperature. Again no tempering is involved
in the conventional sense but austempering represents a short, economical heat
treatment cycle that provides a good combination of strength and toughness.
The cooling rate must be fast enough to avoid the formation of pearlite and
the isothermal treatment must be long enough to complete the transformation to
bainite.
Although they proved to be extremely useful in gaining a better understanding
of austenite decomposition and transformation kinetics, it became apparent that