Steels_ Metallurgy and Applications, Third Edition

Engineering steels 205

isothermal transformation diagrams were only of limited value in the evalua-
tion of continuous-cooling processes. Although attempts were made to convert
isothermal transformation diagrams to a continuous-cooling format, these were
only partially successful. Attention turned therefore to the direct determination
of continuous-cooling transformation diagrams.

Continuous-cooling transformation (CCT) diagrams

CCT diagrams are generated from a series of temperature-length curves. A spec-
imen is heated slowly into the austenitic range and, as illustrated in Figure 3.4,
the heating curve provides the facility for the determination of the Acl and Ar
temperatures. The specimen is then cooled at a prescribed rate and the start and
finish of transformation can be determined respectively from the initial deviation
from the cooling curve and the subsequent conformity to the heating curve. This
exercise is repeated for a series of cooling rates, ranging from the simulation of
water quenching in a small-diameter rod to that experienced in furnace cooling.
CCT diagrams can be presented in two ways:

1. Temperature-time plots in which the cooling time is plotted horizontally on
   a log scale.


2. Temperature-bar diameter plots, the latter representing different bar sizes
   cooled at rates simulative of air, oil and water cooling.

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Temperature

Figure 3.4 Length changes in heating and cooling