Continuous
Cooling Transformation (CCT) Diagrams
The application
of isothermal transformation methods, such as austempering and martempering, in
the steel processing industry is somewhat limited. Most of steel heat
treatments involve austenitizing the material at an appropriate temperature,
followed by continuous cooling to room temperature or the desired temperature.
Thus, the transformation of austentite does not occur isothermally, as assumed
in the TTT diagram, but over a certain period during which the temperature
drops from, say, T1 to T2. The average
temperature of the transformation (T1 + T2)/2
is therefore lower during continuous cooling than during isothermal cooling. As
a result, the transformation of austenite will be somewhat delayed. This will
cause the TTT curve to be shifted toward lower temperatures and longer
transformation times during continuous cooling as compared to isothermal
cooling. This type of transformation behaviour is best described by the use of continuous
cooling transformation (CCT) diagrams.
Simply stated, a
CCT diagram a TTT diagram shifted to lower temperatures and longer
transformation times. Below are CCT diagrams for an eutectoid steel (AISI-SAE
1080 steel). Different steels have their own CCT diagrams. Note that whereas
the TTT diagram is interpreted by reading from left to right at a constant
temperature, the CCT diagram is read along the cooling curves from the top left
to bottom right.
Fig: CCT diagram
for a SAE1080 steel (solid lines) compared with the TTT diagram (dashed lines).
Fig; CCT Diagrams for Hypoeutectoid Steels with carbon content of c = 45 %
Fig; CCT Diagrams for Hypereutectoid Steels with carbon content of c = 1 %
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