Part VI : Heat treatment
Module 2 : Formation and evolution of microstructures
 
2.3.1 CCT and TTT diagrams and the rule of Scheil

Even though TTT diagrams are very useful for studying microstructures, in most of the cases of practical interest, the transformations take place not isothermally but during continuous cooling. Hence, the so-called Continuous Cooling Transformation diagrams (CCT diagrams are very important).

It is possible to go from TTT diagrams to CCT diagrams under certain circumstances, namely, if we assume that the kinetics of the transformation depends only one the fraction of the phases transformed and the temperature. In Figure 7 we show how to calculate the CCT diagram from the isothermal transformation curve, schematically; the steps involved are as follows:

Figure 7: CCT and TTT diagrams.
  1. Divide the temperature range into steps of size $\Delta t$;
  2. Calculate the time spent in each temperature range; this calculation is carried out by dividing the time step $\Delta t$ by the time of the isothermal transformation $T$ which corresponds to the average temperature \begin{displaymath} f = \sum_{T_e}^{T} \frac{\Delta t}{t} \end{displaymath} corresponding to the given $\Delta t$;
  3. The cumulative phase fraction, then is given by
    $f \approx 1$ (1)

  4. The transformation begins when $\frac{70-62}{97-62} = 0.228$.

The Eq. 1 is known as the additive rule of Scheil.