Heat treatments use temperature and time to develop the microstructure of the steel. The microstructure changes due to phase transformations. These are usually controlled by diffusion, which takes time. The data for the rate of transformation, which is used to design a heat treatment for a particular steel, is described by the TTT (Temperature-Time-Transformation) diagram.
The characteristic C shape of the isothermal TTT curve arises due to the combined effect of
- increased driving force for the phase transformation from austenite to pearlite with decreasing temperature, and
- decreased rates of nucleation and growth of pearlite from austenite due to decreased diffusion rates.
If the steel is cooled sufficiently quickly to avoid the nose of the C-curve, then the austenite transforms to martensite by a displacive, diffusionless phase transformation. Quenching a partially transformed steel will also transform the remaining austenite to martensite.
In order to study the effects of temperature and time, the transformation of austenite is studied isothermally at temperatures below the critical temperature, providing different values of undercooling to provide the driving force for the transformation.