Role of Prior Austenite Grain Size on the Thermodynamics and Kinetics of Its Transformation

The role of prior austenite grain (PAG) size on martensitic transformation has been qualitatively reported in earlier studies. However, a quantitative relationship between PAG size, thermodynamic driving force, and transformation kinetics remains insufficiently understood. Dilatation data were investigated in the current study to determine the temperature and amount of martensite formed as a function of temperature, along with transformation kinetics of austenite. It is established that the grain size refinement led to a decrement in martensite start ( M s ) temperature, and high initial transformation rates followed by a progressive reduction. The current results establish an empirical relationship to estimate the change of free energy for martensitic transformation as a function of PAG size. An expression for dependence of volume fraction of martensite ( f α ′ ) on the prior austenite grain size was also developed.