Thermodynamic Approach to Describe the Martensite Phase Transformation Kinetics via the Stabilization of Austenite

This study investigated the spread of the martensite transformation, i.e. , the extent of the transformation as a function of the temperature, via the development of a model focusing on the stabilization of residual austenite along the transformation rather than describing the nucleation processes of each individual unit of martensite. The stabilization of the retained austenite is described in a thermodynamic framework accounting for the chemical composition, grain sizes, stress state, dislocation density, and temperature effects. In situ X-ray diffraction experiment was performed at synchrotron PETRA III (Hamburg, Germany) on a low-alloyed steel to support the theoretical development. Among others, the martensite transformation kinetic as well as the lattice parameter and dislocation density of austenite were measured during the transformation. In order to describe the refinement of austenite grains during the transformation the initial prior austenite grain size distribution was determined by thermal etching. A good agreement of up to 60 to 70 pct of the martensite transformation upon cooling is found between the results of the model and the experiment. The results show that the major contribution to the spread of the transformation is the austenite refinement during the transformation itself. For higher martensite fractions the discrepancy observed is attributed to the simple description of austenite refinement used. In addition, the spread of the martensite transformation is shown to be linked with the change of the chemical driving force related to the steel composition, with the C showing the highest effect. New coefficients, based on numerical calculations, for the rate parameter ( α ) of the martensite transformation were proposed. Graphical Abstract