Effect of Ausforming Temperatures on Bainitic Transformation During Isothermal Quenching of 42CrMo Steel

The influence of ausforming temperature on the isothermal bainitic transformation behavior of 42CrMo steel was systematically investigated using thermo-mechanical simulation, dilatometric analysis, and electron backscatter diffraction (EBSD). The results show that ausforming significantly accelerates the bainitic transformation kinetics, whereas lower ausforming temperatures lead to a progressive reduction in the final bainite fraction. This apparently contradictory behavior originates from the competitive interaction between deformation-induced mechanical stabilization of austenite and dislocation-assisted heterogeneous nucleation of bainitic ferrite. Lower ausforming temperatures result in higher retained dislocation densities, which promote early-stage nucleation while simultaneously increasing resistance to transformation interface migration and hindering carbon redistribution. As a consequence, the bainitic ferrite microstructure is markedly refined, exhibiting reduced lath thickness and length. Crystallographic analysis reveals that the bainitic ferrite predominantly follows the Kurdjumov–Sachs orientation relationship with prior austenite, and that strong variant selection is induced by ausforming, particularly at lower deformation temperatures. The reduced variant multiplicity within individual prior austenite grains further contributes to the refinement and preferential orientation of the bainitic microstructure. These findings highlight the critical role of ausforming temperature in governing the coupled evolution of transformation kinetics, phase fraction, and crystallographic characteristics during bainitic transformation and provide guidance for microstructural control of bainitic steels through temperature-dependent thermo-mechanical processing.