Modelling transformation kinetics in spatially graded martensitic–bainitic microstructures of quenched and tempered steel AISI 4140

Inductive short-time austempering allows the creation of homogeneously distributed and spatially graded martensitic–bainitic microstructures by using a precise temperature control during sample cooling. This study first applies Van Bohemen’s bainitic transformation kinetics created for various different steel alloys, to homogenously distributed martensitic–bainitic microstructures in AISI 4140 steel on the laboratory scale. Furthermore, it is transferred to both homogenously distributed and spatially graded martensitic–bainitic microstructures at the component scale. However, the kinetics of bainite transformation exhibit a pronounced dependence on the parent austenite grain size. Consequently, its accurate representation was essential to accurately simulate the surface hardening process. Consistent with established models and their theoretical foundations, the model can be applied to both homogeneous and spatially graded microstructures produced by inductive short-time austempering. This demonstrated its suitability for use at the component scale and provides a solid foundation for the design and optimisation of industrial heat treatment processes.