Bainite Growth Retardation Due to Mechanical Stabilisation of Austenite

Abstract

This study shows the first ever attempt to calculate bainite kinetics based on characteristics of nucleation as well as displacive growth. A model for bainite kinetics is proposed whereby in conjunction to nucleation the displacive growth of laths is described by a growth factor which is assumed to vary linearly with the net driving force and thereby ensures consistency with the thermodynamic growth criterion. Furthermore, the growth factor is supposed to have an inverse dependence on the elastic strain energy to account for retardation of the growth due to mechanical stabilisation. When a bainitic lath is formed the surrounding austenite is work hardened due to the plastic accommodation of the shape change associated with the transformation. The dislocation strengthening is assumed to mechanically stabilize the austenite against subsequent transformation to bainite and/or martensite. Geometrical constraints are also assumed to contribute to the mechanical stabilisation of untransformed austenite in partially bainitic microstructures. The overall degree of mechanical stabilisation has been quantified for low-Si steels by measuring the decrease in start temperature of the second phase martensite after partial bainitic transformations. Based on these experiments an empirical equation has been developed to describe the decrease in the growth factor with progressing transformation. Subsequently, the model has been validated using numerous fraction curves of 10 different low-Si steels. Fits have been optimized by adjusting only the autocatalysis factor for each holding temperature. Owing to the growth factor the model accurately describes the strong retardation in kinetics in the second stage of transformation.