Kinetics of the bainite transformation in steels

Abstract

Kinetics models for the bainitic transformation in steels are reviewed with the objective of summarising the progress and assessing the limitations of current methods. Little is known about the nucleation of bainite except that the activation energy for nucleation is directly proportional to the driving force for transformation. This is expected from the theory for martensite nucleation, although carbon must partition into the austenite during bainite nucleation. The lengthening rate of bainite sheaves and “sub-units” is, for all reported data, found to be much faster than that calculated assuming growth is controlled by the diffusion of carbon in the austenite ahead of the interface. It is argued that the growth of bainite is diffusionless, any excess carbon being rejected from the ferrite subsequent to transformation. The review also includes an assessment of overall transformation kinetics, the influence of trace elements and the effect of stress on the rate of the bainitic reaction.