Possible mechanisms of the formation of bainitic colonies

The possible mechanisms of bainitic transformation in steels are discussed. According to the known models of the growth of Widmanstatten ferrite, an acicular shape of bainitic lathes is due to anisotropy in the surface energy. However, the lath replication mechanisms in upper and lower bainite presumably differ from each other. Upper bainite results from the diffusion-controlled transformation, at which the pearlitic autocatalysis due to the formation of cementite at the interface with ferrite takes place. Lower bainite is formed at a smaller temperature via the diffusionless mechanism, when the branching of precipitates or the autocatalysis of lathes can be provided by a decrease in the system energy due to the disposition of structural defects at the interfaces of precipitates, so the existence of a characteristic lath size is energetically stipulated (Weissmüller effect). The combined effect of different autocatalysis mechanisms leads to a variety of possible bainite modifications.