Bimodal size-distribution of bainite plates

There are two well-known phenomena associated with the bainite reaction, both of which have been exploited here to enhance the mechanical behaviour of steel. Firstly, the bainite plate size decreases as the transformation temperature is reduced. Secondly, it is bad to have large regions of untransformed austenite in the microstructure; this is because they can transform, under the influence of external stress, into corresponding large regions of untempered, brittle martensite. By adopting a two-stage heat treatment in which coarse bainite is produced by isothermal transformation at a high temperature, followed by isothermal transformation at a lower temperature, it has been possible to eliminate blocks of austenite. This induces a microstructure containing an organized dispersion of fine plates of bainitic ferrite in the regions between the coarse plates. The mechanical properties of this mixture are shown to be better than those of bainite obtained by transformation at any single temperature. The experiments have been conducted in the context of very strong steels, where the strength and hardness can exceed 2.5 GPa and 650 HV, respectively.