On modifying shear mechanism of bainitic formation for Fe-C alloys

This paper modified Ko's shear mechanism so as to explain one such phenomenon quite satisfactorily. The authors deal with one thermodynamic difficulty in bainitic formation encountered if shear mechanism is used. The temperature range of bainitic formation is higher than Ms temperature; consequently its driving force of transformation is relatively small and thus can not overcome the shear resistance. The difficulty of insufficient driving force can be overcome by modifying Ko's mechanism. Experiments done by the authors indicate that there is a pre-bainitic transformation during incubation period. In this prestage, the carbon-poor zone and the carbon-rich zone form because of diffusion of carbon atoms in parent austenite. In the carbon-poor zone, as is well known, the driving force is no longer insufficient and is larger than the critical driving force (absolute value) which, at the Bs temperature, is 400-1200 J/mol. Therefore, the shear formation of bainitic ferrite can be realized within the whole temperature range of bainitic formation.