Bainite transformation under the effect of large stress in a carbide-free bainite steel

The effects of large stress on bainite transformation and microstructure were investigated by dilatometry, SEM and EBSD. The results show that the effect of large stress on bainite transformation includes two parts: mechanical driving force and prior deformation. A new finding is that bainite transformation is inhibited by large stress when the negative effect of prior deformation surpasses the positive effect of mechanical driving force. In addition, the bainite transformation kinetics model established by van Bohemen and Sietsma is modified to describe the bainite transformation under the effect of stress more accurately. Moreover, significant variant selection occurs under the effect of large stress, but the selected bainite variants in adjacent austenite twins are different. Furthermore, the hardness of the steel increases with the stress because of the increase in dislocation density and martensite amount.