New insights into the effects of deformation below-MS on isothermal kinetics of bainitic transformation

Abstract Both ausforming and austempering below the martensite start temperature (MS) have been shown to accelerate the bainitic transformation. This work investigates the combined effects of deformation and austempering below the MS on the isothermal transformation kinetics, microstructure, and hardness in a medium-carbon bainitic steel. The results show that, deformation below the MS can delay the onset of the baintic transformation compared to the case in which no deformation is applied. The larger the amount of martensite presents prior to the deformation step, the greater the delay in the onset of bainitic transformation. This behaviour is attributed to the development of strong misorientation gradients within austenite during the co-deformation of the austenite-martensite microstructure. These large misorientation gradients make the pre-existing martensite a less effective nucleation site for bainite.