Isothermal bainite transformation in low-alloy steels: Mechanism of transformation

Abstract Effect of carbon concentration of parent austenite on isothermal transformation to bainite has been studied. It has been shown that when the carbon concentration of parent austenite is low and the transformation temperature is higher than the Zener ordering temperature, the carbon content of bainitic ferrite is low. On the other hand, it is unusually high when parent austenite has higher carbon and the transformation temperature is less than the Zener ordering temperature. The results have been rationalised on the basis of Zener ordering of carbon atoms resulting in the formation of BCT ferrite with higher solubility of carbon. Similarly, the carbon content of untransformed austenite in the former case was found to closely follow the T 0 curve line whereas it was higher than T 0 in the latter case. A theoretical framework based on the equilibrium between BCT ferrite and austenite has been proposed to explain the shift of the T 0 curve to higher carbon concentration. Finally, the revised limits of carbon concentration of bainitic ferrite and austenite have been suggested for the calculation of the relative fractions of these phases when transformation temperature is less than the Zener ordering temperature.