Effects of Undercooling and Transformation Time on Microstructure and Strength of Fe-C-Mn-Si Superbainitic Steel

A metallographic method, dilatometry, and X-ray diffraction were applied to investigate the effects of undercooling and holding time on bainitic transformation, microstructure, and strength of Fe-C-Mn-Si superbainitic steel. The results indicate that the strength of the samples decreases and the elongation increases with the isothermal transformation time, resulting in an increase in the product of the tensile strength and the total elongation. Therefore, the prolongation of the transformation time can improve the comprehensive property of the sample. In addition, the morphology of bainite changes from granular bainite to lath-like one with a decline in the isothermal transformation temperature, leading to an increase in strength. The elongation first increases and then decreases with a decrease in the isothermal transformation temperature. Finally, the sample shows the best comprehensive property when it is austempered at an intermediate temperature (350 degrees C). The data presented here are instrumental in optimizing the technology of austempering treatment in industrial production.