Refined Bainite Microstructure and Mechanical Properties of a High-Strength Low-Carbon Bainitic Steel Treated by Austempering Below and Above M-S

Four isothermal heat treatment schedules are designed to study the effects of transformation temperature below and above martensite starting temperature (M-S) on bainitic transformation kinetics, microstructure, and mechanical properties of a low-carbon bainitic steel. The results indicate that the product of tensile strength and elongation (PSE) of tested steel do not show any improvement when the samples are austempered at a temperature below M-S. Finer bainite microstructure can be observed when the samples are austempered at a temperature below M-S, but the highest PSE is obtained in the sample austempered at a temperature above M-S (400 degrees C). Lower PSE of the samples transformed below M-S is mainly attributed to less bainite and retained austenite (RA) amount and more amount of athermal martensite, as well as the existence of carbide. In addition, with the decrease of the isothermal transformation temperature, the PSE first increases and then decreases. Different from austempering above M-S, when the sample is austempered below M-S, PSE decreases with the decrease of transformation temperature. Moreover, whether the treatment below M-S can improve mechanical property of bainitic steels depends on the composition of steels and transformation temperature.