Bainitic Transformation and Mechanical Properties of Low-Carbon High-Strength Bainitic Steels with Mo Addition

Abstract

Two low-carbon high-strength bainitic steels with/without molybdenum (Mo) addition were designed to investigate the effect of Mo on bainitic transformation, microstructure and properties. The results show that during austempering, Mo addition increases the amount of isothermal bainite transformation and slightly accelerates the bainite transformation kinetics. It is mainly attributed to the fact that there is the ferrite transformation in Mo-free steel but not in Mo-added steel even cooled at a very high cooling rate before austempering. In Mo-free steel, the formation of ferrite leads to the higher carbon content in untransformed austenite with higher stability and thus retards the isothermal bainite transformation kinetics. The ferrite transformation controlled by carbon diffusion is strongly inhibited in the Mo-added steel because Mo addition decreases obviously the diffusion coefficient of carbon and grain boundaries energies. In addition, the promoted effect of Mo addition on isothermal bainite transformation is stronger at lower austempering temperature. Mo addition leads to the more and finer bainite plates and increases the strength of low-carbon bainitic steel. Moreover, the volume fraction of retained austenite first decreases and then increases in Mo-free steel with the increase in austempering temperature, whereas it presents the inverse trend in Mo-added steel. Similarly, during continuous cooling process, Mo addition effectively retards the formation of high-temperature products and thus improves the tensile properties of low-carbon steels.