Effect of prior martensite on bainite transformation and microstructure of high-carbon nano-bainitic steel

Bainite transformation kinetics, microstructure, and mechanical properties were comparably investigated by X-ray diffraction, scanning electron microscopy, electron backscatter diffraction, transmission electron microscopy observation, and tensile and impact tests for two treatments: with and without quenching before isothermal bainite transformation at 300 °C. Both the incubation time and growth stage of bainite transformation are accelerated by the introduction of prior martensite, especially the former. In addition, the bainite microstructure around the prior martensite is refined and shows a similar orientation with the martensite due to the strain field caused by the existence of prior martensite. The finer bainite microstructure and prior martensite improve the strength and hardness of the high-carbon nano-bainitic steel. Although the presence of prior martensite and low misorientation relationship between the martensite and the adjacent bainite microstructure are prejudicial to impact toughness, the nanoscale bainite microstructure can inhibit the crack propagation and then improve the impact toughness of the high-carbon nano-bainitic steel.