Effect of rapid tempering at high temperature on microstructure, mechanical properties and stability of retained austenite of medium carbon ultrafine bainitic steel

Rapid tempering at high temperature is beneficial to release residual stress. However, the evolution of microstructure and mechanical properties of ultrafine bainitic steel after rapid tempering at high temperature is still unclear. In this research, the effect of rapid tempering at high temperature on microstructure, mechanical properties and mechanical stability of retained austenite (RA) of a medium-carbon ultrafine bainitic steel were studied in detail. Results show that after a rapid tempering at 550 degrees C, the volume fraction of RA decreased gradually, and the carbon atoms in bainitic ferrite (BF) gradually diffused out of the lattice and finally formed carbides, and the BF plate gradually coarsened. When the tempering time is only 2 min, the carbon clusters formed, strengthening the matrix and stabilizing the RA, which result in the highest yield strength, and the highest impact toughness of the steel. With the extension of tempering time, nano-carbides precipitated and gradually grew up, and the dislocation density in steel decreased, resulting in a gradual decrease in the yield strength of the steel and the mechanical stability of RA. However, the deformation ability and strain hardening ability of the BF matrix gradually increase, so that the tensile strength of the steel gradually increases. The decomposition of RA and the formation of brittle martensite making the impact toughness of the steel gradually decrease. This study can provide theoretical support for the development of rapid tempering process of ultrafine bainitic steel.