Revisiting rapid tempering of martensitic steel: the key role of tempering time over heating rate in carbide refinement and precipitation strengthening

Rapid tempering process can effectively improve the mechanical properties of martensitic steels such as strength, fracture toughness and impact toughness, which is mainly attributed to the promoted density and refined grain size of carbides. However, the mechanism behind is still controversial, particularly regarding the relative contributions of rapid heating rate and short tempering time to the refinement process. In the present study, the effects of heating rate (10-300 °C/s), tempering time (1-240 s), and tempering temperature (300-600 °C) on the microstructure and mechanical properties of martensitic steel were systematically investigated. The results indicated that the carbide precipitation of the tempered martensite was almost independent of the heating rate. It is the tempering time that greatly influences the density and size of carbides, which means tempering with a very short tempering time at a low heating rate can also significantly improve the strength of tempered martensitic steels compared to the rapid tempering approach. It was also observed that the block size, dislocation density, and carbon content of the tempered martensite also exhibited minimal dependence on the heating rate. Therefore, we proposed an ultrashort-time tempering approach as an alternative to the rapid tempering for improving the mechanical property of martensitic steels, which has high feasibility for industrial implementation.