Microstructural evolution during tempering and intrinsic strengthening mechanisms in a low carbon martensitic stainless bearing steel

Abstract

Herein, the microstructural evolution and corresponding change in tensile properties in a low carbon martensitic stainless bearing steel during tempering were studied. The results showed that there was no δ-ferrite in the studied steel after austenitizing at 1000∼1100 °C, and the volume fraction of austenite increased with increasing the tempering temperature from 470 °C to 590 °C due to the formation of reverted austenite. According to the EDS measurements, the reverted austenite was rich in C, Cr and Mo, other than previously assumed Co and Ni. On top of that, a considerable number of nanosized Mo-rich carbides with globular and rod-like morphologies were found after tempering at 530 °C, and further coarsened after tempering at 590 °C. Along with the change of phase constituents, the dislocation density decreased, and martensite block width increased as the tempering temperature increased. All the evolution of microstructures jointly contributed to an initial increase and then a decrease in the strength, and various strengthening mechanisms contributed to the change of yield strength were quantitatively analyzed and discussed in the study.