Study on the mechanism of excellent strength and toughness combination of a high nitrogen martensitic stainless steel treated by Q-C-P-T process

In this study, a high nitrogen martensitic stainless bearing steel with ultra-high strength and excellent toughness combination treated by quenching-cryogenic cooling-partitioning-tempering (Q-C-P-T) process was obtained. The yield strength, ultimate tensile strength, uniform elongation and the impact energy of the sample after partitioning at 200 °C for 1 h followed by tempering at 400 °C for 1 h are up to 2034 MPa, 2217 MPa, 5.83 % and 58.73 J respectively. The microstructure characterization shows that the interstitial atoms of C and N in martensite diffuse into adjacent austenite during the partitioning process, resulting in the high stability of austenite. Strain-induced martensitic transformation was suppressed in maximum, effectively avoiding the formation of hard martensite enriched in C/N and dimensional instability caused by volume expansion of martensitic transformation, which is crucial for bearing steels. And nano-scale precipitates in the martensite matrix effectively compensate for the strength loss of the martensite caused by lower solid solution of carbon and nitrogen, thus obtaining excellent comprehensive mechanical properties. This work paves a new way for the development of advanced bearing steels.