Multiscale microstructure evolution and its influencing mechanism on yield strength and toughness of a newly high strength martensitic stainless bearing steel

The multiscale microstructure of a new type of high-strength martensitic stainless steel (HSMSS) used in aerobearings under different solid-solution temperatures is quantitatively characterized. Their relationships with strength and toughness are systematically studied based on experimental characterizations and theoretical calculations. The results indicate that the increasing solid-solution temperature leads to a decrease in the yield strength and an increase in the impact toughness. The optimal solid-solution temperature is accordingly determined to be 1100 °C. Via the multiscale characterizations by SEM, XRD, EBSD, and TEM, it shows that the sizes of high-angle grain have little variation with the solid-solution temperature, with an average size of approximately 1.8 μm. In addition, the increase in the solid-solution temperature leads to the dissolution of M23C6 carbides at 1050 °C and of M6C carbides at 1100 °C. The dislocation density remains consistent across levels after tempering in the solid-solution temperature range of 1000–1150 °C. Nanoprecipitated Mo-enriched phase and spinodal decomposition also occur after post-tempering, resulting in a significant increase in strength. The size of Mo-enriched phase increases as the solid-solution temperature. By quantitively comparing the various strengthening mechanisms for yield strength, it was found that dislocation strengthening is the most key mechanism, accounting for more than 40% of the total increase. Yet, the impact toughness of HSMSS is jointly determined by the undissolved carbides and Mo-enriched phase. This study is, for the first time, to clarify its multi-scale microstructure key factor for mechanical properties in HSMSS and thus lays a foundation for fabricating excellent temperature-resistant aerobearing steel.