Effect of solid-solution temperature on the microstructure and mechanical properties in low-cobalt secondary hardening ultra-high strength steel

The effects of solid-solution temperature on the microstructure and mechanical properties of low-cobalt M₂C and NiAl co-precipitated secondary hardening steel were investigated. With increasing solid-solution temperature, the microstructure undergoes complex evolution, characterized by an increase in retained austenite, dissolution of primary M₇C₃ carbides, and growth of prior austenite grains. Correspondingly, the yield strength and impact toughness in both quenched and aged conditions initially improve. This is attributed to the dissolution of the primary M₇C₃ carbides, which enhances the precipitation strengthening of M₂C carbides and inhibits crack nucleation, thereby improving both strength and toughness. However, when the solid-solution temperature exceeds 900 °C, the coarsening of prior austenite grains leads to a reduction in Hall-Petch strengthening and decreased crack propagation resistance, resulting in a decline in yield strength and toughness in the aged state. The optimal solid-solution temperature is found to be 900 °C, where an excellent balance between strength and toughness is achieved.