Tailoring carbide evolution, strengthening, and plasticity in 45Cr9Si3 martensitic heat-resistant steel through optimized heat treatment strategies

The microstructural evolution and strengthening mechanisms of 45Cr9Si3 martensitic heat-resistant steel were systematically investigated using a combination of transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), and atom probe tomography (APT). An L9 (3⁴) orthogonal heat treatment design was employed to optimize the mechanical properties, revealing that quenching and aging temperatures are the most significant factors affecting both room and high-temperature performance. The optimized heat treatment process (1000 °C/24 min/oil cooling +610 °C/1.5 h/air cooling +650 °C/2.0 h/air cooling) yielded a tensile strength of 1143 MPa, yield strength of 943 MPa, elongation of 18.5 %, and reduction of area of 45.3 % at room temperature. At 500 °C, the steel exhibited a tensile strength of 766.7 MPa and a yield strength of 661.7 MPa. The dissolution of coarse M7C3 carbides during quenching and the subsequent precipitation of fine M23C6 carbides during tempering were found to play critical roles in enhancing mechanical properties. The synergistic effect of precipitation hardening and dislocation strengthening accounted for approximately 65 % of the yield strength. Notably, APT analyses revealed nanoscale carbide precipitation along martensitic lath boundaries, effectively inhibiting dislocation motion and contributing to the observed strengthening. These findings provide new insights into the microstructural control of martensitic heat-resistant steels and establish a framework for optimizing heat treatment strategies to enhance their mechanical performance.