Improvement on Strength‐Elongation Product of CSS‐42L Steel Achieved by Isothermal Quenching & Tempering

Enhancing both strength and ductility in high‐performance bearing steels remains a key challenge for next‐generation aerospace applications. In this study, the microstructural evolution and mechanical response of CSS‐42L aerospace bearing steel subjected to isothermal quenching and tempering (IQT) were systematically investigated, with the conventional quenching & tempering (CQT) as the contrast. The isothermal quenching at 260°C produced a refined bainite/martensite dual‐phase microstructure characterized by a smaller effective grain size (~1.26 μm), homogeneously dispersed nanoscale Mo‐ and Cr‐rich carbides. These synergistic features led to a superior strength‐elongation product of 23.6 GPa%, which was superior to that achieved by CQT. Microstructural analysis revealed that the low‐temperature bainitic transformation provided enhanced thermal stability of substructures, while tempering promoted uniform carbide precipitation within the matrix and along grain boundaries without excessive coarsening. The improvement in tensile properties arose from the combined effects of grain boundaries, carbide precipitation, and dislocation strengthening, together with the suppression of intergranular embrittlement through carbide refinement. The findings demonstrate that the low‐temperature IQT route offers a promising pathway to achieve a balanced strength–ductility trade‐off in CSS‐42L steel, providing new insights for designing heat‐treatment strategies in high‐performance aerospace bearings.