The Effect of Quenching and Tempering Temperatures on the Microstructure and Properties of a New Low-Alloy Ultra-High-Strength Martensitic Steel

This study systematically investigates the influence of quenching (850-910 °C) and tempering (160-280 °C) temperatures on the microstructural evolution and mechanical properties of a novel low-alloy ultra-high-strength martensitic steel (UHSMS). Comprehensive microstructural characterization combined with mechanical testing demonstrates that quenching at 880 °C results in the finest martensitic laths and the highest dislocation density, leading to an excellent strength-toughness balance. Subsequent tempering treatments reveal that the specimen tempered at 200 °C achieves an optimal combination of properties, with a yield strength of 1517 MPa, ultimate tensile strength of 2017 MPa, elongation of 10.4%, and impact toughness of 80.3 J/cm². This optimum is mechanistically linked to a cooperative effect where the fine tempered martensitic structure and stable film-like retained austenite (RA) enhance toughness and ductility, while the nano-scale precipitates (forming during the ε→θ carbide transition) simultaneously provide substantial precipitation strengthening, thereby minimizing the strength sacrifice typically associated with improved toughness. Furthermore, the 200 °C tempered specimen exhibits the largest shear lip on the tensile fracture surface and the maximum dimple size on the impact fracture surface, indicative of a high plastic strain capacity and excellent crack propagation resistance.