Mechanistic Study on the Effect of QLT Process on Microstructure and Properties of 9Ni Steel

In this article, the effects of quenching + tempering (QT) and quenching + intercritical annealing followed by quenching + intercritical annealing followed by quenching + tempering (QLT) heat treatments on the microstructure and mechanical properties of 9Ni steel are systematically investigated. By integrating microstructure analysis with mechanical property testing, the mechanism by which heat treatment influences the strength and toughness of 9Ni steel is elucidated. The results indicate that after QLT heat treatment, the impact absorbed energy for V‐notch (Akv) increases from 24.65 to 49.47 J, representing a 100% enhancement, while the tensile strength decreases by 6.8%. Microstructure analysis reveals that the significant improvement in toughness following QLT heat treatment is primarily attributed to the substantial refinement of the lath martensitic structure induced by intercritical annealing followed by quenching. Furthermore, electron back scattering diffraction analysis of the specimens demonstrates that high‐angle grain boundaries effectively reduce the crack growth zone, markedly enhancing crack growth resistance, and thereby significantly increasing the impact energy of the test steel.