Effect of deep cryogenic treatment on microstructure and mechanical properties of 18Ni (200) cryogenic maraging steel

The study investigated the effects of different cryogenic treatment time at −196 °C on the mechanical properties, microstructure, and fracture behavior of 18Ni (200) maraging steel. The results showed that deep cryogenic treatment (DCT) resulted in enhancements in both the strength and hardness of the maraging steel, as well as a notable improvement in cryogenic toughness. Specifically, the −196 °C impact energy (AKV) of the specimen subjected to an 18-h treatment was measured at 45 J, reflecting a 21% increase relative to the untreated specimen, thereby achieving optimal strength-toughness matching. Microstructural analysis revealed that DCT process facilitated the transformation of retained austenite to martensite after quenching, enhancing the stability of the microstructure and increasing dislocation density. Calculated results of lattice parameters and lattice strain indicated that DCT process, especially for 18 h, significantly affected the tetragonality of the martensite, leading to lattice contraction and distortion, which increased internal stress and deformation potential. After aging treatment, the release of internal energy allowed for rapid diffusion of elements, enhancing the nucleation driving force of Ni3(Ti, Mo) and film-like reversed austenite, thereby increasing the density of precipitates and the volume fraction of austenite, reducing texture strength and improving the uniformity of the material.