Aging temperature-dependent microstructure and mechanical behavior of martensitic special steel under −196°C extreme cryogenic conditions

Maraging stainless steel exhibited remarkable strength and toughness in cryogenic environments owing to its transformation-induced strengthening mechanism and adjustable reversed-austenite fraction. In this work, the microstructural evolution and mechanical response of maraging stainless steel were systematically examined under three processing conditions: solution-treated, aged at 500 °C, and aged at 550 °C. Microstructural characterization using SEM, TEM, and EBSD revealed that the aging temperature critically governed the morphology and stability of austenite. Aging at 500 °C produced 11.7 vol% of highly stable film-like austenite, while aging at 550 °C yielded 38.3 vol% of low-stability blocky austenite. Tensile testing at room and cryogenic temperatures demonstrated that deformation at cryogenic conditions markedly promoted the austenite-to-martensite transformation, particularly under simultaneous low-temperature and tensile loading. The transformation sensitivity of film-like austenite was higher than that of blocky austenite. Consequently, the mechanical stability of austenite was identified as the governing factor controlling the strength–ductility synergy. The 500 °C aged specimen achieved balanced mechanical properties through moderate transformation strengthening and sustained work-hardening capability, whereas excessive instability of blocky austenite at 550 °C led to premature transformation and degraded performance. These findings provided theoretical and experimental guidance for optimizing aging temperature and related heat-treatment conditions to achieve a desirable combination of strength and ductility in maraging stainless steels under extreme environments.