Tailoring Strength-Toughness Synergy in 18Ni (200) Maraging Steel by Adjusting Aging Temperature and Time

To simultaneously enhance the strength and toughness of 18Ni (200) steel, the aging parameters (temperature and time) are systematically investigated to optimize the microstructure and mechanical properties. The results indicate that the steel aged at 550 °C exhibits optimum yield strength and ductility compared with that at 530 and 580 °C for the same duration. At 550 °C, strength decreases but ductility and Charpy impact energy increase with increasing time and the steel aged at 550 °C for 4 h shows an excellent combination of strength and toughness (exhibiting 1531.5 MPa yield strength and 80.2 J impact energy). With prolonged aging time and elevated aging temperature, the specimens exhibit decreased dislocation density alongside progressive coarsening of precipitates and reverted austenite. The aging process employed in this study simultaneously preserves elevated dislocation density, high precipitate density, and substantial reverted austenite content, while concurrently suppressing coarsening of precipitates and reverted austenite. Crucially, such microstructural optimization facilitates the synergistic effects of precipitation, dislocation, and grain boundary strengthening, combined with a high-volume fraction of austenite, achieving an exceptional strength-toughness balance in maraging steel.