Evaluation of heat treatment effects on the microstructure and mechanical properties of PBF-LB maraging steel through ex-situ and in-situ characterizations

The 18 wt% Ni 300-grade maraging (Mar18Ni) steel is a high-strength alloy with an excellent combination of strength and fracture toughness. However, despite the widespread use of this alloy in additive manufacturing, the literature still lacks a systematic understanding of how distinct post-processing routes, particularly those involving aging, austenite reversion, and their combinations, affect microstructure, austenite stability, and mechanically assisted transformation during deformation. This gap is especially relevant because maraging steels produced by PBF-LB can exhibit metastable austenite, heterogeneous defect structures, and complex precipitation behavior that differ from those of their conventionally processed counterparts. To address this issue, this study evaluates how targeted heat treatments tune the balance between strength, ductility, and TRIP activity by coupling ex-situ characterization with in-situ XRD-tensile experimental results. The outcomes indirectly depict that aging at 480 °C promotes nanoscale intermetallic precipitation, enhancing strength, whereas reversion-range treatments (610–670 °C) increase ductility through recovery, partial recrystallization, and the formation of reverted austenite. Mixed reversion-plus-aging routes yield intermediate behavior, offering a balanced strength-ductility combination. The Johnson-Mehl-Avrami-Kolmogorov (JMAK) analysis highlights relative differences in transformation kinetics among conditions, ranging from rapid, continuous transformation in the as-built state to slower, more stable behavior in the reversion-treated alloy. Spatially resolved XRD mapping confirms that TRIP activity is broadly distributed in the as-built condition but becomes localized as austenite stability increases in the heat-treated material. Altogether, the findings demonstrate that controlled metastability, rather than maximized stability, remains essential for optimizing the TRIP effect and achieving a superior mechanical balance in PBF-LB maraging steels.