In-situ observation of precipitation and austenite reversion in additively manufactured 18 wt. % Ni maraging steel

Maraging steel with 18 wt.% Ni (Mar18Ni) has attracted growing interest for additive manufacturing (AM), particularly through powder bed fusion with a laser beam (PBF-LB), owing to its high strength and heat-treatability. However, the microstructural characteristics introduced by PBF-LB can significantly influence phase transformation pathways during post-processing. While conventionally processed maraging steels have been extensively studied, the precipitation behavior and austenite reversion kinetics in AM-produced Mar18Ni steel remain insufficiently understood. This study investigates the phase transformations in PBF-LB Mar18Ni steel using in-situ transmission electron microscopy (TEM). Samples were subjected to isothermal aging at 480 °C and austenite reversion heat treatments at 650 and 670 °C. The evolution of intermetallic precipitates and the nucleation and morphological development of austenite were systematically characterized. The first precipitation sequence involved Fe 2 (Mo,Ti) and Ni 3 Mo after ∼1 h at 480 °C, followed by Ni 3 Ti formation after a crystallographic rearrangement between martensite and austenite. Unlike the conventionally processed alloy, Ni 3 Mo forms prior to Ni 3 Ti in this PBF-LB steel. During aging, precipitation initially competes with austenite reversion as Fe, Ni, and Ti are consumed from the matrix, reducing austenite stability. With prolonged exposure, however, austenite persists and coarsens. At 650 °C, twinning-mediated reversion begins after ∼12 min, whereas at 670 °C, reversion proceeds entirely via the Kurdjumov-Sachs orientation relationship. Al-Ti-rich nano-oxides act as preferential heterogeneous nucleation sites for austenite, promoting reversion but limiting further precipitation. Overall, the results reveal a thermokinetic balance between precipitation and reversion in PBF-LB Mar18Ni steel, providing mechanistic insight into its microstructural evolution.