Tailoring Microstructure and Hardness in Maraging 300 Steel via 2.4 wt.% Niobium Addition and Controlled Aging Treatments

This pioneering study demonstrates, for the first time, the transformative potential of incorporating 2. 4 wt. % niobium into commercial Maraging 300 steel. Following a solution annealing treatment optimized for fracture toughness, controlled aging at 480°C and 560°C for 3 h reveals Nb's profound influence on microstructure and mechanical performance. Multitechnique characterization (X‐ray diffraction, Scanning electron microscopy–energy‐dispersive X‐ray spectroscopy, electron backscatter diffraction (EBSD), and dilatometry) uncovers significant lattice distortion from Nb's larger atomic radius and 4 d configuration, driving enhanced solid‐solution strengthening and modified dislocation dynamics. Dilatometry identifies distinct transformation regimes—precipitation (428°C–554°C) and austenite reversion (556°C–754°C), with Nb elevating the martensite start temperature to 210°C (vs. ~170°C in standard alloys). Aging at 480°C delivers peak hardness (54. 7 HRC), matching or exceeding conventional Maraging 300, through fine Nb–Mo‐rich precipitates without reverted austenite formation. At 560°C, ~1. 92 vol. % reverted austenite emerges in Ni‐enriched bands, reducing hardness to 51. 0 HRC but enabling potential transformation‐induced plasticity‐like ductility. EBSD reveals weak 101 BCC texture at 480°C and FCC emergence at 560°C. These findings position high‐Nb maraging steels as a breakthrough in tunable ultra‐high‐strength alloys, offering a cost‐effective, resource‐leveraging alternative to Ti‐heavy compositions.