This study investigates the fatigue behavior of 18Ni300 maraging steel fabricated via Laser Powder Bed Fusion (LPBF) and subjected to a novel, cost-effective two-stage heat treatment performed in an air atmosphere. The specimens underwent solution annealing at 940°C for 1 h followed by aging at 490°C for 6 h. Monotonic tensile tests revealed that this thermal treatment significantly enhances mechanical strength, increasing the yield strength by 86% and the ultimate tensile strength by 70% compared to the as-built condition, although ductility decreases from 4.7% to 2.6%. Fatigue test results demonstrated superior fatigue resistance compared to similar datasets from literature for both as-built and conventionally heat-treated conditions. Microstructural analysis confirmed that the studied air-atmosphere thermal process effectively dissolved the laser-induced melt pool boundaries, resulting in a homogenized martensitic matrix, but with a notable fraction of reverted austenite. Fractographic examination identified that fatigue failure was driven predominantly by non-metallic inclusions located just beneath the surface. The findings suggest that while air-furnace heat treatment is a viable, low-cost method for restoring static strength, the fatigue life of AM maraging steel remains sensitive to oxide inclusions which persist as stress concentrators within the hardened matrix.
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