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Enhancing the hardness and wear resistance of selective laser melted (SLM) maraging steel is challenging. In this work, plasma nitriding was developed to improve the surface mechanical properties of SLM 18Ni300 maraging steel based on the effect of simultaneous aging and nitriding. The nitriding behaviors of the as-fabricated sample and heat-treated (i.e., solution, aging and combined solution-aging treatments) samples were systematically analyzed using microstructure and property characterizations as well as first principles calculations. The results show that the hardness and wear resistance of all samples were notably enhanced by a nitrided layer consisting of an outermost N-rich bright white layer and an inner Mo-rich diffusion layer. Among all nitrided samples, the directly nitrided sample without heat treatment possesses the best mechanical properties, which are closely related to the strongest Mo segregation and a novel nitriding structure induced by additive manufacturing. The novel nitriding structure is an interlaced lamellar structure consisting of coarse γN lamellae along the prior SLM subgrain interfaces and parallel fine γN lamellae in the αN matrix, which obeys the typical Kurdjumov-Sachs orientation relationship between αN and γN. Compared to the traditional nitriding structure originated from the homogenized maraging steel, the novel nitriding structure benefits strengthening and toughening and encourages the formation of an Fe-O-Si self-lubricating film. The abovementioned results suggest that direct nitriding of the as-fabricated sample without heat treatment is the best method to obtain the best surface mechanical properties.
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•The mechanical properties of SLM maraging steel are notably improved by nitriding.•Direct nitriding of as-fabricated sample is the best design.•Typical SLM structure are beneficial to N penetration and Mo segregation.•A novel nitriding structure is induced by the typical SLM structure.•The novel nitriding structure is helpful to improving mechanical properties.