The Influence Mechanism of Nitriding Time on NH ₃ Gas Nitriding of Laser Powder Bed Fusion 18Ni300 Steel

NH 3 gas is employed in this study to perform nitriding treatment on laser powder bed fusion (LPBF) 18Ni300 steel, followed by testing and analysis of the surface layer by means of scanning electron microscope, optical metallographic, energy disperse spectroscopy, X‐ray diffraction, the surface roughness and hardness measurements. Results indicate that, with an increase in the nitriding time, the grain directionality of LPBF 18Ni300 gradually diminishes, and the thickness of the nitrided case rises following a parabolic law, and the surface roughness gradually decreases and tends to stabilize. The diffusion of N atoms from the surface of LPBF 18Ni300 steel occurs mainly in several phases: 1) nonequilibrium initial deposition state; 2) more severe lattice distortion caused by the penetration of N atoms; 3) extensive precipitation of supersaturated solid solution, allowing rapid penetration of nitrogen atoms; 4) gradually saturated N atom content. In the third phase, the diffusion coefficient of N atom is the highest (2.496 × 10 −13 m 2 s −1 ), resulting in the fastest diffusion rate. In local regions, the concentration of N element in fine grains is lower than that in larger grains. The improvement in surface hardness is attributed to the precipitation of strengthening phases and the solid solution strengthening of nitrogen atoms.