Mechanical properties and fracture micromechanisms in 316L stainless steel subjected to ion-plasma treatment with mixture of N, H and Ar gases

Phase composition, tensile properties, and fracture micromechanisms of austenitic stainless steel Fe-17Cr-13Ni-1.7Mn-2.7Mo-0.5Si-0.01C (wt %) specimens after ion-plasma nitriding in the mixture of gases (30% nitrogen and 62% argon) with the addition of 8% hydrogen were experimentally studied. Nitrided layer is characterized by the heterogeneous phase composition, which includes nitrogen-enriched austenite (Fe-γN), ordered nitrogen-enriched (Fe4N) phase and a small amount of CrN and Fe-α phases. The diffusion layer is characterized mainly by Fe-γN phase. The addition of hydrogen in saturating gas enhances the diffusion of nitrogen during ion-plasma treatment, and deeper nitrogen-assisted surface and subsurface layers are formed in comparison with specimens treated in H-free gas mixture. Ion-plasma nitriding leads to an increase in the yield strength of the specimens and a decrease in ductility. Fracture surface in nitrided specimens contains several characteristic regions fractured in transgranular mode: surface quasi-brittle layer with cleavage-like facets, subsurface nitrogen-assisted diffusion layer with elongated dimples and numerous traces of deformation, and nitrogen-free ductile matrix with dimple fracture micromechanism.