Nitriding of steels of various structural classes manufactured by laser additive technologies

The authors have conducted a comparative analysis of diffusion layers of steels of various structural classes manufactured by complex technology including laser remelting of powder material and plasma nitriding. Parameters of diffusion layers of bainitic steel (Fe - 0.09 % C - 1 % Cr - 2 % Ni - 1 % Mo - 1 % Cu) and martensitic steel (Fe-0.25 % C - 13 % Cr-2 % Ni) manufactured by direct laser deposition (DLD) and austenitic steel (Fe-0.03 % C-17 % Cr-14 % Ni-3 % Mo) manufactured by selective laser melting (SLM) were investigated. During plasma nitriding at 540 °C for 24 h of martensitic and austenitic steels, diffusion layer of 140-160 μm was formed, additionally maximum microhardness of surface layer was 800 HV0.1 and 1050 HV0.1 and it is almost constant on thickness of 100 μm. Diffusing layer of bainitic steel is 900 μm and its microhardness monotonously decreases from the surface. Reinforcing phases of nitrided layer were determined by X-ray analysis: γ' (Fe4N) is fixed in the bainitic steel, γ' and CrN are fixed in martensitic and austenitic steels. Moreover on the surface of austenitic steel solid nitrided layer is formed. The influence of heat treatment after laser remelting of powder material was also studied. It was determined, that despite decreasing of crystal structure defects after heat treatment, the thickness of nitrided layer changes slightly. Also the authors have investigated the influence of porosity of austenitic steel on the thickness of nitrided layer. It was shown, that porosity of 0.5-2.0 % doesn't result in changing of diffusion layer's thickness. © 2020 National University of Science and Technology MISIS. All rights reserved.