Anode plasma electrolytic nitrohardening of medium carbon steel

In this work, we investigate the structure of the medium carbon steel which is formed as a result of plasma electrolytic nitriding with following hardening in the same electrolyte. The cross-sectional microstructure, composition and phase constituents of modified layer under different processing conditions were characterized. It is shown that the electrolyte that contained ammonia chloride and ammonia nitrate provides the saturation of steel with nitrogen up to 5.6 wt.% and formation Fe4N и Fe2-3N nitrides. The nitrogen diffusion decreases the austenitization temperature and results in formation of martensite after the sample cooling in the electrolyte. The aqueous solution that contained 15 wt.% NH4Cl allows one to obtain the layer microhardness up to 1060 HV during 5 min at 750 °C. Modified layer thickness that contained iron nitrides, martensite and retained austenite is determined by nitrogen diffusion which depends on the thickness of the surface oxide layer. In its turn, the oxide layer thickness is affected by anode dissolution and oxidation of the steel substrate. It is found that ammonia chloride promotes the decrease in the oxide layer thickness owing to the anode dissolution that results in the growth of the modified layer. The increase in the concentration of ammonia nitrate as a donor of nitrogen leads to the oxidation and the decrease in the modified layer thickness. Surface roughness of samples treated under different processing modes and concentration of electrolyte components is increased due to the insufficient rate of anode dissolution. The minimal roughness is observed after nitrohardening at 700 °C in the electrolyte with 10 wt.% NH4NO3.