Abstract: Nitrided layers on the 3Cr13 steel after nitriding for 4, 8 and 12 h in ammonia and in (ammonia+nitrogen) mixed atmosphere respectively at 450 ℃ were compared. The cross section microstructure, microhardness, phase composition and corrosion resistance of the nitrided layers were characterized by metallographical microscope, microhardness tester, X-ray diffractometer and electrochemical workstation. The nitrided layer obtained in ammonia is composed of a compound layer and a bright white layer. However, there is no bright white layer in the nitrided layer obtained in ammonia and nitrogen mixed atmosphere. The surface hardness of the nitrided layer (12 h) in ammonia is 1050.0 HV0.05. After ammonia nitriding, the compound layer's main phase is ε-Fe_2-3N, the secondary phase is γ′-Fe₄N, a small amount of CrN appears, and the phase composition of the bright white layer is γ′-Fe₄N. After nitriding in ammonia, the passivation zone of the polarization curve widens, the self-corrosion current density decreases and the corrosion resistance increases. The surface hardness of the nitrided layer (12 h) in ammonia and nitrogen mixed atmosphere is 998.0 HV0.05. Because of the increase of nitrogen concentration in the mixed atmosphere, the CrN content increases in the nitride layer after nitriding for 8 h, and the secondary phase transforms from γ′-Fe₄N, which is secondary phase after nitriding in ammonia for 8 h, to CrN. The self-corrosion current density of the nitrided layer decreases when the nitriding time prolongs to 12 h in ammonia and nitrogen mixed atmosphere, the width of the passivation zone widens, and the corrosion resistance slightly improves.

Key words: 3Cr13 steel, plasma nitriding, atmosphere, microstructure, properties