Study On Microstructure And Properties Of Plasm Nitriding Layer On F51 Duplex Stainless Steel

The F51 duplex stainless steel,whose micro structure consisted of ?-austenite and ?-ferrite,was plasm-nitrided at two different temperature of 450? and 550?.The micro structure of the nitrided layer of F51 duplex stainless steel was observed by optical microscopy(OPM)and scanning electron microscopy(SEM).The phases present and the phase composition variation of the nitrided layer from surface to the inner were determined by X-ray diffraction(XRD).The microchemical composition in different depths and areas was evaluated by X-ray energy dispersive spectrometer(EDS).Based on the above experimental results,the phase compositions,their formation mechanism and the diffusion behavior of N atoms in the nitriding process at 450? and 550? were discussed.The microhardness and the wear resistance were evaluated by microhardness tester and friction wear tester respectively.The surface wear tracks were observed by laser scanning confocal microscope(LSCM).The effect of nitriding treatment on the friction and wear properties of F51 duplex stainless steel was analyzed.Through the polarization curve of the nitrided samples and untreated sample,the surface corrosion resistance of F51 stainless steel and nitrided samples in 3.5wt%NaCl solution was evaluated comparatively.The low-temperature(450?)nitrided layer of F51 duplex stainless steel was mainly composed of ?N and a small amount of ?N.The phase distribution from the surface to the inner was ?N+?N(a small amount)??N+?N(more amount)??+?(matrix),and the thickness of nitrided layer was about 7?m.The high-temperature(550?)nitrided layer was mainly composed of CrN + ?N.The phase distribution from surface to the inner was CrN+?N?aN+?N??+y(matrix),and the thickness of nitrided layer was about 19?m,The average surface-hardness of the low-temperature nitrided sample was 943.5HV100,while the surface-hardness of high-temperature nitrided sample was 1079.8HV100.The surface-hardness was increased by 2.5-3 times compared with the untreated samples.The tribological properties of the low-temperature and the high-temperature nitrided layers were significantly improved compared to the untreated F51 duplex stainless steel.The skin-friction coefficient of untreated sample was 0.71.While the skin-friction coefficients of low-temperature and high-temperature nitrided samples were greatly reduced,which were 0.24 and 0.17 respectively.The width and the depth of surface wear tracks of the nitrided samples were reduced obviously compared with the untreated sample.The nitrided surfaces did not exhibited adverse effect in the corrosion resistance but slight improvement in the 3.5 pct NaCl solution.