Wear And Corrosion Properties Of Plasma Source Nitrided AISI420Martensitic Stainless Steel

AISI420martensitic stainless steel had been nitrided by plasma source nitriding in pure nitrogen at the range temperature of35O-55O℃for the processing time of6h. The phase structure, metallurgical structure and composition of the modified layer were characterized by X-ray diffraction (XRD), optical metallography (OM) and electron probe microanalysis (EPMA). The wear and corrosion properties of the modified sample were investigated by using microhardness measurement, scan electron microscopy(SEM), tribological test and anodic polarization measurement. Relation between the modified layer and technical characteristics was discussed by changing the potential state and sample position of plasma source nitriding.The modified layers were consisted of αN and s-(Fe,Cr)2-3N phases at350℃, αN, ε-(Fe,Cr)2-3N and γ’-(Fe,Cr)4N at450℃, α, CrN and γ’-(Fe,Cr)4N at550℃. The nitrogen concentrations were24at.%,24at.%and20at.%at350℃,450℃and550℃, respectively. The thicknesses of the modified layers were2μm,18μm and45μm, respectively. The diffusion mechanism of nitrogen was discussed by analyzing the ratio of Cr to Fe. It could promote the diffusivity of nitrogen in the modified layer.The microhardness of the modified layer ranged from HV0.25N10.5GPa to13.5GPa,4-5times higher than that of untreated sample. Tribological properties of the modified layer on a ball-on-disk tribometer against an Si3N4ceramic counterface under a normal load of4N with a sliding speed of0.22m/s. A transition of the wear mechanisms from abrasive wear to oxidative wear was found. The increased wear resistance of the modified layer was obtained. A typical course from self-passivation to pitting corrosion of the modified layer was observed, the pitting corrosion resistance of the modified layer was improved in comparison with that of the untreated stainless steel with non-anodic passivation at the range nitriding temperature from350℃to450℃by analyzing anodic polarization measurement in3.5%NaCl aqueous solutions. However, the precipitation of CrN phase at550℃in the modified layer led to the cost of the deterioration of pitting corrosion resistance.The phase structure and properties of the modified layer could be influenced by changing the distance between screen and sample. Changing the sample potential couldn’t influence the phase structure, but could influence the thickness and microhardness.