| 17-7PH precipitation hardening stainless steel was plasma nitrided at the low temperature of 450?,six hours.Its matrix is composed mainly of a large number of martensite,a small amount of austenite,and ferrite along the rolling direction.The phase composition,chemical composition and microstructure of the nitrided layer were investigated by X-ray diffraction(XRD),optical microscope(OPM)and scanning electron microscopy(SEM)and transmission electron microscope(TEM)and energy dispersive spectrometer(EDS).Based on the above experimental results,the effects of different matrix phases and matrix microstructures on the phase composition,phase structure and the diffusion behavior of N were discussed.The tribological properties of nitrided and untreated alloy were evaluated comparatively.Study on corrosion resistance of nitrided layer in 3.5wt%NaCl solution by polarization curve.The following conclusions are reached:The nitrided layer was mainly composed of expanded austenite(?N),expanded martensite(?'N)and a small amount of(Fe,Cr)2-3N which was formed on the surface of the nitrided layer.During nitridation martensite tended to transform into austenite.In general,the interface between matrix and nitrided layer was straight,and the average thickness of the nitrided layer was about 16?m,but in ferrite grain,the nitrided layer's thickness reached 21?m.The XRD results indicate that the lattice expansion of ?N was relatively great,but no obvious lattice expansion was found in ?'N.The Ad/d of ?N(111),?N(200)and?N(220)in nitride layer were different.And the content of N calculated by XRD result was greater than actual content measured by EDS.After low-temperature plasma nitriding,the surface-hardness was four times as hard as the original,and the friction coefficient reduced?from 0.8 to 0.25.Suggesting that the hardness and wear-resistance of surface had been improved greatly.After nitriding,the corrosion resistance of the sample has no obvious change. |
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