The Study On Low Temperature Plasma Nitriding Of Several Stainless Steels

Austenitic stainless steels are among the most used engineering materials in modern industry mainly because of their superior corrosion resistance.However,low hardness and poor wear resistance often shorten considerably their life expectancy.Conventional nitriding would improve the wear resistance,but will compromise the corrosion resistance,because high temperature?T>500??induced precipitation of chromium-rich nitrides at the surface of workpiece would lead to chromium depletion.Low temperature plasma nitriding can solve this issue well,for it can improve wear resistance while maintaining corrosion resistance.In this study,low temperature plasma nitriding?LTPN?were performed on three austenitic stainless steels AISI 316L,AISI 321,AISI 904L and one duplex stainless steel SAF 2205.In order to investigate the influence of nitriding temperature on microstructure,corrosion resistance and wear resistance of nitriding layer,some analytical techniques such as Optical Microscope?OM?,Scanning Electron Microscope?SEM?,Energy Dispersive Spectroscopy?EDS?,X-Ray Diffractometer?XRD?,Electrochemical Workstation,Microhardness Tester and Friction and Wear Tester were used.It has been found that:?1?The process of LTPN at 360?can induce formation of a single S-phase layer on the surface of AISI 316L,AISI 321 and AISI 904L.S-phase is a interstitial supersaturated expended austenite and has a white appearance in OM.The N concentration in S-phase can reach 22.2at.%.The lattice parameter increased by 9.7%.The?200?plane expended 11.7%more than other planes.Due to the lattice distortion the hardness of the S-phase can reach 868HV_0.05.05 which is three times over the substrate.?2?The wear resistance of stainless steel was considerably improved due to the wear mechanism transformation from adhesive wear to oxidative wear after LTPN.The width of wear track was reduced to 1/3 of the untreated samples.With the increase of nitriding temperature there would be the formation of precipitation and increase of layer thickness,which would cause the continual improvement of hardness and wear resistance.The wear resistance of AISI 904L was improved most.?3?The corrosion resistance especially pitting corrosion resistance was enhanced in the presence of S-phase.The pitting potential of AISI 316L was increased from 0.37V to 0.59V after nitriding at 380?,the characteristic of pitting corrosion of AISI 321 was disappeared.S-phase is metastable,with the increase of nitriding temperature it will decompose into?,?,and nitrides,which will compromise the corrosion resistance.The S-phase decomposition rate of these three austenitic stainless steeel increases in the order of AISI 321,AISI 316L,AISI 904L.?4?Considering the corrosion resistance and wear resistance,the optimum nitriding temperature for AISI 316L,AISI 321 and AISI 904L is 380?,360?and450?respectively.?5?The N has the tendency to form nitrides with Cr during LTPN of SAF 2205due to the low solid solubility in ferrite,which will make the difficulty to form S-phase and cause depletion of Cr,in hence the decrease of corrosion resistance.