Research On Influence Of Plasma Nitriding On Corrosion Resistance Of 304 Stainless Steel And Its Application

Austenitic stainless steel has good corrosion resistance,but its application is greatly limited due to its low hardness and poor wear resistance.The hardness and wear resistance can be significantly improved by conventional plasma nitriding,however,its corrosion resistance is obviously reduced.Therefore,it is significantly to research the effect of plasma nitriding on the corrosion resistance of austenitic stainless steel,while improving its hardness and wear resistance without reducing its original corrosion resistance.In this paper,the influence of gas bombardment before ion nitriding and the nitriding parameters in plasma nitriding on the corrosion resistance of the 304 stainless steel and its causes were analyzed.The best ion nitridation process was obtained and production verification was carried out.The conclusions are as follows:（1）when Ar,H₂ and the combination of the two bombarded the sample,the temperature rise rate of the sample,change in surface morphology,composition of the sample,corrosion resistance and semiconductor properties of the passivation film of the sample were discussed.According to the glow discharge theory,the bombardment energy of the gas to the sample was derived.The results show that the electric field energy in the cathode drop region obtained by Ar⁺is larger than H⁺,so the bombardment effect of Ar⁺is stronger,and Ar⁺preferentially sputters on Cr atoms.Therefore,the higher the proportion of Ar in the bombardment gas,the higher the heating rate of the sample,the rougher the surface of the sample,the greater the loss of Cr on the surface,resulting in the lower in the Cr₂O₃ content in the passivation film,the more oxygen hole defects of the passivation film,which lead to the worse corrosion resistance.It is concluded that the preferred bombarding gas is Ar:H₂=1:3,which hardly reduces the corrosion resistance of 304 stainless steel.（2）The effect of process parameters of plasma nitriding on the performance of the nitride layer was analyzed.The thickness,phase structure,hardness and corrosion resistance of the nitrided layer were investigated.And based on the plasma discharge theory,the reasons for the influence of nitrogen-hydrogen ratio,nitriding temperature and cathode voltage on the nitride layer were derived.The results show that when H₂⁺collides with N₂,the energy conversion efficiency is very high,so N₂ can easily dissociate the active nitrogen atom.However,the proportion of N₂ in the nitriding atmosphere is too low,resulting in the active nitrogen atom reducing and the lower permeation rate.Through orthogonal test,the preferred process parameters are N₂:H₂=1:3,holding time 6 h,and pressure of 600 Pa.On this basis,the nitriding temperature and the cathode voltage were further studied.The diffusion coefficient of the active nitrogen atom is exponentially related to the nitriding temperature.At the same time,the electric field energy obtained by H₂⁺and N₂⁺increases with the increasing of the cathode voltage,which causes N₂ dissociate more active nitrogen atoms,therefore,the nitride layer thickens as the nitridation temperature and the cathode voltage increase.When the nitriding temperature is lower than or equal to 450°C,and the cathode voltage is higher or equal to 550 V and lower than or equal to 650V,the nitrided layer is a high-hardγ_N phase with excellent corrosion resistance.However,at 480°C and at 750 V,the nitride layer forms CrN,making the substrate"Cr-depleted",resulting in obviously decreasing in corrosion resistance.It is concluded that the preferred nitridation temperature is 450°C and the cathode voltage is650 V.（3）The optimal plasma nitridation process was carried out in actual production.The optimizest plasma nitriding process is as follows:bombardment gas Ar:H₂=1:3,plasma nitriding process N₂:H₂=1:3,holding time 6 h,gas pressure 600 Pa,nitriding temperature450°C,The cathode voltage 650 V.The verification shows that the nitrided layer of the case product is 13.5μm and the surface hardness is 1340 HV_0.05.There is no rusty phenomenon after the 48 h salt spray test,which meets the design requirements.The optimal plasma nitridation process has achieved production application.