Microstruture And Mechanical Properties Of Plasma Nitrided Surface Layer Of DZ2 Steel

High-speed rail engineering has become one of the technical services exported to the world.However,the key parts of the train are still dependent on imports.In order to realize the independent replacement of high-speed axles for EMUs and improve the level of equipment manufacturing,China has independently developed DZ2 steel.In view of the imperfect performance indicators and insufficient toughness of DZ2 axle steel,the domestic research team is conducting research under the support of national key research and development projects.In this paper,DZ2 axle steel was treated with plasma nitriding at different preparing conditions including temperature,time and atmosphere.The thickness and microscopic shape of the nitrided layer were observed by metallographic microscope（OM）,scanning electron microscopy（SEM）and X-ray diffraction（XRD）.The appearance and phase structure were studied.In addition,the mechanical properties such as hardness distribution,wear resistance and corrosion resistance of the nitrided layer were tested by means of microhardness tester,friction and wear tester and electrochemical workstation.The results show that the original DZ2 steel is treated at 900°C and then nitrided at different temperatures to form nitride layer.DZ2 steel is nitrided at 490°C and520°C for 4,8,and 12 hours.After 16 and 24 h,the thickness of the layer was parabolic with time,and the modified layer formed after nitriding at 520°C under the same nitriding time was thicker.The nitrogen and hydrogen ratios were researched at the condition of nitriding at 520°C for 8 h.X-ray diffraction analysis shows that the surface of the nitrided layer was transformed from a solid solution-state??-Fe phase to?-Fe₃N and??-Fe₄N phase at different nitriding temperatures,nitriding time and nitriding atmosphere conditions.Moreover,with higher nitriding temperature and longer the nitriding time,the content of strontium??-Fe₄N phase increases.By analyzing and comparing the surface morphology of the samples after nitriding in different process plasmas,the nitrides formed on the surface are relatively uniform under low temperature and short-time nitriding conditions.After high temperature long-time nitriding,the nitrides formed on the surface aggregate,which results to the higher roughness of surface.According to the hardness distribution curve and friction and wear test of the sample surface under different processes,the optimal plasma nitriding process of DZ2steel is the two experimental processes of plasma nitriding at different temperatures for 8 h and 520°C.Compared with the solid solution sample,the surface hardness of the sample was doubled at 460°C for 8 h or 490°C for 4 h.From the polarization curves of plasma nitriding at different temperatures for 8h and 520°C for different time,it can be seen that after 8 h of 550°C plasma nitrid-ing,the corrosion potential is larger than the solid solution state and the corrosion r-esistance is better;After nitriding at 520 ^oC,the polarization curve showed a passive zone.