| The low surface hardness and bad wear resistance of stainless steels are to the disadvantage of applying in the tools,bearings and so on.After nitriding,a nitrided layer with high hardness can form on stainless steels,which can significantly improve the wear resistance of stainless steels.So in this study,the typical 2Cr13 martensitic stainless steel and 0Cr18Ni9 austenitic stainless steel were selected for the substrate to study a gas nitriding of stainless steel.The effects of nitriding time,nitrided temperature,cooling way and subsequent tempering on the microstructure and properties(mechanical properties,corrosion property and wear property)of nitrided samples were systematically studied by means of different characterization measures including transmission electron microscopy,scanning electron microscope,optical microscope,X-ray diffractometer,microhardness tester,mechanics of materials testing machine,electrochemical workstation and friction tester.The followings can be concluded:(1)With the increase of nitriding temperature,the microstructure of 2Cr13 steel nitrided layer has been varying.For 450?nitrided specimens,there is not compound layers,and the thickness of the nitrided layers is uneven.For 500?nitrided specimens,the compound layers are compact and even,and the thickness of compound layers do not change with the extension of time,but the diffusion layers are gradually thickening with the extension of time.the compound layers of 550?nitrided layers contains a large number of pores and cracks.The microstructure of diffusion layers of2Cr13 steel nitrided at 500? mainly consisted of martensite,which contains a large amount of dislocation and dislocation cell structure,and a large amount of Cr N nano-precipitates.The Cr N nano-precipitates dispersedly distribute in the interior of grains and at the boundaries of dislocation cells,and the Cr N nano-precipitates at the boundaries of dislocation cells are more density and larger comparing to that of grain interior.There are a large amount of ?-Fe2-3N or(Cr,Fe)2N precipitation at the grain boundaries of lath martensite.When the nitriding temperature increased to 550?,the martensite of the diffusion layer recovered itself,and the Cr N precipitates lined up into form Cr N lamellas.During nitriding at 550?,the microstructure of diffusion layers have transformed into lamellar pearlite consisting of the ?-Fe matrix and the Cr N phase.(2)After nitriding,the surface hardness,wear-resistance,bending strength and tensile strength of 2Cr13 steel are great improved,but the corrosion resistance decreases a little and the ductile of 2Cr13 steel decreases great.With the increase of nitriding temperature,the surface hardness of nitrided layers of 2Cr13 steel first increased and then declined,and the peak hardness reached at 500?.The cooling methods and subsequent tempering almost have no influence on the hardness and microstructure of diffusion layers.And the hardness of the diffusion layers of the500?nitrided specimens with or without tempering keeps more than 1000HV0.1.An electro-less Ni-Co-P plating can improve the corrosion resistance of the nitrided samples in acidic conditions.(3)After gas nitriding at 450 – 610 ? for 4 h,the microstructure of the nitride layers of a 0Cr18Ni9 steel is changing with the nitriding temperature.When the nitriding temperature was less than 500?,the nitrided layers were very thin(about10?m),and the microstructure of the surface layer almost consisted fully of the S phase.When nitriding temperature increased to 520?,the S phase of the surface layer had transformed into the Cr N and ??-Fe4 N phase,and the content of Cr N and??-Fe4 N phase and the thickness of the nitrided layers were increased with the nitriding temperature.With the increase of nitriding temperature,the surface hardness of nitrided layers first increased and then declined,and the peak hardness reached at500?.For 0Cr18Ni9 stainless steels,the corrosion resistance of the samples nitride at450? is best among all of the nitrided samples. |
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