Effect Of Salt Bath Nitriding And QPQ Technology On Surface Microstructure And Properties Of Hyper Duplex Stainless Steel

SAF3207 hyper duplex stainless steels are newly developed material with high contents of N and Cr.In comparison with the traditional duplex stainless steels,it has higher strength and better corrosion resistance.But its low hardness and poor wear resistance limit applications in the field of high requirement of wear resistance（e.g.corrosion,wear and over loading）.QPQ technology envolues salt bath nitriding and salt bath oxidation.Some researchers have used it into improveing wear resistance of stainless steels and achieved good results.However,they are mainly used for single phase such as austenite and marensite stainless steels.Few literatures report its application on duplex stainless steel.In this paper,salt bath nitriding and QPQ technology are used for surface modification of SAF3207 hyper duplex stainless steel with N contents of 0.51%and 0.72%.The effects of salt bath nitriding and QPQ technology on microstructure and properties are investigated by means of OM,SEM,XRD,Electron probe spectrμm analysis,Micro-hardness and wear resistance test.Microstructure observation showes that the surfaces layer of SAF3207 sample after salt bath nitriding is composed of compound layer and diffusion layer.The surface of the sample after QPQ treated forms an oxide layer outside of the compound layer.The thicker compound layer increases with the concentration of nitrogen on the substrate.The surface layers of ferritic and austenite are quite different after QPQ treated.The surface layer of ferritic and the main phase of oxide layer is Fe3O4.The main phase of compound layer is composed of Fe2-3N,S,αN,CrN,and γ’,diffusion layer are composed of Fe2₃N,S,αN,CrN,γ’,Fe4N and a.The surface layer of austenite and the main phase of oxide layer are composed of Fe3O4.The compound layer are composed of Fe2-3N,S,CrN and y’,and the diffusion layer are composed of Fe2-3N,S,γ’,CrN and y.In Comparison with the ferrite,austenite has a thicker compound layer and a thinner diffusion layer.SAF3207 sample is treated by salt bath nitriding.The results show that oxygen content of permeability layer is low.The amount of nitrogen decreases with the increase of distance to the surface.In the comparison with the compound layer of ferritic,nitrogen in the austenite is lower.The iron in the oxide layer is significantly lower than substrate.The contents of chromium in the compound layer and diffusion layer are lower than that on the substrate.SAF3207 sample after QPQ treated shows that the amount of oxygen element increases first and then decreases on the oxide layer of ferritic.While in the oxide layer of austenite,the amount of oxygen element is low and stable.The amount of nitrogen element on the ferritic compound layer increases first and then decreases.The amount of iron on the surface of the sample after QPQ treated is fewer than salt bath nitriding.Distributions of chromium in the compound layer and diffusion layer are basically identical.The hardness on the surface of the sample is significantly improved after salt bath nitriding and QPQ treated.QPQ treatment sample surface hardness first increases and then decreases.The maxium hardness of high N sample is higher than low N sample,but the width of the maxium hardness is narrower.The results of dry-sliding test show that the mass loss of salt bath nitriding sample is much lower than the substrate,but higher than the mass loss of QPQ treatment sample.The friction coefficient values of the sample is significantly reduced.Fe3O4 film that is formed on the surface of the sample after QPQ treated contributes to lower friction coefficient.The low N sample has better wear resistance as the result of the thicker compound layer after salt bath nitriding and QPQ treated.The wear mechanisms of the substrate and salt bath nitriding are abrasive wear and adhesive wear.The wear mechanism of the QPQ treated sample is abrasive wear.