| As a material with excellent comprehensive properties,high nitrogen austenitic stainless steel is used in aerospace,oil exploitation,medical implantation and many other fields.How to obtain stainless steel with high relative density and high nitrogen content is an urgent problem to be solved in the preparation of high nitrogen stainless steel by powder metallurgy.In this study,high nitrogen stainless steel was prepared by powder metallurgy method of two-step nitriding.Stainless steel with different nitrogen content were obtained by vacuum holding for different times,and treated by solid solution.The effects of process parameters on nitrogen content,relative density and microstructure of the stainless steel were studied.The precipitation behavior of nitrides was analyzed.The effect of solution treatment on the mechanical properties of stainless steel with different nitrogen content and the effect of nitrogen content on the corrosion resistance of solid solution stainless steel were studied.The main conclusions were as follows:（1）Increasing the nitriding pressure could increase the nitrogen content of the stainless steel,but reduce the relative density;Increasing the sintering temperature or prolonging the sintering time could increase the relative density of the stainless steel,but reduce the nitrogen content.Overall,the better process route was 20 k Pa low-pressure nitriding at600℃;When the temperature rose to 1280℃,increasing the nitrogen pressure to 80 k Pa for high-pressure nitriding and keeping it for 2 hours until the end of sintering.High nitrogen stainless steel with nitrogen content of 1.41 wt.%and relative density of 97.94%could be prepared by this process.After furnace cooling,the matrix grain was austenitic phase,Cr N and Cr2N phase were cellular structure,which precipitates preferentially at the intersection of grain boundary and grain boundary,and then grew into the grain in a bundle until it ran through the whole grain.（2）When the nitrogen content of sintered stainless steel increased from 1.15 wt.%to 1.41 wt.%,its tensile strength increased from 813 MPa to 951 MPa,and the elongation had no obvious change law.After solution treatment at 1200℃for 2 h,the microstructure was mainly austenitic phase with a large number of annealing twins,with less precipitation of Cr2N.The tensile strength increased from 936 MPa to 1123 MPa,the elongation increased from 4.3%to 53.1%and then decreased to 35.9%,and the hardness increased from 293 HV to 385 HV,that was,the solid solution strengthening effect of nitrogen could significantly increase the strength of stainless steel.（3）When the nitrogen content of the solid solution stainless steel increased from 1.11 wt.%to 1.37 wt.%,the nitrogen in the matrix could consume the H+produced by solution hydrolysis and hinder the corrosion.In Cl-solution immersion corrosion,the weighted average corrosion rate increased from 9.02×10-6 g/（mm2·d）reduced to 3.97×10-6 g/（mm2·d）,corresponding self-corrosion current density from 6.88×10-7 A/cm2reduced to 3.14×10-7 A/cm2.In the immersion corrosion of PBS slow-release solution,the weighted average corrosion rate increased from3.05×10-7 g/（mm2·d）reduced to 2.35×10-7 g/（mm2·d）,corresponding self-corrosion current density from 3.88×10-7 A/cm2 reduced to 2.26×10-7A/cm2.The comprehensive results showed that the corrosion resistance became better with the increase of nitrogen content.There are 49 figures,18 tables and 152 references... |
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