Optimization Of Grain Boundary Character Distribution And Its Effect On The Corrosion Behavior Of Fe-18Cr-18Mn-0.63N High-Nitrogen And Ni-Free Austenitic Stainless Steel

High-nitrogen and Ni-free austenitic stainless steel having excellent properties can satisfy the engineering application requirements,such as high strength,high ductility,good corrosion resistance and no magnetism etc.At the same time,compared the austenitic stainless steel containing Ni,it owns obvious cost advantage.In addition,human organs exhibit no irritability to it owing to no containing Ni.Therefore,it also becomes one of the research focuses in the metallic biomaterials.However,the high nitrogen content may cause the precipitation of nitride during thermal processes in this advanced steel material.The nitride precipitation can cause serious intergranular corrosion(IGC),which directly limits the practical applications of such advanced steel materials.In the present work,the Fe-18Cr-18Mn-0.63N high-nitrogen and Ni-free austenitic stainless steel were cold rolled and then annealed,and it is found that cold-rolling by 7%followed by annealing at 1150 ? for 10 min is the most feasible for the optimization of GBCD in the experimental steel under the current experimental conditions.The proportion of special boundaries increases from 47.3%for the solid solution treated high-nitrogen austenitic stainless steel specimen to 83.3%for the specimen cold-rolled by 7%and then annealed at 1150 ? for 10 min.The fraction of the special boundaries(SBs)increases 36%and the optimization effect is very remarkable.At the same time,grain-cluster microstructure of the large size is observed.The SBs of the experimental steel mainly consists of ?3,-9 and ?27 boundaries and ?3 boundaries are major.The precipitates are Cr2N phases in the experimental steel.The second phase precipitation obviously decreases in the specimens owning SBs of high proportions,which shows the characteristics of SBs inhibiting the precipitation at the grain boundaries.The tensile test at the room temperature shows that GBCD optimization almost has no effect on the strength and ductility of the experimental steel.The effects of GBCD optimization on the intergranular corrosion(IGC)were investigated by oxalic acid electrolytic corrosion experiment and a ferric sulfate-sulfuric acid test.After GBCD optimization,the IGC of the experimental steel is remarkably improved.Only ?3 boundaries are almost resistant to IGC and other low-ECSL boundaries,containing?9 and ?27 boundaries are severely corroded.E3 grain boundaries of high proportion effectively interrupt the connectivity of no corrosion-resisting grain boundaries(?9,?27 and general high angle ones etc.)network,inhibiting the corrosion extension along no corrosion-resisting grain boundaries.In addition,large size grain-clusters with high proportion of the ?3n boundaries effectively slow down the corrosion extension towards the inside of grains and hinder the dropping of grains,which improves the IGC resistance of the high-nitrogen austenitic stainless steel.