Study On Corrosion Resistance And Mechanical Properties Of Cold-worked High Nitrogen Austenitic Stainless Steel

Compared with conventional stainless steels,high nitrogen austenitic stainless steel(HNASS)acquires good austenite stability,high comprehensive mechanical properties,excellent corrosion resistance and excellent biocompatibility.As a new type of metallic biomaterials,HNASS has highly regarded application prospects as biomaterials for orthopedics,interventional implanted devices and so on.Considering that HNASS implants are often used in a cold worked state,so it is important to make sure that cold-worked HNASS has good corrosion resistance and mechanical properties.The following issues exists in cold worked HNASS,including that the pitting behavior mechanism of cold worked HNASS is still unclear;severely cold worked HNASS with a relatively low nitrogen content has poor pitting corrosion resistance;it is difficult to strengthen severely cold worked HNASS,and cold deformation affects corrosion fatigue of HNASS.In present work,the research of HNASS focuses on the following four main aspects.1.Systematic electrochemical experiments were used to study the pitting behavior and critical pitting temperature(CPT)of HNASS with different cold worked levels,and the mechanism by which cold deformation affects the critical pitting temperature of HNASS by affecting the pitting behaviors was also studied.The main results are as follows:Cold work has different effects on critical pitting temperature of HNASS with different nitrogen contents.Increasing cold work level significantly reduces the critical pitting temperature of HNASS with relatively low nitrogen content;while for HNASS with relatively high nitrogen content,moderate cold work increases the CPT of HNASS,and further increasing the cold work level,CPT obviously decreases.CPT of HNASS increases with the increase of nitrogen content.In addition,we found that cold work will increase the probability of pitting nucleation of HNASS with relatively low nitrogen content;while for HNASS with relatively high nitrogen content,moderate cold work reduces the probability of pitting nucleation.And further increasing the cold work level,the probability of pitting nucleation increases.Cold work promotes the growth of metastable pitting corrosion,the transformation from metastable pitting to stable pitting and inhibit the repassivation process of HNASS.Cold deformation affects the pitting behavior of HNASS with different nitrogen contents is the reason why cold deformation affects its critical pitting temperature.2.Chemical and electrochemical methods were used to investigate the effect of nitric acid passivation on the pitting corrosion resistance of severely cold worked HNASS.The related mechanism was also analyzed by analyzing changes in passive film after nitric acid passivation.The main results are as follows:The pitting corrosion resistance of severely cold-worked HNS was significantly improved by a nitric acid passivation.The improvement was obviously enhanced by increasing passivation temperature.Moreover,passivation at 75? showed much better improvement.The improvement was attributed to the increase of Cr oxide enrichment and the decrease of cation vacancy density,while the dissolution of MnS inclusions had no effect on this improvement.Cold-worked HNS showed a weaker passive film formation ability than solution-treated matrix.3.By controling precipiting behaviors of Cr2N in severely cold-worked HNASS,we improved the strength of HNASS by cold deformation and the second phase with no sacrificing its plasticity and excellent pitting corrosion resistance.The main results are as follows:After unidirectional multipass cold rolling,microstructure of cold-worked HNASS maintains the full austenite structure which is composed of a large amount of deformation band zone and a small amount of deformation twin zone.Width distribution of the deformation band is from several hundreds nanometers to a few microns.After low temperature aging,a large number nano-sized spherical Cr2N are precipitated in cold-worked HNASS.Cr2N are aggregated in deformation band and no Cr2N was precipitated in deformation twin zone or out of deformation band.In addition,HNASS after peak-aged at 550? maintains excellent pitting corrosion resistance as severely cold-worked HNASS.In more aggravating aging conditions,Cr2N growed which induced a severe Cr depletion zone.And the pitting corrosion resistance of HNASS decreased significantly.By controlling precipitating behavior of nano-sized Cr2N in deformation band zone,strength of severely cold-worked HNASS significantly increased.The peak-aged strength at 550? of HNASS is up to 2160 MPa,and show a 340 MPa increase compared to severely cold-worked state.Meanwhile,plasticity is mostly the same as that of severely cold-worked HNASS.4.Through the fatigue results,the effect of cold deformation on the high cycle fatigue behavior and corrosion fatigue behavior of HNASS and related mechanisms were studied.The main results are as follows:Fatigue resistance and corrosion fatigue resistance of HNASS are better than traditional 316L austenitic stainless steel.Increasing the degree of cold worked level can significantly improve the fatigue resistance of HNASS.Within the proper cold-worked level range,the corrosion fatigue resistance of high nitrogen steel in Hank's solution has been significantly improved.To further improve the cold worked level range,corrosion resistance fatigue performance is basically consistent.