Investigation On Grain Boundary Character Distribution,Mechanical And Stress Corrosion Behavior Of High-Nitrogen And Ni-Free Austenitic Stainless Steel

High-nitrogen and Ni-free austenitic stainless steel having excellent properties can not only satisfy the engineering application requirements,such as high strength,high ductility,good corrosion resistance and no magnetism etc,but also get a remarkable cost advantage by the addition of a large amount of nitrogen to replace nickel,comparing the traditional austenitic stainless steel containing Ni.At the same time,because of not containing nickel sensitized by human body,it becomes one of the research hotspots of biomaterials.Therefore,as a new resource-saving stainless steel,the research about its mechanical properties and stress corrosion resistance own important practical significance.In this paper,18Cr-16Mn-2Mo-0.85N and 19Cr-16Mn-2Mo-0.49N high-nitrogen and Ni-free austenitic stainless steels were selected as the research materials.The deformation behavior and mechanism of 18Cr-16Mn-2Mo-0.85N and 19Cr-16Mn-2Mo0.49N steels at different strain rates was investigated.The effect of thermomechanical treatment on the grain boundary characteristic distribution(GBCD)and the evolution of GBCD of 18Cr-16Mn-2Mo-0.85N steel were studied by electron backscatter diffraction(EBSD).The stress corrosion cracking(SCC)behavior of 18Cr-16Mn-2Mo-0.85N steel was studied by the slow strain rate tensile(SSRT)method.The effects of different grain boundaries(including trigeminal grain boundaries)on stress corrosion were analyzed and the relationship between GBCD and SCC was preliminarily explored.Compared with the traditional Ni-containing austenitic stainless steel,high-nitrogen and Ni-free austenitic stainless steel has a high strength,and low ductility.With the strain rate increasing,the yield strength increases,while the tensile strength and total elongation decrease.With the increase of nitrogen content,the yield strength and tensile strength both increase,while the total elongation decreases.Under the different strain rates and nitrogen contents,the short-range order structure and stacking fault energy have different effects on the deformation.Through their combined action,the deformation follows the path of "dislocation slip/pile-up?planar slip band?twin-like band?deformation twins".After 18Cr-16Mn-2Mo-0.85N steel was subjected to different thermomechanical treatments,it is found that the best technological system of GBCD optimization under the experimental condition is annealling at 1150? for 10min after 5%cold-rolling.The proportion of special grain boundaries(consisting mainly of ?3,?9 and ?27)increases from 49.76%for the solid solution treated to 69.49%for the most feasible system treated.After GBCD optimization,grain boundaries are existed by clusters,while the cluster is composed of the interrupted general high angle grain boundaries containing lots of low coincident site lattice(CSL)grain boundaries inside it.Under the optimal teconology,the extent of random high angle grain boundaries interrupted by special grain boundaries is the most serious and the size of the cluster is the largest,thus realizing the GBCD optimization.The result of SSRT stress corrosion cracking test on 18Cr-16Mn-2Mo-0.85N steel shows that the region near the surface of the sample is corroded along the grain.The cracked grain boundary is mainly the random high angle grain boundary.The crack is mitigated or prevented at J1 or J2 type trigeminal grain boundary,and the front and the vicinity of the crack tip are subjected to severe plastic deformation.