Effects Of Grain Boundary Character Distribution On The Mechanical Properties And Stress Corrosion Crack Behavior In High-Nitrogen And Ni-Free Austenitic Stainless Steel

Compared with traditional austenitic stainless steel,a new steel material replaced nickel by manganese and nitride,namely high nitrogen and nickel-free austenitic stainless steel,not only achieves excellent mechanical properties and local corrosion resistance,but also has obvious cost advantages and good biocompatibility,which has attracted more and more attention from scholars at home and abroad.However,the high nitrogen content makes the new stainless steel precipitate nitride and intermetallic phase when used in welding or high temperature.The precipitation of precipitated phase at grain boundaries will lead to serious intergranular corrosion,which directly limits the development and application of this new advanced steel material.In recent years,grain boundary engineering has been widely concerned as a new method to solve the problem of grain boundary failure such as intergranular corrosion.In this paper,18Cr-16Mn-2Mo-0.85N high-nitrogen and Ni-free austenitic stainless steel was selected as the research materials,the effect of thermo-mechanical treatment on the grain boundary characteristic distribution(GBCD)of the experimental steel was studied by electron backscatter diffraction(EBSD).Then the influence of grain boundary characteristic distribution optimization on the mechanical properties and the influence mechanism was explored by 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 crack were analyzed and the relationship between GBCD and SCC was preliminarily explored.After 18Cr-16Mn-2Mo-0.85N steel is subjected to different thermo-mechanical treatments,it is found that the optimal process of GBCD optimization under the experimental condition is annealling at 1150? for 72h after 5%cold-rolling.The proportion of special grain boundaries(consisting mainly of ?3,?9 and ?27)increases from 47.23%for the BM to 82.85%for the GBE-5-72.After GBCD optimization,the grain size has not changed much but the size of special grain boundary clusters has increased significantly,the J2 and J3 contents in the triple grain boundaries have increased,the extent of random high angle grain boundaries have been interrupted by special grain boundaries,thus realizing the GBCD optimization.For BM and GBE-5-72 specimens tensioning at different strain rates,it is found that the strength of the experimental steel slightly decreases and the plasticity obviously increases.According to the analysis of the local misorientation,Schmid factor and Taylor factor,GBE-5-72 specimen with higher proportion of special grain boundaries owns more homogenous strain distribution during deformation,which can make slip more likely to occur,as a result of higher plasticity.The result of SSRT stress corrosion cracking test on 18Cr-16Mn-2Mo-0.85N steel for BM and GBE-5-72 specimens shows that grain boundary engineering can improve stress corrosion resistance in NACE/A solution corrosion environment at 50 ?.During the process of SCC fracture crack propagation,the fracture cracks of BM specimens are mainly intergranular cracks,with little of micro-transgranular cracks in the grain.The fracture cracks of GBE-5-72 specimens present a mixed model of transgranular and intergranular crack.The intergranular crack mainly propagates along the random large angle grain boundary,and the crack is prevented at J2 or J3 type trigeminal grain boundary.When the expansion of transgranular crack happen into the grain cluster,because of the special boundary and trigeminal grain boundary,the direction of crack propagation will change constantly,the crack will end up inside the grain cluster.