The Effect Of Grain Refinement On Mechanical Properties Of Austenitic Stainless Steels At High Temperature

In response to the low yield strength of commercial austenitic stainless steel and the fact that many of its service conditions are in high-temperature environments,this article prepared ultra-fine grained austenitic stainless steel with both ultra-high yield strength and high plasticity through reverse phase transformation technology.The deformation mechanism of ultra-fine grained austenitic stainless steel during service at room temperature and high temperature was analyzed using microstructure characterization techniques such as scanning electron microscopy and transmission electron microscopy,and the grain size was explored,The mechanism of temperature and strain rate on austenitic stainless steel and strength plasticization.The specific research content and results are as follows:（1）Ultra fine grained austenitic stainless steel（UFG sample）was obtained by 60%cold rolling and 700℃+2min annealing treatment on commercial austenitic stainless steel（CG sample）,and its mechanical properties were compared with the original austenitic stainless steel.The results indicate that the grain size has increased from the original 10.3μm drops to 0.9μm.At room temperature,the yield strength of austenitic stainless steel after grain refinement increased from 384MPa to 704 MPa,while the elongation decreased from 63.8%to 46.3%,and the static toughness increased from39718 MPa%to 43543 MPa%.The deformation mechanism of the two groups of samples at room temperature is dislocation slip,deformation induced martensite and twinning synergism.In the early stage of deformation,the dislocation slip increment in UFG sample is significantly more than that in CG sample.In the middle stage of deformation,UFG sample has obvious twinning effect,while CG sample is weak.In the late stage of deformation,both CG sample and UFG sample have deformed martensite.At the time of fracture,the deformed martensite content in CG sample（39%）is higher than that in UFG sample（10%）.（2）CG and UFG samples were selected for tensile testing at different high temperatures（650-800℃）under strain rate conditions of 10^-3/s to study the effect of temperature on the mechanical properties of ultrafine grained austenitic stainless steel.The research results indicate that as the temperature increases from 650℃ to 800℃,the yield strength,tensile strength,and static toughness of CG and UFG samples show a slow decreasing trend;The elongation of the two groups of samples showed a trend of rapid decrease and then increase,with UFG samples being particularly prominent.As the temperature increases,the deformation mechanism of CG and UFG gradually shifts to mainly grain boundary migration.Due to grain refinement,the temperature at which the deformation mechanism of UFG samples undergoes transformation is lower than that of CG samples.Grain refinement can advance and deepen the dynamic recrystallization temperature of the material.（3）CG and UFG samples were selected to undergo slow strain(10^-5/s)stretching at high temperature,and the effect of strain rate on the mechanical properties of ultra-fine grained austenitic stainless steel at high temperature was studied by comparing the results with those obtained at 10^-3/s.The results show that the yield strength of the sample decreases with the same temperature drop and low strain rate,while the elongation of CG sample does not change significantly,while the elongation of UFG sample increases significantly,exhibiting superplasticity at 750℃ and 800℃.This is due to the decrease in strain rate leading to an advance in the recrystallization temperature.At 750℃,the deformation mechanism of CG samples changes to grain boundary migration,while at650℃,the deformation mechanism of UFG samples has changed to grain boundary migration.