Research For Structural Transformation And Low Temperature Mechanical Properties Of Strain-Reinforced Austenitic Stainless Steel For Pressure Vessels

Based on the strain strengthening technology,lightweight design and manufacture of austenitic stainless steel pressure vessel and cryogenic vessel has been applied in engineering.This technology mainly utilizes the good plasticity,toughness and low temperature properties of the austenitic stainless steel material to improve the yield strength by sacrificing some redundant plasticity of the material,which can effectively increase the bearing capacity of austenitic stainless steel pressure vessels,reduce the wall thickness and the weight-to-volume ratio,and improve the economic benefits.However,how to balance the strength increase and redundant plastic loss caused by the microstructure transformation and reveal the microstructure transformation mechanism and mechanical properties of strain-reinforced austenitic stainless steel is of great significance for the further promotion and application of strain-strengthened austenitic stainless steel in pressure vessels.Based on this,the normal temperature mechanical behavior,microstructure transformation behavior and mechanism,impact and tensile properties in the cryogenic temperature environment(77K?4.2K)of strain-strengthened austenite stainless steel under the interaction of strain strengthening and strain strengthening with low temperature aging were investigated in this artical for S31603and S30408.Based on the mechanical behavior of strain reinforced materials after cryogenic temperature aging treatment,the phenomenon of"self-strengthening"in the cryogenic temperature zone of strain reinforced austenitic stainless steel was further expounded.The specific research contents are as follows:(1)The phase composition and microstructure transformation of strain-reinforced S31603 austenitic stainless steel were investigated by XRD,SEM and TEM,and the mechanism of deformation twins after strain enhancement was analyzed from the point of dislocation.(2)The mechanical properties of S31603 Austenite stainless steel after strain strengthening at room temperature were studied.It was proposed that the strain rate of8%and the strain rate of 10^-3s^-1 are suitable strain strengthening parameters.The mechanical behavior and mechanism of mechanical behavior of S31603 austenite stainless steel after strain strengthening was analyzed by Ludwigson model and microstructure transformation.The results show that Ludwigson model can accurately describe the plastic rheological behavior of strengthened S31603 austenite stainless steel.(3)According to the measurement method of diffraction peak integral intensity,the volume fraction of strain-induced martensite is calculated.The variation of??martensite and?martensite content with strain variables at different temperatures and the variation of total martensite content with cryogenic temperature at different strain strengthening levels were obtained.(4)The ferromagnetic phase content in strain-reinforced austenitic stainless steel was measured by Ferritescope,which is approximately equal to ferromagnetic??martensite content in austenitic stainless steel.The total content of strain and low temperature induced martensite was calculated by the diffraction peak integral strength measurement method.The numerical model of"total transformation variable of martensite-ferrite meter measurement value"was further established.By using the model,engineers can use Ferritescope to estimate the total amount of martensite transformation in strain-reinforced austenitic stainless steel,which was applied to engineering easily.(5)The microstructure and phase composition of S30408 austenitic stainless steel under the combined action of strain strengthening and strain strengthening&low temperature aging were investigated.The phase evolution and martensite transformation mechanism of S30408 stainless steel were analyzed.The effect of strain strengthening technology on martensite transformation was expounded from the transformation kinetics,and the dynamic model of strain induced martensite transformation proposed by Olson and Cohen was modified to make it more suitable for engineering practice.(6)Based on the temperature return test and the ANSYS simulation of sample surface temperature,the main difficulties and causes in the process of cryogenic and low temperature impact test were pointed out quantitatively.The thermal insulation effect of perlite,degreasing cotton and Si O₂ aerogel in the cryogenic low temperature impact test was compared,which provided data support for the establishment of cryogenic low temperature impact test methods and standards.(7)The effects of strain strengthening on the low temperature tensile and the impact properties of S30408 austenitic stainless steel and the low temperature impact properties of S30408 welded joints were studied by using the cryogenic low temperature tensile and impact test equipments,and the cryogenic low temperature impact test method was established.The mechanical properties of S30408 austenitic stainless steel in different low temperature environment were obtained.When the temperature was lower than 77K,the decreasing of impact absorbed energy tended to be smooth,and showed a"platform"phenomenon.(8)The self-strengthening phenomenon of austenitic stainless steel was discussed,and the design of S30408 austenitic stainless steel low-temperature storage tank used in77K environment was carried out based on the principle of low-temperature self-strengthening.The results show that the allowable stress after low temperature self-strengthening was increased by 37.4%compared with that at 20?,the wall thickness was reduced by 25%.The weight reduction effect was obvious.