S30408 Strain Strengthening Mechanism And Welding Technology Of Cryogenic Vessel Stainless Steel

This topic is based on the project requirements of the petrochemical equipment company,which realized the welding of S30408 stainless steel storage vessels of different specifications by the automatic submerged arc welding technology.Treat the welded joints with strain strengthening technology and evaluate the mechanical properties of the strengthened joints,and the organization evolution of the welded joints during strain strengthening was studied.The difference of strengthening effect in different areas of the joint and the influence of strain strengthening on the corrosion resistance and residual stress of the joint were analyzed,which provides theoretical and experimental basis for the production of austenitic stainless steel storage vessels.In this paper,the austenitic stainless steel test plate was welded by the automatic submerged arc.Strain strengthening at room temperature with a strain variable of 9%was applied on welded joints.The results show that the microstructure of weld metal were ferrite and austenite.There were two typical forms,lathy and skeletal ?-ferrite.The deformation-induced martensite was formed in parallel and cross arrangement within austenite grain boundaries after strain strengthening.The martensitic transformation is non-diffusion phase transition.Deformation martensite mainly occurs in the heat affected zone and base metal zone,there is a small amount of martensite at the fusion line,and no martensite in the weld.The electrochemical corrosion test of welded joints shows that the pitting corrosion in the weld zone is not obvious,and the corrosion resistance of the weld zone is better than that of the base material.After strain strengthening,due to the structural transformation of the base material area.Martensite with low corrosion potential is preferentially corroded,leading to pitting corrosion and reduce the corrosion resistance.Through the analysis of the mechanical properties of the welded joints,all welded joints meet the pressure vessel manufacturing standards.Both the yield strength and the tensile strength of the joints were improved after strain strengthening,and the effect of strain strengthening is achieved.The study of fracture characteristic at-196? low temperature with strain strengthening joint showed that the tensile fracture is quasi cleavage morphology.The weld metal fracture after impact test was mainly composed of dimples zone,and there are tearing dimples and second phase particles.Cleavage steps,secondary cracks and river pattern were been found in HAZ,which are typical quasi-cleavage fractures.The deformation rate distribution of the welded joint during the strain strengthening process was studied through the deformation rate test.The results show that the deformation rate of joint in the various regions was disparity during strain strengthening.The weld deformation rate is the lowest,the heat affected zone is second,and the base material zone is the highest.Microhardness analysis shows that the hardness value of the welded joint is significantly improved due to the formation of high hardness martensite after strengthening.Dislocation strengthening occurs mainly in the weld zone,and phase transformation strengthening occurs in the HAZ and the base material zone.The trend of microhardness is the same as that of deformation rate.The results of finite element simulation in the welding process show that there is a large post-weld residual stress in the welded joint.After strain strengthening,the residual stress in the welded joint is released,and the stress distribution is uniform and stable at the low stress level.with the increase of the amount of strain,the residual stress in the joint basically does not change.And the transverse stress in the direction perpendicular to the weld has a tendency to increase due to the same direction as the tensile loading.