Study On Mechanism And Elimination Of Welding Residual Stress In Austenitic Stainless Steel

Welding is widely used in equipment manufacturing in the petrochemical industry.The welded joints are mostly in the discontinuous parts of the equipment,and the operating environment of the petrochemical industry equipment is harsh,the stress at the welded joint is higher than other parts,and it is more likely to cause failure events.Welding quality is affected by welding design,welding parameters,base metal properties,operators,environment and other factors.The reason for the stress concentration of the welded joint is due to the welding defect,and the welding defect is also the starting point of the welding crack initiation,which leads to the safety hazard of the welded joint.The welding process is accompanied by an intrinsic phenomenon,that is,residual stress.The large welding residual stress will lead to the reduction of the bearing capacity of the welded joint,and at the same time,the deformation of the welded joint will affect the subsequent processing technology.Therefore,eliminating the residual stress of harmful welding can not only improve the mechanical properties and service life of the welded joint but also has important significance for reducing the accident of welding joint failure.At present,there are many measures and methods to eliminate welding residual stress,such as heat treatment,vibration aging method,mechanical stretching method,and ultrasonic impact treatment,and ultrasonic impact treatment is a new process to improve the residual stress of welded joint after welding.The ultrasonic impact treatment has many advantages such as lightweight,low noise,high efficiency,low cost,and good energy saving.Therefore,studying the influence of ultrasonic impact treatment on the residual stress of welded structure and its changing law is a test work with application value,and also an important index to evaluate the process measures.Using finite element software ABAQUS,based on the double ellipsoid heat source model,the temperature field distribution of the butt welding 316L specimen and the 304 lifting lug pad plate was analyzed.The distribution characteristics of the welding residual stress were calculated as the applied variable load.Based on the theory of transient contact mechanics,the distribution of stress field after the ultrasonic impact of 3 16L samples was analyzed.The 6 mm thick 316L austenitic stainless steel plate was prepared.The residual stress of the butt welded joint of 316L plate was measured by blind hole method.The welded joint was treated by ultrasonic impact to observe the microstructure of the welded joint before and after the ultrasonic impact.And compared with the residual stress test results to verify the accuracy of the simulation results.The results show that the weld microstructure consists of austenite and ?ferrite,and the ferrite is mainly distributed in the worm-like shape on the main axis of the dendrite.Before the ultrasonic impact treatment,the fusion line is clearly visible,the grain in the heat-affected zone near the fusion line is coarse,and there is a slip line inside the grain,and the part is plastically deformed during the welding process.After the ultrasonic impact treatment,the weld line becomes blurred and the residual stress of the joint is greatly reduced.The transverse residual stress of the joint is mainly tensile stress,and the maximum stress is at the weld fusion line;the maximum longitudinal residual tensile stress appears near the weld and its heat affected zone.The simulation results of the transverse residual stress are in good agreement with the blind hole method test results,and the deviation from the test results is less than 20%.Analysis of fracture failure of 304 lifting lug pad plate.Based on the investigation of the working environment of the lifting lug pad plate,the microstructure and fracture morphology was observed by an optical microscope and scanning electron microscope.Using the finite element method to predict the welding residual stress and thermo-mechanical stress at the weld between the lifting lug pad plate and the cylinder.The results show that the high temperature,welding residual stress and thermo-mechanical stress synergistic effect are the main factors leading to the mixed fracture of toughness and brittleness.