Residual Stress And Stress Corrosion Cracking In Repair Welding Joint Of 304 Stainless Steel

Pressure vessels would inevitably produce all kinds of dangerous defects in the manufacturing,installation and service process,repair welding is usually used to repair the equipments to recover their structural integrity.But in the process of repair welding,the repair welding joint would inevitably produce a large residual stress due to the uneven heat.Residual stress usually has a great effect on the service life of repair welding equipments,especially under the combined effect of both corrosive medium and residual stress,the equipments are prone to stress corrosion cracking.Therefore,it is of great scientific research value to study the residual stress and stress corrosion cracking behavior of repair welding joint.This dissertation focused mainly on repair welding joints of the 304 stainless steel.The distribution rules of residual stress,microstructure and microhardness in the local repair welding parts were studied.In addition,the stress corrosion cracking mechanism of repair welding joints in 3.5% NaCl solution was studied as the same time.The dissertation also discussed the influence of heat input,weld reinforcement and repair length on the residual stress and stress corrosion susceptibility of the local repair welding parts.The results would provide reference for optimization of repair welding technology.The main work and conclutions of the dissertation are summarized as below:(1)The distribution rules of metallographic structure and microhardness in the local repair welding parts of 304 stainless steel were studied.It is shown that metallographic structures in the weld,weld line and HAZ of the welding joint are all composed with austenitic matrix and ?-ferrite.The ?-ferrites in the three regions are corresponding in the form of reticular,slate and stripes.The microhardness value is the largest at the weld,followed by the HAZ,and the microhardness value at the fusion line is the smallest.With the increase of heat input and weld reinforcement,the microhardness and the content of ?-ferrite are decreased in the weld and HAZ.(2)The residual stress distribution of the local repair welding parts was studied by the combination of X-ray diffraction and ABAQUS numerical simulation.It is shown that the results of the finite element calculation are consistent with the experimental results.The large transverse and longitudinal residual stresses are distributed in the weld and HAZ,and the longitudinal residual stress is significantly greater than the transverse residual stress.The transverse residual stress at the starting point of the welding is lower than the stopping point,while longitudinal residual stress has little difference between the starting point and the stopping point.(3)The heat input,weld reinforcement and repair length have a great effect on transverse stress.With the increase of heat input and repair length,the transverse stress in the weld and HAZ is obviously decreased,while weld reinforcement reducing the transverse stress is mainly controlled by the number of weld layers.The heat input,weld reinforcement and repair length have little effect on longitudinal stress.(4)The stress corrosion cracking behavior of the local repair welding part was studied by the combination of slow strain rate tensile test(SSRT)and scanning electron microscopy(SEM)fracture analysis.It is shown that both in the inert medium(air)and the corrosive medium(3.5% NaCl solution),the HAZ are brittle fracture,while the weld and parent metal are ductile fracture.The weld zone is insensitive to the corrosive medium due to the presence of semi-reticular and reticulated ?-ferrite,while the base metal and HAZ have a certain sensitivity to the corrosive medium.In addition,it is found that the original defects edge,the starting and stopping weld points are the dangerous parts of the repair welding equipments.(5)With the increase of heat input,weld reinforcement and repair length,the fracture time and elongation of repair welding specimens accordingly increase in both air and 3.5% NaCl solution.With the increase of heat input and weld reinforcement,the SCC sensitivity in 3.5% NaCl solution of repair welding specimen accordingly decrease.But the SCC sensitivity of repair welding specimen has an increase tendency with the increase of repair length.