A Comparative Study Between Laser Beam Welding And Tungsten Inert Gas Welding Of AISI304 Stainless Steel

As sustainable clean energy,nuclear power has gradually become an important part of modern energy.Plenty of significant structures of nuclear power which are made of 304 austenitic stainless steel,such as spacer column assembly and reactor vessel internals,are usually fabricated by welding techniques.However,conventional fusion welding methods,such as tungsten inert gas welding(TIG),cannot meet the ever-growing requirements of the production efficiency and quality.Laser beam welding(LBW)is a process increasingly used in fabrication industry because of its many advantages over conventional fusion welding methods,such as low heat input,small heat affected zone(HAZ),quick welding speed and deep penetration while it hasn't widely used in nuclear power industry.This study fabricated two kinds of joints made of 304 austenitic stainless steel pipes which are induced by laser beam welding(LBW)and tungsten inert gas welding(TIG)respectively.Microstructure characterization,electron backscattered diffraction analysis,micro hardness test,slow strain rate test,numerical simulation method and a series of research methods were carried out to compare the microstructure,mechanical properties,residual stress and stress corrosion sensitivity of the two joints.The comparison are used to evaluate whether laser beam welding could be utilized in nuclear power instead of TIG welding.The microstructure results show that weld metal of the two joints are both composed of austenite matrix and ?-ferrite dendrites and the dendrites have different growth directions due to the different directions of heat dissipation.The average grain size of LBW joint is higher than that of TIG joint while the residual stress and welding deformation of LBW joint is smaller than that of TIG joint.According to the hardness test results,the micro hardness of TIG joint gradually reduces from weld metal to base material showing obvious hardening characteristics.However,the micro hardness of LBW joint is close to base metal and shows no hardening behavior.The residual stress results obtained by numerical simulation method is consistent with the results measured by XRD method.The axial residual stress and hoop residual stress of the two joints have different trends.The welding residual stress of the joints are different in values and distributions along the thickness direction and on outer surface.Overall,the range of high hoop tensile stress of LBW joint is smaller than that of TIG joint.Moreover,because of the lower total heat input,LBW joint has smaller heat affected zone and welding deformation than TIG joint.The composition and intensities of oxidation films on fracture surfaces of the two joints are similar and the tensile strength of LBW joint is lower than that of TIG joint.The tensile strength of joints in the simulated primary water is slightly lower than that in air condition showing stress corrosion cracking behavior.In addition,fracture micrographs of the joints show that there are both ductile fracture and brittle fracture occurred during fracture process suggesting stress corrosion phenomenon.Although both the joints have good stress corrosion resistance,TIG joint has higher stress corrosion sensitivity than LBW joint.It is because of the relatively high residual stress of TIG joint,especially in the heat affected zone which is lack of corrosion-resistant ?-ferrite.According to the results above,these two joints both have good forming and no defects could be found.The mechanical properties of LBW joint is close to that of base metal.Considering the welding efficiency,welding residual stress,welding deformation and stress corrosion sensitivity,LBW is an advantageous welding method with many superiorities and research space that can meet the requirements of nuclear power industry compared to conventional TIG welding.