Investigation On Microstructure And Properties Of SA553/AISI304 Dissimilar Joints Welded By Autogenous Fiber Laser

As global environmental issues become increasingly prominent,countries around the world are paying closer attention to environmental problems.Natural gas,as an environmentally friendly and efficient clean energy,is increasingly recognized by domestic and foreign markets.At present,the main material of large-scale LNG low-temperature storage tanks is martensitic steel SA553 with a nickel content of 9%,and the pipes and valves connected to the tank use austenitic stainless steel AISI304 as feedstock.The demands for connecting dissimilar steel SA553/AISI304 has been increasing.Compared with arc welding,laser welding has the advantages of high welding quality,fast speed,high efficiency,narrow weld seam,and energy concentration,etc.It has outstanding advantages in the field of dissimilar welding.However,the use of fiber laser for SA553/AISI304 dissimilar steel connections has been still scarce,and it is necessary to conduct in-depth studies on element distribution,microstructure characteristics,and cryogenic performance of SA553/AISI304dissimilar joints.In this project,an autogenous fiber laser welding method was used to obtain a10mm thick SA553/AISI304 dissimilar joints.By adjusting the process parameters and using nitrogen as shielding gas,joints with good formation and no pores in the weld were obtained.The results showed that process pores are related to the intrusion of shielding gas into the molten pool due to the instability and collapse of the keyhole.When N₂ was used as shielding gas,nitrogen reacted with the liquid metal to help the pores dissolve,thereby ultimately inhibiting the formation of pores.The formation of root humps is related to the strained condition of the bottom molten pool.Adding back-side shielding gas can provide upward holding force for the molten pool,and accelerate the cooling rate at the bottom of the molten pool,thereby inhibiting the formation of root humps.The results showed that while using the backside shielding gas,increasing the positive defocus amount helped reduce the recoil pressure of the keyhole metal vapor on the bottom molten pool,which can further suppress the molten pool from sinking and obtain good weld formation.By changing the offset of the incident laser beam,the fusion ratio of the base metal was adjusted.The experimental results showed that with the incident beam moving towards the stainless steel AISI304,the fusion ratio of AISI304 gradually increased,the content of Cr in the weld seam increased significantly,and the retained austenite content also increased.The weld microstructure changed from single martensite to two-phase microstructure of martensite+retained austenite,and to three-phase microstructure of martensite+retained austenite+?-ferrite finally.When the microstructure is martensite+retained austenite,the cryogenic impact energy of the joint was the highest,reaching41.48J,exceeding the 35J stipulated in the BS 7777 part 2 standard.When?-ferrite appeared in the weld,the cryogenic impact energy of the joint decreased.This is because?-ferrite as a brittle phase reduces the energy consumed by crack initiation and propagation,and accelerates the rate of crack initiation and propagation at low temperatures,which is bad for cryogenic toughness of joints.The results showed that the weld formation showed a typical nail shape with wide top and narrow middle when the fiber laser was used to weld SA553/AISI304 dissimilar joints.The composition and microstructure of the weld were more uniform in the middle and lower parts,but there were significant inhomogeneities in the upper part.The Cr content near the SA553 side was significantly lower than that near the AISI304side.Analysis indicated that this inhomogeneity was related to the flow of the molten pool driven by Marangoni convection.Under the function of Marangoni convection,the convection between different regions in the upper part was weakened,which hindered the diffusion and mixing of alloy elements and then caused inhomogeneities of composition,microstructure and hardness,which was detrimental for the performance of SA553/AISI304 dissimilar joints.