Comparison of 304L Stainless Steel Sealing Overlay Welding Processes in Air and Underwater Environments
Nuclear energy has been recognized as one of the best options for meeting the growing demand for electricity.Currently,the cladding of the spent fuel pool of a second-generation nuclear power plant is made of 304L stainless steel.Corrosion cracks,pitting corrosion,and other defects are inevitable because of long-term immersion in a boric acid environment.As the defects further expand,they penetrate the entire stainless-steel wall panel,which may lead to leakage of the spent fuel pool and pose major safety concerns.Spent fuel is radioactive;therefore,construction personnel cannot work in such an environment.Therefore,welding repair in an underwater environment is particularly important.However,relatively little research has been conducted worldwide on welding repair in underwater environments.In this study,an ER-2209 duplex stainless-steel solid welding wire was used to prepare a sealed surface layer on 304 L stainless steel in both air and underwater environments.The results showed that the weld bead formations on the surface of the surfacing layer were continuous and uniform,without defects such as cracks,slag inclusions,weld beadings,or undercuts.The microstructure inside the surfacing layer includes both duplex and austenitic stainless steels,with a regional distribution between the two.The proportion of δ grains in the microstructure of duplex stainless steel in the rapid cooling mode was high,the grain size was large,and a large amount of SF precipitated from austenitic stainless steel.The tensile strength obtained from the stress-strain curve of the transverse tensile specimen of the overlay layer was higher than that of the BM.A martensitic transformation was induced during the deformation process,which increased the strength of the fracture after the stress-strain curve of the BM,and the overall elongation was greater than that of the overlay layer.The crystalline orientation of the austenite grains was deflected at the fusion point between the weld beads and the texture strength of the weld bead was enhanced.In both environments,the top transverse tensile specimen exhibited higher tensile strength and lower elongation.Owing to the rapid cooling effect of the underwater environment,the microstructure of duplex stainless steel,δ,and grain orientation,y,underwent a suppressed transformation,which resulted in larger grain sizes(δ).The stacking fault energy of ferrite was higher,and the hardness of the surfacing layer increased.Rapid cooling promotes the formation of a large amount of flat noodle ferrite distributed along the austenite grain boundaries in austenitic stainless-steel structures.Flat noodle ferrite is distributed regionally,which improves the hardness of austenitic stainless-steel structures.The self-corrosion current density of Al in the activation zone was close to that of the base material,whereas W1 had a higher self-corrosion current density.This is because of the rapid cooling rate during underwater welding,which causes insufficient phase transformation of the weld microstructure and suppresses the diffusion of the corresponding chemical elements.A clear passivation range was observed in the polarization curve,and the surface layer exhibited good corrosion resistance.304L stainless-steel surfacing layers were prepared in both air and underwater environments.The macroscopic morphology,microstructure,and phase composition of the surfacing layer were studied,and the mechanical properties,microhardness,and corrosion resistance of the surfacing layer were analyzed.This research plays an important role in the shallow crack repair process of 304L stainless steel,which has laid a solid theoretical and practical foundation for the practical engineering application of underwater laser welding.
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