Combined with the technologies of underwater local dry laser cladding (ULDLC) and underwater local dry laser remelting (ULDLR), a duplex stainless steel (DSS) coating has been made in an underwater environment. The phase composition, microstructure, chemical components and electrochemical corrosion resistance was studied. The results show that after underwater laser remelting, the phase composition of DSS coating remains unchanged and the phase transformation from Widmanstätten austenite + intragranular austenite + (211) ferrite to (110) ferrite occurred. The ULDLR process can improve the corrosion resistance of the underwater local dry laser cladded coating. The corrosion resistance of remelted coating at 3 kW is the best, the corrosion resistance of remelted coating at 1kW and 5kW is similar and the corrosion resistance of (110) ferrite phase is better than grain boundary austenite phase. The ULDLC + ULDLR process can meet the requirements of efficient underwater maintenance, forming quality control and corrosion resistance. It can also be used to repair the surface of S32101 duplex stainless steel in underwater environment.

Taking S32101 duplex stainless steel as the research object, underwater laser wire filling welding technology was used for U-groove filling welding. The influence of different shielding gas compositions on the ferrite content, microstructure, mechanical properties and pitting corrosion resistance was studied by simulating a water depth of 15 m in the hyperbaric chamber. The results show that, under the same process parameters, the size and proportion of austenite in the weld when using pure nitrogen as the shielding gas are larger than those protected by other shielding gases. In a mixed shielding gas, the increase in nitrogen content has little effect on the strength and toughness of the weld. Regardless of the shielding gas used, the base metal was the weakest part of the weld. At the same time, intermetallic inclusions have an adverse effect on the impact toughness of the weld. The pitting corrosion resistance of the welds depends on the Cr₂N content in the heat-affected zone. The precipitation and enrichment of Cr₂N causes local chromium deficiency, which is the main factor for the weak pitting corrosion ability of the heat-affected zone. Pure nitrogen protection has a better corrosion resistance than other gas protection.

To address the deficiencies in the fillet welding position of the clad plates in the simulated spent fuel pool, an underwater test platform was designed for local dry laser fillet welding under both normal and high-pressure environments. This study focused on multilayer and multipass (MLMP) welding repairs, specifically examining the local underwater laser dry fillet welding process using filler wire made of duplex stainless steel S32101 for third-generation nuclear power stations in air environment (AE), underwater environment (UE), and high-pressure underwater environment (HPUE). The analysis included the microstructure, ferrite content, phase composition, chemical composition, microhardness, and localized corrosion resistance of the fillet welds across the three environments. The main conclusions are as follows: under HPUE, the grain size is the smallest, the cooling rate is the fastest, and the weld consists of intragranular austenite (IGA), grain boundary austenite (GBA), and Widmanstätten austenite (WA); the ferrite content in the heat-affected zone (HAZ) is greater than that in the weld zone, with the highest ferrite content observed in the HPUE; the full width at half maximum (FWHM) of δ (110) and γ (111) varies across different environments; the manganese (Mn) content in HPUE is low; the hardness of the weld zone is the highest, followed by the HAZ, while the base metal (BM) exhibits the lowest hardness; and the pitting corrosion resistance in HPUE is the strongest, attributed to the rapid cooling rate and the pressure of nitrogen (N2) gas velocity.