Microstructure and properties of underwater wet laser welded 0Cr25Ni6Mo3N duplex stainless steel joints

A self-designed welding flux was successfully applied to underwater wet welding of a 5 mm thick 0Cr25Ni6Mo3N duplex stainless steel in 6 mm simulated seawater. After welding, the microstructure of the weld centre, heat-affected zone, and base metal were observed using a metallographic microscope. Further, the microstructural evolution mechanism was derived, and the phase transition process of the metal in the fusion zone was analysed by thermodynamic calculations. After the experiment, the average tensile strength of the specimens was 811 MPa, which reached 95.3 % of the tensile strength of the base metal, and the average elongation was 35.5 %. The microstructure of the fracture showed obvious cleavage steps and a small number of tendons, which belonged to the mixed fracture type. The Vickers microhardness test results were as follows: the average values of the weld centre and the heat-affected zone are 297.7 HV and 278.0 HV. Electrochemical tests showed that the corrosion resistance of the weld were better and the passivation ability was weaker than that of the base metal.