Influence of CaF₂ on microstructural characteristics and mechanical properties of 304 stainless steel underwater wet welding using flux-cored wire

Underwater wet welding (UWW) of 304 stainless steel was performed using a set of wires with various calcium fluoride contents. The appearances, compositions, microstructural characteristics and mechanical properties of welding joints were analyzed. As the proportion of CaF₂ rose, the welding appearances had been improved owing to the increasing coverage of slag. The depth-width ratio descended from 33.5% to 15.7% with an increase of CaF₂ from 0% to 65% due to the reduction of the molten pool heat. Microstructural observations showed that the fully austenitic weld metal was consisted of four regions and the proportion of each region varied with the content of CaF₂. The t_8/5 cooling times of the heat affected zone were prolonged from 4.4 s to 6.1 s as the CaF₂ content increased from 0% to 65%, which contributed to the microstructure variation. Besides, the uppermost element in the welds was Ni (at least 65%) whose content gradually went up with more and more CaF₂ in the wire. Under the same conditions, the concentration of the solutes in the Ni-based welds lowered, encouraging the dislocation to be smaller and more dispersive. It was 20% CaF₂ that the wire contained when the weld reached the best mechanical property with 522 MPa tensile strength and 132.74 J/cm² impact toughness.