Microstructure and mechanical properties of underwater laser deposition remanufactured 316LN stainless steel at a pressure of 0.3 MPa

Abstract

The nuclear power steel 316LN stainless steel (SS) was first repaired by underwater laser directed metal deposition (UDMD) technique with various processing parameters in a simulated 30 m underwater environment. The microstructure and mechanical properties of the as-repaired samples were systematically characterized. The results were compared with those of the samples repaired by in-air directed metal deposition (in-air DMD). The results indicate that the sizes of the columnar dendrites and equiaxed grains in the UDMD samples increased with linear laser energy. The dendrite size in the UDMD samples was smaller than that in in-air DMD samples due to the water quenching effect involved in the UDMD process. During UDMD, many oxide inclusions were formed in the repaired zones due to the influence of the water film in the underwater local dry area. The number of inclusions decreased with the increase of linear laser energy. The microhardness of the UDMD samples decreased with the increase of linear laser energy. The microhardness of the UDMD samples was higher than that of the in-air DMD samples due to the fact that high densities of dislocations, inclusions and M7C3 carbides were distributed in the UDMD samples. For the UDMD samples, the increase of linear laser energy led to decreasing tensile strengths and increasing impact energies. The mechanical properties of the UDMD samples were superior to those of the in-air DMD samples. The results obtained from this study could provide useful guidance for repairing underwater nuclear power materials by UDMD.