Underwater laser directed energy deposition of NV E690 steel

Powder-based laser direct metal deposition (DMD), one of the directed energy deposition, was applied in air and underwater to repair pre-machined NV E690 steel plates. Systematic investigations on the effects of underwater environment and ambient pressures (0.01–0.35 MPa) on the microstructure evolution, phase transformation, and mechanical properties were conducted. The water quenching effect refined the grain size and increased the dislocation density and lath martensite content. The theoretical models of the underwater pressurized nitriding process and the precipitation kinetics of (Ti, V)N particles were established. Moreover, the microstructure evolution and the mechanical properties of other underwater DMD repaired samples did not show obvious relation with the underwater ambient pressures. This investigation not only provides a candidate for the underwater restoration technique but also bridges marine engineering and emerging DMD technology.