High-quality remanufacturing of HSLA-100 steel through the underwater laser directed energy deposition in an underwater hyperbaric environment

Underwater laser directed energy deposition (UDED) can be employed to repair and maintain the offshore engineering structures due to its advantages of flexible adjustment of feedstock materials and controllable heat input to the structures. For the first time, preprepared HSLA-100 steel plates were successfully remanufactured by UDED at an ambient pressure of 0.3 MPa (water depth of 30 m). The relationships between the hyperbaric underwater environment, solidification process, microstructures and mechanical properties of the HSLA-100 steel were clarified. The results show that the cooperation of surrounding water, central gases and gas curtain gas with large flow rates increased the cooling rates of the underwater melt pool. A lath martensitic microstructure with high dislocation densities and a number of inclusions was formed in the sample remanufactured by UDED. The in-situ precipitation of Cu-enriched nanoparticles was caused by the unique intrinsic heat treatment involved in the underwater deposition process. The average diameter of the Cu-enriched nanoparticles increased with increasing laser energy density. The microstructure of the sample remanufactured by UDED was harder than that of the sample remanufactured by in-air DED. The Charpy impact toughness and tensile properties of the samples remanufactured by UDED were close to those of the sample remanufactured by in-air DED. This work demonstrates the feasibility of high-quality remanufacturing of HSLA-100 steel via UDED in a hyperbaric underwater environment. The results obtained in this study could provide useful guidance for the application of UDED to offshore engineering structures. •The HSLA-100 is firstly repaired by UDED in an underwater hyperbaric environment.•The high ambient pressure influences the metallurgical processes and microstructures.•In-situ precipitation of Cu is determined by the unique underwater thermal cycles.•HSLA-100 steel repaired by UDED possesses high strength and excellent toughness.•This work expands the applications of UDED to the hyperbaric underwater environments.