Evolutions of microstructure and mechanical property of high nitrogen steel repaired by the underwater directed energy deposition technique

Abstract

Damaged high nitrogen steel (HNS) plates were repaired by underwater laser direct metal deposition (UDMD) technique at the ambient pressure of 0.3 MPa. Both experiments and finite element method (FEM) simulations were employed to elucidate the influence mechanism of the underwater hyperbaric environment on the microstructure evolution. Carbides were the most typical precipitates in the HNS selected in this research, and their features were dominated by molten pool cooling rate and the intrinsic heat treatment (IHT) effect associated with the thermal cycles. Accelerated cooling rate induced by water chilling effect or the weakened laser energy input promoted the carbide precipitation. Against the build direction, carbides became coarse and the overall content decreased due to the cumulative IHT effect. Such carbide features evolution trend in UDMD repaired sample was more pronounced than that in DMD repaired sample. The shortened high-temperature duration owing to the intense thermal dissipation in the underwater environment highlighted the proportion of intrinsic heat treatment (IHT) effect on carbide features and dendrite size. Owing to the slow air cooling and enhanced thermal accumulation in the ground environment, the prolonged high-temperature duration facilitated the decomposition of unstable austenite into lamellar intragranular carbide and ferrite. Benchmarking mechanical properties against the DMD repaired sample counterpart demonstrated that UDMD repaired samples possessed higher strength and comparable impact toughness.