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.

采用水下激光直接金属沉积（UDMD）技术在0.3 MPa的环境压力下修复损坏的高氮钢（HNS）板。采用实验和有限元方法（FEM）模拟来阐明水下高压环境对微观结构演化的影响机制。碳化物是本研究选择的 HNS 中最典型的析出物，其特征主要是熔池冷却速度和与热循环相关的固有热处理 (IHT) 效应。由水冷效应引起的加速冷却速度或减弱的激光能量输入促进了碳化物的析出。与构建方向相反，由于累积的 IHT 效应，碳化物变得粗大并且总含量下降。这种碳化物特征在UDMD修复样品中的演变趋势比DMD修复样品更明显。由于水下环境中的强烈散热导致高温持续时间缩短，这突出了本征热处理 (IHT) 对碳化物特征和枝晶尺寸的影响的比例。由于空气冷却缓慢和地面环境中热量积累的增加，高温持续时间的延长促进了不稳定的奥氏体分解为层状晶内碳化物和铁素体。对 DMD 修复样品对应的机械性能进行基准测试表明，UDMD 修复样品具有更高的强度和相当的冲击韧性。由于水下环境中的强烈散热导致高温持续时间缩短，这突出了本征热处理 (IHT) 对碳化物特征和枝晶尺寸的影响的比例。由于空气冷却缓慢和地面环境中热量积累的增加，高温持续时间的延长促进了不稳定的奥氏体分解为层状晶内碳化物和铁素体。对 DMD 修复样品对应的机械性能进行基准测试表明，UDMD 修复样品具有更高的强度和相当的冲击韧性。由于水下环境中的强烈散热导致高温持续时间缩短，这突出了本征热处理 (IHT) 对碳化物特征和枝晶尺寸的影响的比例。由于空气冷却缓慢和地面环境中热量积累的增加，高温持续时间的延长促进了不稳定的奥氏体分解为层状晶内碳化物和铁素体。对 DMD 修复样品对应的机械性能进行基准测试表明，UDMD 修复样品具有更高的强度和相当的冲击韧性。由于空气冷却缓慢和地面环境中热量积累的增加，高温持续时间的延长促进了不稳定的奥氏体分解为层状晶内碳化物和铁素体。对 DMD 修复样品对应的机械性能进行基准测试表明，UDMD 修复样品具有更高的强度和相当的冲击韧性。由于空气冷却缓慢和地面环境中热量积累的增加，高温持续时间的延长促进了不稳定的奥氏体分解为层状晶内碳化物和铁素体。对 DMD 修复样品对应的机械性能进行基准测试表明，UDMD 修复样品具有更高的强度和相当的冲击韧性。