Tailoring of the Microstructure and Mechanical Properties of High Nitrogen Steel Repaired by Underwater Laser Direct Metal Deposition

The influence mechanism of shielding gas composition (SG1 with 50 vol pct N 2 + 50 vol pct Ar, SG2 with 70 vol pct N 2 + 30 vol pct Ar, and SG3 with 90 vol pct N 2 + 10 vol pct Ar, the corresponding samples were named as sample SG1, sample SG2, and sample SG3, respectively) on phase transformation and mechanical properties of high nitrogen steel (HNS) repaired by underwater laser direct metal deposition (UDMD) at an ambient pressure of 0.3 MPa was systematically elaborated. Results showed that the elevated nitrogen content in shielding gas increased the nitrogen partial pressure above the molten pool, thereby enhancing the nitrogen solubility inside the molten pool. This led to a reduction in porosity defects from sample SG1 to sample SG2 and their virtual absence in sample SG3. Meanwhile, the elevated nitrogen partial pressure facilitated nitrogen absorption process from heat source space, thereby raising the austenite transition starting temperature and increasing austenite fraction from sample SG1 to sample SG3. Some of the absorbed nitrogen atoms dissolved into the matrix, providing extra solid solution strengthening, while the remaining ones reacted with Cr to form Cr 2 N. Furthermore, the elevated nitrogen content in shielding gas would compromise the protective effect on the molten pool by enlarging the compressed air (drainage gas) backflow area, thereby introducing more oxygen into the molten pool, and resulting in more oxide formation. Additionally, with the increase of nitrogen with higher thermal conductivity in shielding gas, the molten pool cooling rate would be accelerated. Consequently, the refined grain and denser dislocation network were expected. Approximately, 70 vol pct of nitrogen content in shielding gas was recommended, as it effectively inhibited porosity defects and regulated the formation of intermetallic compounds (oxides and Cr 2 N), leading to superior mechanical properties (sample SG2).