Effects of MnN powder on the microstructure and properties of high nitrogen steel joint via laser-arc hybrid welding

The welding of high nitrogen steel (HNS) was accompanied with N loss and porosity, leading to the mechanical properties degradation. In this study, laser-arc hybrid welding of HNS with MnN powder was evaluated. The experimental results indicated that the addition of MnN powder could increase the N content by 17% compared with the non-nitride joints. And no MnN particles were detected in the joint. There were mainly two kinds of pores in the weld of HNS. The irregular pore was suppressed by adjusted welding speed and laser power. The degree of austenitization and the N pores in joint increased with the increase in the N content, because the MnN powder could promote the formation of N pores. The reactive pore was originated from the thermal decomposition of the MnN powder with a temperature more than 800℃. And the micro-pore was precipitated along the interface between austenite and ferrite because of the supersaturated N in ferrite. Adding Ti could give priority to the formation of the TiN phase and inhibit the formation of N pores. The tensile strength of the joint was increased to 1072 MPa. The impact property of the nitrided joint was also slightly improved. However, the tensile strength of the arc zone was better than that of the laser zone due to the uneven distribution of N.