Research On High Nitrogen Austenitic Stainless Steel By PMIG Welding Robot

The purpose of this paper is to research the weldability of high nitrogen austenitic stainless steel by PMIG welding with 307Mo weld wire. The methods to increase the nitrogen content of the weld metal, the nitrogen behavior, microstructure and mechanical properties of the HNS weld joints are researched.In the welding of HNS by PMIG, the N-content of the weld metal increases as the nitrogen in the shielding gas containing Ar and N2 increases under the same heat input. The highest content of N in the weld can be 0.42%. The welding heat input can also impact on the nitrogen content in the weld. By using Ar+N2 shielding gas, the lower the heat input is, the higher N-content in the weld metal will be obtained.Adding some active O2 in the shielding gas, the N-content of the weld metal can be very high. The content of nitrogen in the weld can increase to 0.60%(67% N-content of the HNS parent metal).The nitrogen pore will increase when the N-content of the weld metal is higher. When the N-content of the weld metal lower than 0.20%, there is no nitrogen pore in the weld. When the content of nitrogen is higher than 0.38%, nitrogen pores can be found in the weld.With the same welding heat input, the nitrogen content in the weld has significant effect on the weld solidification mode and microstructure. When the N-content of the weld metal is lower than 0.24%, the solidification mode is FA and the microstructure is austenite and ferrite. The higher N-content in the weld metal, the little ferrite in it. When the N-content of the weld metal is higher than 0.30%, the solidification mode is A and the microstructure is single austenite. The austenite dendrites become progressive coarse while there is a higher nitrogen level. No nitrides and hot cracks can be found in the weld joints.Hardness test shows that the weld has higher hardness with the higher nitrogen level. The weld has a good tensile strength, about 892MPa, it is 75% of the HNS’s level. The higher N-content in the weld also can benefit the toughness, but if the N-content is too high, nitrogen pores can damage the toughness of the weld.