The Investigation Of Welding Procedure And Microstructure And Properties Of Welding Joint Of High Nitrogen Steel

Compared with Ni-Cr austenitic stainless steel（γ） which is traditional and had been widely used, the high nitrogen austenitic（γ） has many excellent properties, such as high strength, high toughness, high strain hardening ability, strong corrosion resistance and low magnetization and so on. So it gradually developed into a new type of structural materials,whether it can be widely used to a certain extent depends on its welding performance.Because of the high content of nitrogen in the high nitrogen steel, the loss of nitrogen emission, the formation of nitrogen compounds, the formation of pores and hot crack may occur in the welding process. These defects lead to a poor welding performance and the welding joints lose the superior performance of the original high nitrogen steels. Therefore,the research on the welding process of high nitrogen steel is of great theoretical significance and practical value.In this paper, gas metal arc welding（GMAW） tests were carried out on high nitrogen nickel-free austenitic stainless steel.The effects of the welding parameters, gas composition,welding wire composition and different processing conditions on the microstructure and properties of welding joints of high nitrogen steel were studied; The optical microscopy,scanning electron microscopy, energy spectrum analysis, XRD analysis and other equipment were used for the analysis of organization and phase of the welding joints. The mechanical properties of welding joints were tested by micro-hardness test and impact toughness test.The electrochemical corrosion properties of high nitrogen steel in 3.5%NaCl aqueous solution and the corrosion resistance of high nitrogen steel in aqueous solution of10%H₂SO₄ and 5%NaCl of three kinds of process state of casting, hot rolling and welding seam was investigated.The experimental results within the range of the experimental parameters show that:（1）The microstructure of high nitrogen welding joint tend to be consistent, dendritic austenite（γ）+a small amount of precipitation of δ ferrite; the banded structure is dispersed in the equiaxial austenitic（γ） matrix of the heat affected zone（HAZ） due to structure heredity;the grain size of austenite in HAZ is the same as that of base metal since no grain growth occurs; the trasition of the fusion zone between the weld and the base metal is good, and the grain boundary becomes wider and the δ ferrite percipitates.（2）The hardness of the weld increased with the increasing of the volume fraction of N₂ in Ar shielding gas, the hardness of the HAZ and the hardness of the weld shielded by Ar is higher than that of base metal, the toughness of HAZ is better due to the improvement of the bonding state between the banded structure and the austenitic matrix.（3） The hardness of welded joints of the hot rolled high nitrogen steel, as cast high nitrogen steel and as solid solution high nitrogen steel tend to be consistent. There are not softening zone in the welded joints. With the increase of heat input, the hardness of the HAZ increases while the hardness of the welded joints and the fusion zone decreases. In comparison with base metal, the hardness of HAZ is higher which can be mainly attributed to a large number of regions enriched with Cr-N hindering the movement of dislocation in the austenitic matrix;（4） The electrochemical test results of the as cast high nitrogen steel, hot rolled high nitrogen steel and weld tend to be consistent. Compared with 316 L, the passivation zone is relatively wide, with the increase of potential, the current first decreased and then increased slowly, “the second passivation phenomenon” occurred when the potential is higher, the electrochemical corrosion was significantly better than 316L; The Electrochemical corrosion resistance of HNS is related to the attractive effect of N on Cr in the inner layer of passivation layer was the strongest, which lead to the compactness of Cr₂O₃ in the inner layer of passivation layer higher than the surface, But the corrosion resistance in 3.5% NaCl aqueous solution reduced. This is related to the microstructure of high nitrogen and the inhomogeneous distribution of alloy elements which decreased the corrosion resistance.