The influence of nitrogen in shielding gas on the 316LN austenitic stainless steel welded joints

Abstract

To ensure the stable operation of fusion reactors and synchrotron radiation facilities, the magnetic permeability of the weld joints in 316LN austenitic stainless steel (ASS) must be μ_r ≤ 1.03. Therefore, controlling magnetic permeability during welding is essential. This paper examines the impact of nitrogen in the shielding gas on the magnetic permeability, microstructure, and mechanical properties of 316LN welded joints. This was achieved by using argon-nitrogen mixed shielding gas to weld the test plates. The findings indicate that the primary microstructures of the welded joints consist of austenite and a small amount of ferrite. The magnetic permeabilities are measured at 1.026, 1.015, 1.010, 1.007, 1.004, and 1.005, respectively. The main factor contributing to the decline in magnetic permeability with increased nitrogen content in the shielding gas is the reduction in ferrite content. Additionally, adding nitrogen to the shielding gas improves the mechanical properties of the welded joints at 4.2 K. The ultimate tensile strengths of the welded samples using pure argon gas and argon-nitrogen mixed gas (1 % and 2 % nitrogen) were 1423.73 MPa, 1465.49 MPa, and 1546.45 MPa, respectively. The impact energy of the samples was 38 J, 139 J, and 137 J, respectively. Analysis of the fracture surfaces reveals that the argon-nitrogen mixed gas welded samples exhibit ductile fracture with dimples and micropore morphology. In contrast, the pure argon gas welded samples show the combination of ductile and brittle fracture. It is proposed that fine grain strengthening and the dispersion strengthening effect of N is the main factors in improving strength. This study provides an effective guide for the application of the welded joint of 316LN ASS at cryogenic temperature.