Mechanism of shielding gas nitriding improving fatigue performance of 2205 duplex stainless steel pipe joint

P-Tig welding method was adopted to connect UNS S32205 duplex stainless steel (DSS) thin pipe, 2205 DSS pipe welded joint with excellent performance was obtained under different N 2 percentages of shielding gas. The effects of nitriding of shielding gas on microstructure, two-phase distribution, and dislocation distribution of welded pipe joints were analyzed by a series of microstructural characterizations. The experimental results show that N 2 in the shielding gas will enter the austenite grain interior in the form of interstitial atoms after arc ionization, which plays a role in strengthening austenite, reducing the two-phase hardness difference, among which the two-phase hardness difference of the welded joint under the 4 vol.% N 2 shielding gas is the smallest. Additionally, at low stress levels, dislocation proliferation mainly occurs within the austenite grains. The reduced dislocation accumulation at the grain boundaries of welded joints under 4 vol.% N 2 shielding gas slows down the initiation of micro-cracks at the grain boundaries, which is the fundamental reason for the improvement in the fatigue life of welded joints.