Abstract
Duplex stainless steels are widely used in oil and gas pipelines both offshore and onshore due to their good mechanical properties and corrosion resistance. The microstructure of duplex stainless steel consists of two phases: austenite and ferrite. Phase balance between austenite and ferrite is directly affected by welding parameters; and thus, may lead to the deterioration of the corrosion resistance and mechanical properties of the welds. This research is aimed to study the effects of increasing nitrogen content into argon backing gas mixture ratios (100%Ar, 5%N2+95%Ar, 10%N2+90%Ar, 15%N2+85%Ar and 100%N2) on the changes in microstructure and pitting corrosion resistance in gas tungsten arc welded duplex stainless steel ASTM A790 pipes using ER2209 filler metal and 100%Ar shielding gas. During welding, heat input was maintained below 1.5 kJ/mm and the interpass temperature not exceeding 120°C. After welding, nitrogen contents, hardness, microstructure, and pitting corrosion resistance were measured. The results showed that the pitting potential was highest in the welded specimen using 100%N2 backing gas. In addition, the presence of secondary austenite was found to be highest in specimen welded with 100%Ar backing gas, and decreased with increasing nitrogen content in the backing gas.
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