Effect of nitrogen in backing gas on duplex root weld properties of heavy-walled pipe

Duplex stainless steels are increasingly used for offshore subsea components as the technology allows for deeper recovery of oil and gas reserves. Alloy UNS S31803 (EN 1.4462/UNS S32205) combines high strength with good resistance to pitting and crevice corrosion. This grade is alloyed with nitrogen and has good weldability. For heavy-walled process piping intended for subsea sour service, the properties of the root pass are of high importance. For this reason, the effect of using nitrogen in the backing gas was investigated. Test pieces were manually welded with the gas tungsten arc welding (GTAW) process using an ER2209 filler wire. Tensile, bending and hardness testing was carried out and showed acceptable results across all specimens. Higher contents of nitrogen in the backing gas showed more austenite formation and a significant increase in pitting corrosion resistance when compared with pure argon. With Ar + 2 % N2, there was no pitting, but a small weight loss of 1.7 g/m2 was measured. 10 % N2 was required to pass both the corrosion tests ASTM G48 Method A (4 g/m2) and ASTM A923 (1 g/m2). The highest average impact toughness was achieved with pure nitrogen as the backing gas.