The effect of back shielding gas types and contained oxygen level on the corrosion resistance of weld was investigated for gas tungsten arc welding (GTAW) of UNS S32750 super duplex stainless steel (SDSS) pipes. The SDSS pipes were welded with three types of back shielding gases of nominal 100% Ar, 100% N2 and a mixture of 90% N2+10% H2 along with controlled O2 contents at 100, 500 and 2,000 ppm as pre-mixed gas. It was found that 100% N2 back shielding gas improves the corrosion resistance compared to 100% Ar, however, a mixture of 90% N2+10% H2 had an adverse effect. In addition, the specimen employing 100% N2 back shielding gas with 2,000 ppmO2 was acceptable in corrosion test. It was observed that the corrosion resistance of SDSS weld joint is mainly affected by depletion of nitrogen in the surface of root bead and less affected by oxygen content levels. The nitrogen depletion behavior is affected by the type of back shielding gas.
Duplex stainless steel has a dual phase microstructure consisting with ferrite and austenite. With the two phases appropriately balanced, it shows better resistance to pitting corrosion and stress corrosion cracking and also higher mechanical strength than austenitic stainless steel. Due to its better cost performance, application of duplex stainless steel to pressure vessels and piping has been expanding in oil and gas industries in recent years.
The weldability of duplex stainless steel is generally as good as austenitic stainless steel, however, in order to achieve the desired corrosion resistance and mechanical properties, ferrite and austenite phases in weld metal and heat affected zone (HAZ) shall be properly balanced by controlling welding parameters such as cooling rate and the use of nitrogen-containing shielding gas. When its phase ratio is extremely imbalanced, due to overheating during welding for example, the weld joint indicates insufficient ductility and low corrosion resistance. Oxidation on metal surface may also greatly affect the corrosion resistance.