Influence of shielding gas composition on nitrogen retention, phase balance, and corrosion performance of super-duplex stainless steel weld overlays produced by mechanised GMAW

The selection of shielding gas plays an important role in controlling weld metal chemistry, phase balance, and corrosion performance of super duplex stainless steel (SDSS) weld overlays produced by mechanised gas metal arc welding (GMAW). This study compares five representative industrial shielding gas mixtures (Ar, Ar + 1%O₂, Ar + 30%He + 1%O₂, Ar + 15%He + 2.5%N₂, and Ar + 1%N₂) for SDSS overlays deposited on SA-516 Gr.70 carbon steel using an ER309LMo barrier layer and ER2594 filler metal. The overlays were characterised by optical emission spectroscopy, ferrite measurements, metallography, hardness, guided bend testing, and ASTM G48 pitting and crevice corrosion tests. Shielding gas composition and associated welding thermal conditions influenced weld chemistry, nitrogen retention, PREN, ferrite–austenite balance, hardness, and localised corrosion behaviour. Nitrogen-containing gases generally promoted the required nitrogen content and PREN at lower overlay thicknesses than oxygen-containing and pure argon gases. All overlays exhibited acceptable duplex microstructures without detrimental intermetallic phases. Hardness ranged from 260 to 283 HV, and all bend specimens satisfied ASME Section IX requirements. Ar + 15%He + 2.5%N₂ provided the most favourable overall combination of nitrogen retention, ferrite balance, PREN, and corrosion resistance under the investigated conditions.