Welding of ferritic steel tube material T91 (9Cr-1Mo), was used because of high temperature withstanding, mechanical properties, compatibility with chemicals and adequate weldability. The main objective of this project is to find out near optimum welding parameters with appropriate shielding gas in gas tungsten inert gas welding (GTAW) of T91 tube. The quality and productivity of the weld joint strongly depends on welding parameters. The perfect arc during welding is produced when the welder uses the optimum welding parameters. The composition of shielding gas plays an important role ensuring sound quality of welds. The composition of shielding gas with the molten pool has a marginal influence in the depth of penetration. The optimized parameters with three samples were prepared with different purging gases Viz. 100% helium, 100% Argon, Argon 50% plus Helium 50% for welding of 2.9 mm thick T91 tube with ER 505 filler wire. © 2020 Faculty of Mechanical Engineering.