He and He + CO₂ double shielded TIG welding process

He and He + CO₂ double shielded TIG welding process was proposed to study the electrode protection and weld shape variations of 0Cr13Ni5Mo low carbon martensite stainless steel used for hydraulic turbine runners. Pure inert gas(He) as inner layer,avoids the electrode directly contacting the outer layer active gas(CO₂) and being oxidized. Addition CO₂ to the outer layer shielding gas provides a surface active element(oxygen) which dissolves into the weld pool to change the Marangoni convection mode and weld shape. The results show that the electrode is successfully protected, and Marangoni convection changes from outward to inward to produce a deep and narrow weld when the weld oxygen content is between 0.0076%~0.012% in weld metal. The double shielded TIG welding is more than twice the welding efficiency of the conventional TIG welding.