Shielding gases for TIG and plasma-arc welding
The normal gas for TIG welding is argon (Ar). Helium (He) can be added to increase penetration and fluidity of the weld pool. Argon or argon/helium mixtures can be used for welding of all grades. In some cases nitrogen (N2) and/or hydrogen (H2) can be added in order to achieve special properties. For instance, the addition of hydrogen gives a similar, but much stronger effect as adding helium. However, hydrogen additions should not be used for welding martensitic, ferritic or duplex grades.
Alternatively, if nitrogen is added, the weld deposit properties of nitrogen alloyed grades can be improved. Oxidizing additions are not used because these destroy the tungsten electrode.
Recommendations for shielding gases used in TIG welding of different stainless steels are given in the table. For plasma-arc welding, the gas types with hydrogen additions in the table are mostly used as plasma gas, and pure argon as shielding gas.
|Parent metal (type of material)|
|High-alloy austenitic stainless steel||Nickel alloys|
|Ar + Hea||●||●||●||●||●||●a|
|Ar + (2-5)% H2a,b||●b||-||-||-||●b||●b|
|Ar + (1-2)% N2||-||●||●||-||-||-|
|Ar + 30% He + (1-2)% N2||-||●||●||-||-||-|
a) Improves flow compared with pure Ar.
b) Preferably for automatic welding. High welding speed. Risk of porosity in multi-run welds.