Inert shielding noble gas

When welding with inert gases like argon or helium, it's about completely shielding the molten pool from the surrounding air. The noble gases don't react with the material; they just displace oxygen and nitrogen, which would otherwise form oxides and degrade the structure.

If you slack off and use too little gas or there's a draft in the hall, you'll notice it right away on the weld: it becomes porous, the surface is dull and discolored, and the weld loses strength. The gas flow is only set correctly when the weld looks clean and shiny and no temper colors appear.