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One of my most popular videos does a break down with visuals of proper vs improper
TIG Welding 304 Stainless Exhaust
As mentioned by others the same shielding gas (argon) protects the weld from the inside of the tube by eliminating atmospheric gases which allow unspeakable horrific things to happen aka sugaring
Interesting stuff. I won't be welding anything anytime soon, but it's cool to see how it's done.