Stainless Steel — What It Is and Why It Matters
Stainless steel is a family of corrosion-resistant alloys containing at least 10.5% chromium. The chromium forms a passive oxide layer on the surface that self-heals when scratched, providing the characteristic stain resistance. Common welded grades include 304 (general purpose), 316 (marine and chemical resistance), and 430 (ferritic, lower cost).
Welding stainless requires more care than mild steel. Heat input must be controlled to prevent carbide precipitation (sensitization) in austenitic grades, which destroys corrosion resistance in the heat-affected zone. Low-carbon "L" grades (304L, 316L) are designed to minimize this problem. Back-purging with argon is recommended on the root side of stainless welds to prevent sugaring (oxidation that creates a porous, weak root).
Standard filler metals match the base: ER308L for 304 stainless, ER316L for 316 stainless, and ER309L for joining stainless to carbon steel. TIG welding is the preferred process for thin-gauge stainless work; MIG with tri-mix gas (helium-argon-CO2) is common for production.
Frequently Asked Questions
Why does stainless steel discolor when welded?
The rainbow colors (straw, gold, blue, purple) around a stainless weld are oxide layers formed by heat exposure to atmospheric oxygen. Heavier discoloration indicates higher heat input. Minimizing heat input, using proper gas coverage, and back-purging significantly reduce discoloration.
What gas is used for TIG welding stainless?
Pure argon is the standard shielding gas for TIG welding stainless steel. For back-purging the root side, argon or a specialized forming gas (argon-hydrogen mix) is used to prevent oxidation on the backside of the weld.