How to Weld Stainless Steel: MIG, TIG and Stick

Weld stainless steel with TIG when the parts are thin or the weld must look clean, with MIG for speed on thicker sections, or with stick for outdoor and repair work. Match the filler metal to the steel: ER308L or E308L for 304 stainless, 316L for 316, and 309L when joining stainless to mild steel. Then control the heat, keep carbon steel contamination away, and clean the weld when you’re done, so the joint keeps the corrosion resistance that made you choose stainless.

Which process for stainless?

Process	Best for	Gas / polarity	Notes
TIG (GTAW)	Thin sheet, tube, food and visible work	100% argon; DC electrode negative	Cleanest, most control; slowest
MIG (GMAW)	Thicker sections, faster production	Tri-mix (helium, argon, a little CO2) for short-circuit; argon with 1 to 2% oxygen or CO2 for spray	Needs stainless wire and the right gas
Stick (SMAW)	Outdoor, repair, thicker parts	None; DC electrode positive with most rods	Electrodes such as E308L-16 and E316L-16
Flux-cored	Thicker sections, outdoor	Usually gas-shielded stainless flux-cored wire	Higher deposition

If you’re new to MIG, what is MIG welding explains the machine, wire feed and gas before you move on to stainless.

Filler metal: match the base metal

Base metal	Filler
304 / 304L	308L
316 / 316L	316L
309 / joining stainless to mild steel	309L
347 / 321 (stabilized grades)	347

The “L” means low carbon, which reduces carbide precipitation in the heat-affected zone and helps the weld keep its corrosion resistance. Standard carbon steel rods such as 7018 welding rod are for mild steel, not stainless.

Heat control

Stainless conducts heat poorly and expands more than mild steel, so heat stays near the weld and the part warps more easily.

• Use the lowest heat that gives good fusion, and short, steady welds.
• Tack often and weld in short sections, moving around the part to spread heat.
• Clamp to a heavy fixture or a copper or aluminum backing bar to pull heat away.
• Let the part cool between passes. Too much heat leaves dark discoloration and lowers corrosion resistance.

Back purging for full penetration

When the weld goes through to the back of tube or pipe, the back side oxidizes into a rough, dark crust (“sugaring”) unless it’s shielded. Fill the inside with argon, a back purge, using tape or purge dams to hold the gas in. For sheet, a backing bar or a purge box does the same job.

Avoid contamination

Iron particles from carbon steel tools rust on the surface of stainless and look like the stainless itself is rusting.

• Use stainless-only wire brushes and grinding discs.
• Don’t grind mild steel next to stainless parts.
• Clean the joint with acetone before welding.

Cleaning after welding

Remove heat tint and restore corrosion resistance by brushing with a stainless brush, using a pickling paste, or electrochemical weld cleaning, depending on the finish you need. Follow the safety instructions on pickling paste: it contains strong acids.

Drilling stainless for brackets and fittings has its own speed and bit rules; see best drill bits for stainless steel.