Can 304 stainless steel be welded?
Yes. 304 welds well by TIG, MIG and resistance welding with 308L filler. Use 304L for heavy sections, back-purge tubing and plan for distortion.
304 is among the easiest metals to weld. It needs no preheat and no post-weld heat treatment, it has no hardenability problem, and the austenitic weld metal is tough and ductile. TIG (GTAW) is the default for thin sheet and tube, MIG (GMAW) for plate and structural fabrication, and spot welding is common in sheet-metal assemblies. The standard filler is ER308L (AWS A5.9), which matches the base metal composition and carries a small amount of ferrite that resists hot cracking. ER308LSi is used for MIG when a smoother bead is wanted.
The one metallurgical concern is sensitization. Between about 425 and 870 °C (800 and 1600 °F), chromium carbides precipitate at grain boundaries and rob the surrounding metal of chromium, which can lead to intergranular corrosion in the heat-affected zone. Thin sections cool fast and rarely have a problem. For plate over about 6 mm (0.25") and for parts that will see corrosive service, specify 304L (0.03% max carbon) and an L-grade filler, and keep interpass temperature under 150 °C (300 °F).
Shielding matters more than on carbon steel. The root side of a tube or vessel weld needs an argon back-purge, or the inside will sugar (oxidize into a rough, corrosion-prone crust). After welding, remove the heat tint by pickling, electropolishing or a citric passivation; blue and brown oxide left on a weld is where 304 rusts first. Distortion is the other practical issue: stainless conducts heat about a third as well as carbon steel and expands about 50% more, so tack more, weld in short balanced passes and use fixtures.
Welding for 304 parts: where to send them
What to specify
- Base metal 304 or 304L (call out 304L for sections over 6 mm or for corrosive service), and the form spec (ASTM A240 sheet and plate, A276 bar, A269 or A312 tube)
- Filler: ER308L for TIG and MIG, E308L-16 for stick; ER316L is acceptable if that is what the shop stocks
- Welding procedure to AWS D1.6 for structural work or ASME IX for pressure parts
- Back-purge on all tube and full-penetration welds that are visible or in fluid contact
- Post-weld cleaning: pickle and passivate, or electropolish; state whether heat tint is acceptable on hidden welds
- Weld size, length and whether welds are continuous, intermittent or seal welds; note cosmetic welds to be ground flush
- Flatness or straightness tolerance after welding, so the shop knows to fixture
Pitfalls
- Sensitization in thick sections welded with 304 instead of 304L, showing up as intergranular corrosion later
- Grinding with wheels that were used on carbon steel; embedded iron rusts within days
- No back-purge on tube welds, leaving a sugared root that traps contaminants
- Distortion on thin sheet from long continuous welds; use stitch welds or a copper backing bar
- Heat tint left on welds in wet service is a corrosion starter
- Magnetic pickup surprises: 304 weld metal is slightly magnetic because of its ferrite
Frequently asked questions
- What filler rod is used for 304 stainless?
- ER308L for TIG and MIG, E308L electrodes for stick. It is the matching filler for 304 and 304L. ER316L also works and is often used when one filler covers both grades in a shop.
- Does 304 stainless need preheat or post-weld heat treatment?
- No. Austenitic stainless is welded at room temperature with no post-weld heat treatment. Keep interpass temperature under about 150 °C (300 °F) to limit sensitization and distortion.
- Why does 304 rust at the welds?
- Usually heat tint or carbon-steel contamination. The oxide left by welding is chromium-depleted and porous; remove it by pickling or electropolishing, then passivate. Sensitization from slow cooling in thick sections is the other cause; 304L avoids it.
