How to Weld 304 and 304L Stainless Steel

Why welding 304 is about keeping the corrosion resistance

A sound austenitic weld matches the base-metal strength easily, so 304 joints rarely fail on mechanicals. The real risk is losing the corrosion resistance across the weld and the heat-affected zone (HAZ). 304 is an austenitic stainless steel, UNS S30400, with 304L as the low-carbon variant S30403. It relies on roughly 18 to 20% chromium for a passive film, which gives it a PREN of about 18, lower than 316. Weld it carelessly and the joint sensitizes, tints, or pits long before the plate ever would. Every rule that follows serves one goal: a finished weld as corrosion-resistant as the metal around it.

Before you strike the arc

Are you welding 304 or 304L?

304 and 304L carry the same chromium and nickel. The difference is carbon. Standard 304 allows up to 0.08% C, while 304L caps it at 0.03% C or less. That low carbon is the whole point: it starves the chromium-carbide reaction that drives sensitization during welding. For any welded fabrication held in or cycled through the sensitizing range, specify and buy the low-carbon variant so you avoid sensitization without post-weld annealing. Much mill stock is dual-certified to both 304 and 304L, with carbon held at or below 0.03% while still meeting 304 strength, so you can weld with confidence and still satisfy a 304 callout. Compare the two in 304 vs 304L, or go straight to the 304 and 304L grade hubs.

Choosing filler metal

Match an over-alloyed low-carbon consumable to the base. Use ER308L for GTAW and GMAW, and E308L-16 or -17 for SMAW, on both 304 and 304L base metal. ER308L runs slightly richer in chromium and nickel than the base so that, once it dilutes into the weld pool, the finished metal lands back within base-metal chemistry. It also carries a small, deliberate amount of delta ferrite that suppresses hot cracking as the weld solidifies. For dissimilar joints, 304 to carbon steel or to a dissimilar stainless, step up to a 309L or a nickel-base ERNiCrMo consumable to absorb dilution. The reasoning is covered in weld decay and filler selection.

Process and shielding gas

GTAW (TIG) suits thin sheet, root runs, and hygienic work where a clean root matters. GMAW (MIG) gives faster fill on thicker sections. SMAW (stick) fits site and positional welds, and FCAW earns its place when deposition rate drives the schedule. The metallurgy does not change across processes; only heat input and shielding do. Use pure argon for GTAW and an argon-rich mix for GMAW, such as argon with 1 to 2% oxygen. Critically, run an argon back-purge on the root side. Skip it and the exposed root oxidizes into granular "sugaring," a chromium-depleted surface that becomes a corrosion initiation site. The background theory is in welding stainless and nickel alloys.

Cleanliness and tooling

Keep a dedicated set of stainless-only wire brushes, flap discs, and grinding wheels. Tools that have touched carbon steel leave free iron embedded in the surface, which rusts and sets up galvanic cells. Degrease the joint fully and remove marker, oil, and shop grime before the first pass.

Step by step: how to weld 304 and 304L

1. Clean and degrease the joint. Strip oil, crayon, cutting fluid, and iron contamination with a stainless brush and solvent. Contaminants carbonize in the arc and seed corrosion later.
2. Fit up, tack, and set the back-purge. Tack with ER308L, fit a purge dam, then flood the root cavity with argon until oxygen is displaced. This protects the first and most vulnerable pass.
3. Do not preheat. Austenitic stainless needs none, and preheat only adds time in the damaging temperature band. Run stringer beads at low heat input rather than wide weaves.
4. Weld the root with gas backing. Lay the root while the back-purge holds, so the underside stays bright instead of sugared and oxidized.
5. Control interpass temperature. Let each pass cool to roughly 150 °C or below before the next, keeping cumulative time in the 425 to 815 °C sensitization range as short as possible.
6. Fill and cap with matching filler. Continue with ER308L, or E308L electrodes, and hold total heat input through the joint to a minimum.
7. Clean between runs. De-slag and brush with a dedicated stainless wire brush only, never one that has touched carbon steel.
8. Pickle and passivate. After welding, remove heat tint and the chromium-depleted layer, then rebuild the passive film so the weld resists corrosion like the parent metal.

Avoiding sensitization: the whole point of low heat input

Between 425 °C and 815 °C, chromium and carbon pair up into chromium carbides along the grain boundaries. That leaves the metal beside them stripped of chromium and open to intergranular attack, the failure the old hands call weld decay. It shows up in the HAZ, not the weld bead itself, and 304 is more exposed to it than 316 because it has no molybdenum margin to fall back on.

Three defenses stack up against it. First, low carbon: 304L at 0.03% C or less simply does not have enough carbon to form damaging carbide networks. Second, low heat input and controlled interpass temperature, which limit how long the HAZ dwells in the sensitizing band. Third, where a heavy section or a critical IGC service demands it, post-weld solution annealing dissolves any carbides that did form. For the mechanism and the fixes, see how to prevent sensitization and intergranular corrosion and sensitization.

Post-weld: restore the corrosion resistance

Welding leaves heat tint, the straw-to-blue oxide band, and a thin chromium-depleted layer beside the bead. Both are weaker than the surrounding passive film and pit preferentially, so they must be removed, not just wiped. Mechanically remove loose scale, then pickle with a nitric-hydrofluoric paste or bath to strip the tinted, chromium-poor skin, and passivate per ASTM A380 / A967 to rebuild a uniform chromium-oxide layer. This is the step most often skipped on otherwise perfect welds, and it is the one that most often lets a 304 joint rust in service. Full method in how to passivate stainless steel and the theory in passivation of stainless steel.

Pitfalls and failure modes

Heat tint and the chromium-depleted layer

Heat tint is not merely cosmetic. The oxide and the chromium-poor metal beneath it have lower local pitting resistance, so an uncleaned weld pits where the plate would not, and on low-PREN 304 that margin is already thin. Remove it. More in pitting corrosion.

Chloride stress-corrosion cracking

Austenitic 304 is susceptible to chloride stress-corrosion cracking, and welds bring together the two ingredients it needs, residual tensile stress and chlorides. In hot or coastal chloride service, keep residual stress low and consider a more resistant grade. See stress-corrosion cracking.

Iron and carbon-steel contamination

Embedded free iron from shared tooling, grinding dust, or carbon-steel work in the same bay rusts and creates galvanic sites on the stainless surface. Keep consumables and workspaces segregated.

Distortion and hot cracking

304 expands more and conducts heat less than carbon steel, so it moves more for the same heat. Keep heat input low, balance the sequence, and fixture well. Fully austenitic weld metal also cracks easily, which is exactly why the ferrite number built into ER308L matters. Use the correct matching consumable rather than improvising.

Filler and process matrix

Chemical composition: base metal and filler

Composition per ASTM A240 for base metal and AWS A5.9 for ER308L filler, weight %:

304 carries no deliberate molybdenum, so its pitting resistance rests on chromium alone. The matching ER308L filler runs slightly rich on chromium and nickel so that, once it dilutes into the weld pool, the finished metal still lands within base-metal chemistry while holding enough ferrite to resist hot cracking.

Equivalents

How to verify the weld

A PMI reading on both base metal and weld confirms the material is genuinely low-carbon and that the correct 308L filler went in. An ASTM A262 cut then checks whether the HAZ sensitized. More on this in PMI, NDT, and the mill test certificate.

Related grades and guides

If 304 welds pit or crack in chloride service, the upgrade path runs up the PREN ladder: 304 (~18) < 316 (~24) < 904L (~34). For the dedicated procedures, see how to weld 304L, how to weld 316, and how to weld 316L. To decide whether to step up, read 304 vs 316 and 304L vs 316L, plus the primer on PREN. Ready to buy base metal? Go to 304L plate or 304 plate.

Where welded 304 and 304L is used

Welded 304/304L is the default for general fabrication such as tanks, frames, and enclosures, for architectural railings, cladding, and facades, and for food and dairy equipment. The Nandini Steel Engineering Team sees one recurring spec error: defaulting to 304 for chloride-adjacent or coastal duty, where it pits. Confirm the actual service chlorides before choosing 304 over 316, since stepping up a grade is far cheaper than a failed part. For welded fabrications held at 425 to 815 °C, specify 304L to avoid sensitization without post-weld annealing.

Sourcing weldable, traceable 304 and 304L

All 304 and 304L flat and long products ship ex-stock for immediate dispatch, cut from genuine mill material sourced from nameable mills including Aperam, Outokumpu, Jindal, POSCO, Nippon Steel, and JFE. Every order carries an EN 10204 3.1/3.2 mill test certificate, so you can confirm carbon sits at or below 0.03% before you weld, and PMI and ASTM A262 IGC testing are available on request. Nandini Steel has supplied this material since 2003 and holds ISO 9001:2015 certification (Cert. No. 1283438, Unitas Certification Services UCS-UK, valid to 11 January 2028).

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