Troubleshooting guide › Material guides › Stainless steel welding
Stainless steel is easy to weld and easy to ruin. The arc behaves well; the problems come afterwards: a weld that rusts, a root that looks like sugar, a heat-affected zone that corrodes in service, or a sheet that has buckled. Almost all of it comes from heat, oxygen and iron contamination, and all three can be controlled with cheap discipline. This page covers austenitic grades (304, 316, 321) in detail, with notes on ferritic and duplex.
Reviewed by the Cruxweld technical team, October 2026. Settings are starting points to confirm on a test piece; acceptance is decided by the code or drawing the job is built to. Checks marked “call” are for our technical team.
| Property | What it means at the torch |
|---|---|
| Conducts heat at a third the rate of mild steel | Heat stays in the joint: the pool forms fast, the HAZ stays hot longer, distortion is worse, and current is about 10-20 % lower than for mild steel |
| Expands about 50 % more than mild steel | Thin sheet buckles; tack closely, use chill bars, balance the sequence |
| Protected by a chromium oxide film that needs oxygen to re-form | Heat tint and scale on the surface and inside the pipe must be removed or the steel rusts there |
| Chromium carbides form at 500-800 degrees C | Slow cooling or many hot passes sensitise the HAZ and it corrodes along the grain boundaries (weld decay); use L grades and keep heat input and interpass low |
| Fully austenitic weld metal is crack-sensitive | Fillers 308L/316L are designed to freeze with a little ferrite (5-10 %) to resist hot cracking |
| Iron particles embedded in the surface rust | Dedicated tools: stainless brushes, inox discs, no carbon-steel contact |
The weld and tungsten are still hot when the argon stops; the surface oxidises to straw, blue, then black.
The wider the hot zone, the wider the tint.
The root of a single-sided weld on pipe or sheet is exposed to air and oxidises into a grey, granular, crumbly surface: sugaring. It is weak and it corrodes.
Argon is blown away or diluted.
A straw or light gold tint is usually acceptable on general work; blue, purple and black tints have lost chromium and must be removed for corrosion service.
| Tint colour | Approximate peak temperature | What to do for corrosion service |
|---|---|---|
| None / silver | below 300 degrees C | Nothing |
| Straw / gold | 300-400 degrees C | Usually accepted on general fabrication; clean for food and chemical |
| Brown / purple | 400-500 degrees C | Remove by pickling or brushing |
| Blue | 500-600 degrees C | Remove; the surface has lost chromium |
| Grey / black / sugared | above 600 degrees C in air | Grind out and reweld on pipe roots; remove completely elsewhere |
A steel wire brush, a grinding disc used on mild steel, steel clamps, a steel bench, a chain, or grinding dust from the next bay embeds iron that rusts within days.
The tinted zone has lost chromium and rusts first.
Each one breaks the passive film and starts a pit.
Mild steel wire used by mistake on stainless; the weld itself is carbon steel.
Marker pens, tapes, cleaning chemicals and seawater containing chloride cause pitting and stress corrosion.
A band next to the weld corrodes in service. Chromium carbides formed while the HAZ sat at 500-800 degrees C, leaving the grain boundaries short of chromium.
Fully austenitic weld metal, high dilution, deep narrow beads, crater cracks, contamination with copper, zinc or sulphur.
Dilution of a 308L weld with mild steel gives a crack-sensitive, low-ferrite deposit.
Too fast cooling gives too much ferrite; too slow precipitates brittle phases.
These harden on cooling, unlike austenitic.
| Base metal | TIG / MIG filler | Stick electrode | Note |
|---|---|---|---|
| 304 / 304L | ER308L | E308L | The standard |
| 316 / 316L | ER316L | E316L | Marine, chemical, food |
| 321 / 347 | ER347 | E347 | High temperature |
| Stainless to mild steel | ER309L | E309L | Tolerates dilution |
| 409 / 430 ferritic | ER309L or ER430 | E309L | Keep heat low; grain growth |
| Duplex 2205 | ER2209 | E2209 | Controlled heat input |
| Process | Gas | Flow | Note |
|---|---|---|---|
| TIG | Argon 99.99 %; Ar-2 % H2 for austenitic only | 8-12 L/min, gas lens | Back purge argon 5-10 L/min |
| MIG | Ar-2 % CO2 or Ar-1 % O2 (never more than 2-3 % CO2) | 15-18 L/min | Pure CO2 carburises the weld and darkens it |
| Flux-cored stainless | Ar-20 % CO2 or CO2 as the wire specifies | 18-20 L/min | Slag must be removed fully |
| Spot welding | – | – | One tap lower and shorter time than mild steel |
| Thickness | TIG current (DC-) | Filler | MIG (1.0 mm wire) |
|---|---|---|---|
| 1 mm | 30-45 A, pulse | 1.0-1.6 mm | 50-80 A, 15-17 V, pulse |
| 2 mm | 60-80 A | 1.6 mm | 90-120 A, 18-20 V |
| 3 mm | 80-120 A | 1.6-2.4 mm | 130-160 A, 20-22 V |
| 5 mm | 130-180 A | 2.4 mm | 170-210 A, 23-25 V (1.2 mm wire) |
| 8 mm | 180-240 A, water-cooled torch | 3.2 mm | 220-270 A, 25-28 V (1.2 mm wire) |
About 10-20 % below the mild-steel values. Starting points only; confirm on a test piece and check the back of the weld.
Possibly, but far more often it is iron contamination from steel brushes, discs or benches, or heat tint that was not removed. Clean a small area with a dedicated abrasive, passivate, and wet it for a day; if it stays clean the material is fine.
For anything that carries product (food, chemical, pharma, water) or that is inspected, yes. For structural handrail and furniture, a backing flux paste or a copper bar is often enough, but a sugared root is still weak.
Not for corrosion service: pure CO2 adds carbon to the weld and darkens it. Use Ar-2 % CO2. Flux-cored stainless wires designed for CO2 are the exception.
That is weld decay: the heat-affected zone sat at 500-800 degrees C long enough to form chromium carbides. 304L or 321 with low heat input avoids it.
Stainless needs lower heat input and good gas control. A DC TIG with pulse, down-slope and long post-flow is the standard tool for sheet and pipe; a MIG with pulse makes thin stainless sheet practical at speed. Spot welders need a lower tap and shorter time than for mild steel. None of them replace a clean shop: stainless welded with steel-contaminated tools rusts no matter what machine made the weld.
Send a photo of the weld or cut, your machine model, consumable and plate thickness. Our technical team replies on WhatsApp with the likely causes and the next checks to make.
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