Stainless steel welds well – as long as you remember how it differs from ordinary structural steel. It conducts heat differently, expands more, tolerates contamination and overheating poorly, and its corrosion resistance after welding depends on how the weld and the area around it are treated. This guide takes you through the whole process: from choosing the process and gas, through filler metal and preparation, to post-weld treatment.
How stainless steel differs from carbon steel in welding
The steels welded most often are austenitic – 1.4301, 1.4307, 1.4404, 1.4571, 1.4541. Compared with carbon steel, they:
- conduct heat roughly three times less well – heat stays in the weld zone, making overheating and burn-through of thin walls more likely,
- have about one and a half times the thermal expansion – parts distort more and “pull” as they cool,
- lose corrosion resistance if the weld is overheated, contaminated or left with heat tint,
- do not harden, so they usually need no preheating.
These four traits explain most of the rules below.
Welding processes
TIG (141) — the first choice
Welding with a non-consumable electrode under an inert gas shield gives the cleanest, best-looking welds, good control of the weld pool and virtually no spatter. It is the standard for thin walls, installation pipework, the food and pharmaceutical industries and anywhere the weld is visible. Its drawback is lower productivity.
MIG/MAG (131/135) — productivity on thicker parts
Welding with solid wire (or flux-cored wire – processes 136/138) works well for thicker plate, long welds and series production. For austenitic steels, argon with a small addition of active gas is used, e.g. 2–3% CO₂ or 1–2% O₂. More CO₂ adds carbon to the weld and oxidises the surface more heavily.
MMA (111) — covered electrodes
Covered electrodes are indispensable for site and repair work, outdoors and in hard-to-reach places, because they need no gas cylinder. Expect slag to remove and more finishing work. Store electrodes in a dry place and re-dry damp ones according to the manufacturer’s instructions.
Plasma and laser
Industrial production increasingly uses plasma (15) and laser (52) welding. They give a narrow heat-affected zone and little distortion, but require precise preparation and expensive equipment. This is how continuously produced welded tubes are made, among other things.
Shielding gas and root protection
For TIG, the basic gas is pure argon. Adding helium increases penetration on thicker parts, and a small addition of hydrogen (a few per cent) improves wetting and allows faster welding. Hydrogen is not used, however, for duplex, ferritic or martensitic steels.
Protecting the root is just as important as shielding the face. On the inside of a pipe or on the underside of a weld, the metal is just as hot and reacts with oxygen just the same. Without root purging, the root oxidises into a rough, dark, flaking oxide layer where corrosion begins. Purging uses argon, nitrogen or forming gas – a nitrogen-hydrogen mixture intended for austenitic steels. The space is purged before the arc is struck and the flow is maintained until the weld has cooled. In demanding applications, residual oxygen is checked with a meter.
Choosing the filler metal
Filler metal is matched to the parent metal grade – usually with the same or a slightly richer composition. The most common pairings (designations to EN ISO 14343 and AWS):
| Parent metal | Filler metal | Notes |
|---|---|---|
| 1.4301, 1.4307 (304, 304L) | 19 9 L (ER308L) | standard for 304 |
| 1.4541 (321) | 19 9 Nb (ER347) | niobium-stabilised, as titanium transfers poorly across the arc |
| 1.4401, 1.4404 (316, 316L) | 19 12 3 L (ER316L) | keeps molybdenum in the weld |
| 1.4571 (316Ti) | 19 12 3 Nb (ER318) or 19 12 3 L | stabilised or low-carbon |
| 1.4462 (duplex) | 22 9 3 N L (ER2209) | extra nickel for the right ferrite balance |
| stainless + carbon steel | 23 12 L (ER309L) | dissimilar joints |
Fillers for austenitic steels usually contain a few per cent of ferrite in the weld metal, which protects the weld against hot cracking. A filler that does not match the grade – 308L wire on 316L, for example – gives a weld with no molybdenum, which becomes the weakest point of the whole structure. How to tell these grades apart is covered in how to tell 316 from 304.
Preparation — cleanliness above all
- Dedicated tools. Stainless steel brushes, discs and files used only on stainless. Carbon steel particles embedded in the surface rust and leave rusty stains that look like corrosion of the stainless itself.
- Degreasing. Clean the weld zone with acetone or alcohol – grease, marker pen or remnants of protective film bring carbon and contamination into the weld.
- Careful edge and gap preparation. The stainless weld pool is less fluid than with carbon steel, so it is harder to correct things during welding.
- Closer tack welds. Greater thermal expansion moves the edges more than with carbon steel.
- A separate work area, if possible. Grinding dust from carbon steel settles on stainless and has the same effect as embedded particles.
Heat input — less is more
Overheating is the most common cause of trouble with stainless welds. In the range of roughly 450–850°C, chromium carbides can precipitate at the grain boundaries of austenitic steels. Chromium tied up in carbides no longer protects the steel, and the zone next to the weld becomes susceptible to intergranular corrosion. This is known as sensitisation.
That is why:
- where the weld will work in a corrosive environment, low-carbon grades (L – 1.4307, 1.4404) or grades stabilised with titanium or niobium (1.4541, 1.4571) are used – they contain too little carbon, or have it tied up, so chromium carbides do not form,
- welding is done with the lowest practical heat input: a short arc, a faster travel speed, several narrow beads rather than one wide weave,
- interpass temperature is controlled – for austenitic steels usually no higher than about 150°C.
Low heat input also limits distortion, which is greater with stainless than with carbon steel. A symmetrical welding sequence, backstep welding and clamping fixtures help – ideally with copper backing bars that draw heat away.
Duplex, ferritic and martensitic steels
Duplex steels such as 1.4462 require heat input to be kept within a specific window: too little gives excess ferrite, too much promotes intermetallic phases. Fillers with increased nickel content and gases with added nitrogen, without hydrogen, are used.
Ferritic steels such as 1.4016 are prone to grain growth – they are welded quickly and with low heat input. Martensitic steels such as 1.4021 and 1.4034 harden in the heat-affected zone, so they usually need preheating and post-weld heat treatment according to the recommendations for the grade.
After welding — heat tint, pickling, passivation
The coloured discolouration around a weld – from straw through blue to dark brown – is an oxide layer. The darker it is, the thicker it is, and beneath it lies a layer of metal depleted in chromium. Such a spot corrodes faster than the rest of the surface, even if the weld itself is sound.
Heat tint is removed in three ways:
- chemically – with pickling paste or gel (usually based on nitric and hydrofluoric acid) or in a pickling bath; this is the most effective method, because it also removes the chromium-depleted layer,
- electrochemically – with a weld cleaning unit and electrolyte; fast and free of aggressive pastes, it works well on visible welds,
- mechanically – by brushing or grinding with tools used only on stainless; this removes the discolouration, but it is easy to introduce contamination and leave a rough surface.
After pickling, the surface is rinsed thoroughly with clean water. The passive layer re-forms by itself on contact with air, and where requirements are higher, additional chemical passivation is carried out. Pickling pastes are highly corrosive – working with them requires personal protective equipment and disposal of the waste in line with regulations.
Safety
Fumes from welding stainless steel contain chromium compounds – including hexavalent chromium – and nickel compounds, which are harmful to health. Effective fume extraction at the source is essential, and for longer work in enclosed spaces, respiratory protection as well.
The most common mistakes — a checklist
- welding without root purging,
- a brush or disc that has previously been used on carbon steel,
- too much current and a wide weave,
- a filler that does not match the grade,
- heat tint left without pickling,
- a standard grade instead of a low-carbon or stabilised one where the weld works in a corrosive environment.
Summary
A good weld on stainless steel comes down to four things: cleanliness, the right gas with root protection, a matching filler metal and heat control. Neglecting any of them does not always show straight away – corrosion can appear only after a few months in service.
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