Three alloys, three different jobs
A workshop that cuts and welds stainless steel usually needs only three items in the store. 308LSi for chromium nickel steels of the 18/8 type, 316LSi for steels with molybdenum, and 309L for anything that joins two different materials. The rest of the catalogue covers special cases.
The selection rule is simple: the weld metal has to have a composition close to the parent material or richer, never leaner. Depleting the chromium and nickel in the weld means the joint will corrode first, even if the rest of the structure lasts twenty years.
Selection by steel grade
1.4301 and 1.4307, the classic 18/8
For 1.4301 (X5CrNi18-10, the equivalent of AISI 304) and 1.4307 (X2CrNi18-9, that is 304L) you use a wire of the 308L type. That is the most common case in sheet metal work, on balustrades, water tanks and catering equipment.
The MIG/MAG version carries the designation EN ISO G 19 9 L Si and AWS ER308LSi. The TIG version is W 19 9 L or W 19 9 L Si, depending on whether the manufacturer raises the silicon in rods as well.
1.4401 and 1.4404, the steels with molybdenum
For 1.4401 (X5CrNiMo17-12-2, AISI 316) and 1.4404 (X2CrNiMo17-12-2, that is 316L) you need 316L wire. The molybdenum in the composition raises resistance to pitting corrosion, so this is the material for installations with chlorides, for food processing with chemical cleaning, and for coastal structures.
Do not substitute 308L wire for 316L, because you will lose the molybdenum in the weld and move the point of corrosion to exactly where the joint is. The reverse substitution, 316L on 1.4301, is technically acceptable but more expensive for no reason.
Dissimilar joints and mild steel
309L is a transition alloy. It has about 23 percent chromium and 12 percent nickel, so even after dilution with the parent material it still gives an austenitic ferritic structure that does not crack hot. You use it wherever stainless steel meets mild steel.
Typical uses: welding a stainless flange to a mild steel pipe, repairing linings, a buffer layer before austenitic overlay welding, joining 1.4301 to S235 steel. Without this wire, a 308L weld diluted with iron from mild steel turns martensitic and cracks.
Matching table
| Parent material | Weld metal type | EN ISO | AWS | Böhler product |
|---|---|---|---|---|
| 1.4301, 1.4307 (304, 304L) | 308L | G 19 9 L Si | ER308LSi | BÖHLER EAS 2-IG (Si), Avesta 308L-Si/MVR-Si |
| 1.4401, 1.4404 (316, 316L) | 316L | G 19 12 3 L Si | ER316LSi | BÖHLER EAS 4 M-IG (Si), Thermanit GE-316L Si |
| Stainless to mild steel | 309L | G 23 12 L | ER309L | BÖHLER CN 23/12-IG |
| 1.4301 to 1.4404 | 316L | G 19 12 3 L Si | ER316LSi | BÖHLER EAS 4 M-IG (Si) |
| Difficult to weld, mixed, repair work | 307 | G 18 8 Mn | ER307 (mod.) | BÖHLER A 7 CN-IG |
The last row is the fallback that repair shops reach for. The 18 8 Mn weld metal has a high manganese content, it resists cracking and it tolerates dilution with material of unknown composition. It is worth keeping one spool on the shelf.
Stabilised grades
1.4541 (X6CrNiTi18-10, that is 321) and 1.4571 (X6CrNiMoTi17-12-2, that is 316Ti) contain titanium, which ties up the carbon instead of the chromium. For these a niobium stabilised weld metal is used, type 347 for 1.4541 and type 318 for 1.4571, in the Böhler range SAS 2-IG and SAS 4-IG respectively.
In workshop practice 308L on 1.4541 and 316L on 1.4571 are also accepted, provided the structure does not work at elevated temperature. On installations above 400 degrees do not take that shortcut, because the stabilisation is needed there for the very reason that steel was chosen.
Why the weld metal should contain some ferrite
Pure austenite cracks hot. During solidification, impurities collect at the grain boundaries and, as the weld shrinks, it tears along the axis of the run. A few percent of delta ferrite in the structure breaks that mechanism up and dissolves the sulphur and phosphorus.
That is why 308L and 316L wires are formulated so that the weld metal has roughly 5 to 10 ferrite numbers, and 309L more still. Do not upset that by adding nickel wire to an austenitic joint without need, and do not overheat the joint, because a high interpass temperature lowers the ferrite content.
The practical sign of missing ferrite is a longitudinal crack down the middle of the weld face, usually in the crater at the end of a run. If you see one with the correct wire, first check the dilution with the parent material and how the craters are being filled.
What L means and what Si means
L, the reduced carbon
The letter L means a maximum of 0.03 percent carbon in the weld metal. There is one reason for it: while cooling through the range of about 450 to 850 degrees, carbon ties chromium up as carbides at the grain boundaries. The chromium then stops protecting the steel and the joint corrodes intergranularly, usually along the fusion line.
On thin sheet and a single run the problem is small. On multi-run welds in heavy material, where each new run heats the previous one, the version without L can be a mistake that does not show for the first two years of the installation's life.
Si, the raised silicon
Silicon lowers the surface tension of the weld pool. A wire with Si wets the edges more gently, gives a flat face and less undercut, which is why the Si versions are intended for MIG/MAG. In TIG, where you steer the pool by hand anyway, the benefit is smaller.
That is why in the Böhler range an item with (Si) after the name is a MIG/MAG wire, while the same name without it is a TIG rod. BÖHLER EAS 4 M-IG (Si) carries the classification G 19 12 3 L Si, and BÖHLER EAS 4 M-IG in rod form carries W 19 12 3 L. It is the same alloy in two forms.
Watch one detail: BÖHLER CN 23/12-IG is classified as G 23 12 L and ER309L, with no Si symbol. The 309LSi variants appear in other ranges, so if your WPS calls for ER309LSi, check the code on the label rather than the trade name alone.
The shielding gas, without which the rest means nothing
| Process | Gas | Notes |
|---|---|---|
| MIG/MAG, spray and pulsed arc | argon with a small addition of CO2, of the order of 2 percent (group M12) | a stable arc, minimal carburising of the weld |
| MIG/MAG, alternative | argon with oxygen added, of the order of 2 percent (group M13) | a smoother face, a darker film to be pickled off |
| TIG | argon (I1) | purity of at least 99.99 percent |
| Root purging | argon or a mixture with nitrogen | without it the root oxidises and loses its corrosion resistance |
| What not to use | pure CO2 (C1) | it carburises the weld metal and destroys the effect of the reduced carbon |
Welding stainless in pure CO2 is the most common mistake in workshops changing over from mild steel. The wire may be right and the code on the label may match the documentation, and the joint will still lose its resistance. Set up a separate cylinder and a separate regulator so that nobody mixes them up at a changeover.
Diameters and currents
MIG/MAG wires for stainless steel are made mainly in 0.8, 1.0 and 1.2 mm. Thermanit GE-316L Si also comes in 0.9, 1.6 and 2.0 mm, which is useful in mechanised welding. BÖHLER CN 23/12-IG TIG rods come in 1.6, 2.0, 2.4 and 3.2 mm.
For sheet of 1.5 to 3 mm take 0.8 mm and a current of roughly 60 to 130 A. For 4 to 8 mm take 1.0 mm and 120 to 220 A. Above 8 mm and on multi-run welds, 1.2 mm and 180 to 300 A. Stainless steel conducts heat away less readily than mild steel, so stay at lower values than you would on the same thickness of mild steel.
In the feeder use a PTFE or plastic liner, not a steel one. A steel liner loads the wire with iron filings, which then corrode on the weld face as rust spots. The same goes for the brush: on stainless use stainless wire only, never the same brush as on mild steel.
The three mistakes that cost the most
- The same gas as for mild steel: a carburised weld corrodes despite the correct wire.
- No root purging on pipes and tanks: an oxidised root is a ready made site for pitting corrosion.
- Too high an interpass temperature: with austenite stay below 150 degrees, otherwise the risk of precipitation and distortion rises.
All three show up months later, not straight after welding. That is why on stainless work the procedure matters at acceptance, not the appearance of the weld face alone.
We will match the wire to your grade
Write and tell us your material, thickness, process and gas, and we will point to a specific item from the Böhler Welding range, for which we are the authorised distributor. MIG/MAG wires are in the solid MIG/MAG wires category and rods in the TIG rods category. If you are also looking for a machine for thin stainless steel, have a look at the used TIG category, where every unit is checked in our service centre and carries condition A, B or C.