When to use 309L filler metal
309L is the standard filler metal for joining carbon or low-alloy steel to austenitic stainless steel such as 304L or 316L. Its extra chromium and nickel make up for the dilution from the unalloyed side, so the weld metal stays austenitic with a little ferrite.
Short answer
- Carbon steel to 304L / 316L: 309L (AWS ER309L, EN ISO 14343 23 12 L).
- Carbon steel to 316L where molybdenum matters for corrosion: 309LMo (EN ISO 14343 23 12 2 L).
- Buttering (buffer layer) on carbon steel before welding with a matching stainless filler: 309L.
- Service above about 400 °C, thermal cycling or post-weld heat treatment: a nickel-based filler (for example ERNiCr-3) instead of 309L.
Why not 308L or 316L?
In a dissimilar joint the weld metal is a mix of filler and the two base metals. Typically 20–40 % comes from the parent plates. If you use 308L or 316L, the unalloyed steel dilutes the chromium and nickel so much that the weld can form martensite, which is hard and crack-prone. 309L contains about 23 % Cr and 12 % Ni; after dilution the mix still lands in the austenite-plus-ferrite area of the Schaeffler diagram. The few percent of ferrite also protect against hot cracking.
Practical points
- Keep dilution low: moderate heat input, proper joint preparation and a weaving technique that does not melt too much of the carbon steel.
- Preheat follows the carbon steel side; the stainless side does not need it. Use the preheat temperature calculator for that plate.
- Shielding gas for MAG with solid 309L wire is typically Ar + 2–3 % CO₂ or Ar + 1–2 % O₂; flux-cored 309L wires often use Ar + 18 % CO₂ or 100 % CO₂, as the maker specifies.
- Do not use 309L for joints that will be post-weld heat treated or that run hot for long periods: carbon migrates from the ferritic side and the fusion line can crack. Nickel-based fillers avoid that.
The filler metal and shielding gas selector gives the filler and gas for any pair of base metals, with the reason for the choice.
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