Quick answer: use 309L filler metal (ER309L wire or rod for MIG and TIG, E309L electrodes for stick). It’s designed for joining stainless to carbon steel: its extra chromium and nickel make up for the dilution from the mild steel, so the weld stays crack-resistant. Clean both metals well, keep heat input moderate, and paint or coat the mild steel side afterwards, because it will still rust.
Why stainless to mild steel is different
Welding two different metals together is called a dissimilar metal weld. When you weld 304 stainless to mild steel, the weld pool mixes metal from both sides. This is called dilution.
- If you used mild steel filler, the chromium and nickel from the stainless would make a brittle, crack-prone weld.
- If you used 308L (the normal filler for 304 to 304), the iron from the mild steel can dilute it until it becomes martensitic: hard, brittle and prone to cracking.
- 309L contains more chromium (about 23%) and nickel (about 13%), so even after mixing with mild steel the weld stays austenitic with a little ferrite, which is tough and resistant to hot cracking.
That’s why 309L is the standard filler for stainless-to-carbon steel.
Filler metal by process
| Process | Filler | Notes |
|---|---|---|
| TIG (GTAW) | ER309L rod | The cleanest results, best for thin material and visible welds |
| MIG (GMAW) | ER309L wire | Fast; use a stainless-suitable gas |
| Stick (SMAW) | E309L-16 or E309L-17 electrodes | Good for field repairs and thicker sections |
| Flux-core (FCAW) | E309LT-1 or similar | Thick sections, high deposition |
For some special cases (very thick sections, high temperatures, or other stainless grades) engineers specify other fillers such as 312 or nickel alloys. Follow the welding procedure if one exists.
Shielding gas
| Process | Gas |
|---|---|
| TIG | 100% argon |
| MIG (short-circuit) | A stainless “tri-mix” (helium/argon/CO₂), or argon with about 2% CO₂ or O₂, depending on the wire maker’s recommendation |
| MIG (spray or pulse) | Argon with about 1–2% O₂ or CO₂ |
Avoid standard 75/25 argon/CO₂ for stainless MIG where corrosion resistance and appearance matter. The extra carbon and oxidation can reduce corrosion resistance.
How to weld stainless to mild steel (step by step)
- Clean both pieces. Grind off mill scale, rust and paint from the mild steel, and degrease both with acetone. Use a stainless-only wire brush and grinding disc on the stainless, so you don’t embed carbon steel particles that will rust.
- Fit up tightly. Tight fit-up means less filler and less dilution.
- Set moderate heat. Stainless holds heat and distorts easily. Use the lowest heat that gives good fusion, and keep the travel speed steady.
- Aim slightly toward the stainless side if the mild steel is thicker, so both sides melt evenly.
- Keep dilution low. Use stringer beads, not wide weaves, and don’t run too hot.
- Control interpass temperature. On multi-pass welds, let the joint cool (often below about 300–350°F / 150–175°C, or as the procedure specifies) between passes.
- Clean up afterwards. Remove heat tint from the stainless with a stainless brush, pickling paste or an electrochemical weld cleaner, then protect the mild steel side with primer and paint, galvanising spray or another coating.
Typical starting settings (TIG, 1/8” / 3 mm material)
| Setting | Starting point |
|---|---|
| Polarity | DCEN |
| Amps | About 90–120 A |
| Tungsten | 3/32” (2.4 mm), 2% lanthanated |
| Filler | 3/32” ER309L |
| Argon | 15–20 CFH |
Fine-tune for your machine and the joint. See our GTAW guide for more settings.
Common problems and fixes
| Problem | Cause | Fix |
|---|---|---|
| Cracks in or next to the weld | Wrong filler (308 or mild steel), too much dilution | Use 309L; reduce heat; use stringer beads |
| Rust along the joint | Bare mild steel, or carbon contamination of the stainless | Coat the mild steel side; use stainless-only tools |
| Heavy discolouration on the stainless | Too much heat, poor gas coverage | Lower the heat, check gas flow, clean after welding |
| Warping | Stainless expands more than mild steel | Tack well, balance welds, clamp, and keep heat low |
Do you need to back-purge?
For pipe and tube where the inside surface matters (food, chemical or pressure systems), back-purge the stainless side with argon to stop the root oxidising (“sugaring”). For general fabrication, such as a stainless bracket on a mild steel frame, purging usually isn’t needed.
FAQ
What filler rod do you use to weld stainless to mild steel?
309L (ER309L for TIG and MIG, E309L for stick) is the standard choice.
Can you MIG weld stainless steel to mild steel?
Yes. Use ER309L wire and a gas suitable for stainless, set the heat moderately, and use stringer beads.
Can I use 308L filler for stainless to mild steel?
It’s not recommended. Dilution from the mild steel can make a 308L weld crack-prone. 309L is designed for this joint.
Will the weld rust?
The 309L weld itself and the stainless side resist rust. The mild steel will still rust, so paint or coat it.
Can you stick weld stainless to mild steel?
Yes, with E309L-16 or E309L-17 electrodes on DC electrode positive. Keep the arc short and the rod dry.
How this guide was written: filler selection follows standard dissimilar-metal welding practice and AWS filler classifications. For code work, follow the qualified welding procedure. Related: types of welding processes. See how I test and review.