Picking the right filler for stainless steel comes down to one principle: match the filler to the base metal’s corrosion requirements, not just its strength. A 304 stainless joint welded with carbon steel filler will have full mechanical strength but zero corrosion resistance in the weld zone. The filler metal must deliver the same chemical composition (specifically chromium, nickel, and molybdenum content) as the base metal after accounting for dilution.
This guide covers every common stainless steel base-metal-to-filler combination across all four arc welding processes. Bookmark the master chart and the process-specific tables for quick reference.
Master Filler Selection Chart
| Base Metal | UNS | Matching Filler | Alternative | Notes |
|---|---|---|---|---|
| 301 | S30100 | 308L | 308LSi | Same filler as 304 |
| 302 | S30200 | 308L | 308LSi | Same filler as 304 |
| 304 | S30400 | 308L | 308LSi | Most common combination |
| 304L | S30403 | 308L | 308LSi | L-grade filler mandatory |
| 304H | S30409 | 308H | 308 | Higher carbon for creep strength |
| 309 | S30900 | 309L | 309LSi | High-temp service |
| 310 | S31000 | 310 | None | High-temp, fully austenitic |
| 316 | S31600 | 316L | 316LSi | Molybdenum-bearing |
| 316L | S31603 | 316L | 316LSi | L-grade filler mandatory |
| 317L | S31703 | 317L | None | Higher Mo than 316L |
| 321 | S32100 | 347 | 308L | 347 filler; ER321 hard to find |
| 347 | S34700 | 347 | None | Nb-stabilized |
| 904L | N08904 | 385 (20-25Cb) | ERNiCrMo-3 | Super austenitic |
| 2205 Duplex | S32205 | 2209 | None | Over-alloyed with Ni for phase balance |
| 2507 Super Duplex | S32750 | 2594 | None | Over-alloyed with Ni for phase balance |
| 410 (martensitic) | S41000 | 410 | 309L (for soft deposit) | Preheat required (400-600F) |
| 430 (ferritic) | S43000 | 430 | 309L | Preheat 300-450F, PWHT needed |
Dissimilar Stainless Joints
When joining two different stainless grades to each other, the filler must be compatible with both base metals. The general rule: use the filler that matches the more highly alloyed base metal, or use a filler that’s over-alloyed relative to both.
| Joint Combination | Recommended Filler | Reasoning |
|---|---|---|
| 304 to 316 | 316L | Mo in filler matches 316 side |
| 304 to 321 | 308L or 347 | Either compatible with both |
| 304 to 347 | 347 | Nb stabilization matches 347 |
| 316 to 321 | 316L | Mo content for 316 side |
| 316 to 347 | 316L | Mo content for 316 side |
| 2205 to 304 | 2209 | Phase balance for duplex side |
| 2205 to 316 | 2209 | Phase balance for duplex side |
| 304 to carbon steel | 309L | Over-alloyed for CS dilution |
| 316 to carbon steel | 309LMo | 309L + Mo for 316 compatibility |
| 2205 to carbon steel | 2209 | Phase balance maintained |
Process-Specific Filler Designations
The same filler chemistry comes in different product forms for each welding process. The naming conventions are different but the compositions are equivalent.
TIG and MIG Wire (AWS A5.9)
| Classification | Cr % | Ni % | Mo % | C % (max) | For Base Metal |
|---|---|---|---|---|---|
| ER308L | 19.5-22 | 9-11 | 0.75 max | 0.03 | 304, 304L, 301, 302 |
| ER308LSi | 19.5-22 | 9-11 | 0.75 max | 0.03 | Same + better MIG wetting |
| ER309L | 23-25 | 12-14 | 0.75 max | 0.03 | Dissimilar (SS to CS) |
| ER309LMo | 23-25 | 12-14 | 2-3 | 0.03 | 316 to carbon steel |
| ER316L | 18-20 | 11-14 | 2-3 | 0.03 | 316, 316L |
| ER316LSi | 18-20 | 11-14 | 2-3 | 0.03 | Same + better MIG wetting |
| ER347 | 19-21.5 | 9-11 | 0.75 max | 0.08 | 321, 347 |
| ER2209 | 21.5-23.5 | 8.5-10.5 | 3-3.5 | 0.03 | 2205 duplex |
| ER2594 | 24-27 | 8-10.5 | 3-4.5 | 0.03 | 2507 super duplex |
Stick Electrodes (AWS A5.4)
| Classification | Composition Match | Coating Type | Current |
|---|---|---|---|
| E308L-16 | ER308L | Titania (rutile) | AC or DCEP |
| E308L-17 | ER308L | Titania-silica | AC or DCEP |
| E309L-16 | ER309L | Titania | AC or DCEP |
| E316L-16 | ER316L | Titania | AC or DCEP |
| E347-16 | ER347 | Titania | AC or DCEP |
| E2209-16 | ER2209 | Titania | AC or DCEP |
The “-16” designation means the electrode has a titania (rutile) coating that runs on AC or DCEP. The “-17” variant has a titania-silica coating with slightly different slag characteristics and easier slag removal. Both produce equivalent weld metal chemistry. The “-15” variant (E308L-15) runs on DCEP only with a basic (lime) coating that provides better toughness at low temperatures.
Shielding Gas by Process
| Process | Primary Gas | Alternative | Notes |
|---|---|---|---|
| TIG (GTAW) | 100% Argon | Ar + 2-3% N2 (duplex) | 15-20 CFH, DCEN |
| MIG (GMAW) | 98% Ar / 2% CO2 | Tri-mix (90He/7.5Ar/2.5CO2) | Pulse preferred |
| MIG (GMAW) | 98% Ar / 2% O2 | None | Spray transfer on thicker material |
| Flux-Core (FCAW) | 100% CO2 or 75/25 | None | Gas-shielded only |
| Stick (SMAW) | Self-shielded | N/A | Flux coating provides shielding |
| Back Purge | 100% Argon | Ar + 2-3% N2 (duplex) | 10-15 CFH until O2 < 100 ppm |
Do not use CO2 or Ar/CO2 blends for TIG on stainless. Carbon from CO2 can contribute to sensitization. Pure argon is the only shielding gas for TIG welding stainless steel.
MIG gas selection affects bead profile and spatter. 98/2 Ar/CO2 is the simple, effective choice. Tri-mix with helium produces a hotter arc with better penetration and wetting on thick material but costs significantly more. The 98/2 Ar/O2 blend works for spray transfer in flat position.
Common Filler Selection Mistakes
Using 308L on 316 base metal. The weld deposit won’t have molybdenum, creating a corrosion-weak zone in the joint. The weld is the first thing that pits in chloride service.
Using 309L for 304-to-304 joints. It works but wastes money and over-alloys the weld. The higher chromium and nickel in 309L shift the weld metal composition away from the base metal, which can affect ferrite number and corrosion behavior. Use 308L for same-grade joints.
Using standard (non-L) filler in corrosive service. Standard 308 filler has up to 0.08% carbon, which promotes sensitization in the weld deposit and HAZ. Always use L-grade filler when the weld will contact corrosive media.
Using carbon steel filler on stainless. It happens more than you’d think, usually when someone grabs the wrong spool. The weld will have adequate strength but zero corrosion resistance. The joint rusts within days in any corrosive environment. Label your stainless filler clearly and store it separately from carbon steel consumables.
No back purge on critical applications. The filler is correct, the technique is perfect, but the root side sugars because nobody purged. The 304 vs 316 article covers purging requirements in detail.
Quick Reference: What Filler Do I Grab?
- 304 to 304: ER308L / E308L-16
- 316 to 316: ER316L / E316L-16
- 304 to 316: ER316L / E316L-16
- Any stainless to carbon steel: ER309L / E309L-16
- 316 to carbon steel: ER309LMo / E309LMo-16
- 321 or 347 to itself: ER347 / E347-16
- 2205 duplex: ER2209 / E2209-16
- 2507 super duplex: ER2594 / E2594-16
- MIG version of any above: Add “Si” suffix (ER308LSi, ER316LSi)
- Not sure? ER309L covers more combinations than any other stainless filler. It’s the “universal” choice for dissimilar joints, though it’s not optimal for same-grade joints.
Ferrite Content in Stainless Weld Metal
Austenitic stainless steel weld metal should contain 3-10 FN (Ferrite Number) of delta ferrite to resist solidification cracking. Pure austenite solidifies with a fully columnar grain structure that’s susceptible to hot cracking at grain boundaries. A small amount of ferrite breaks up the columnar structure and provides a crack-healing mechanism during solidification.
L-grade fillers (308L, 316L) are designed to produce controlled ferrite in the deposit. The Schaeffler or WRC-1992 diagrams predict ferrite content based on the chromium equivalent and nickel equivalent of the weld deposit after dilution. If you’re welding to a code that specifies ferrite requirements (common in nuclear, power generation, and petrochemical), verify ferrite content with a calibrated ferrite gauge on production welds.
Too much ferrite (above 12-15 FN) can reduce toughness and corrosion resistance. Too little (below 3 FN) increases hot cracking susceptibility. The correct filler for each base metal, combined with proper technique and dilution control, keeps ferrite in the target range without extra calculations.
Storage and Handling
Stainless steel filler metals require more careful handling than carbon steel consumables:
- Store stainless filler separately from carbon steel wire and rods. Cross-contamination ruins stainless corrosion resistance. Label shelves, drawers, and storage bins clearly.
- Keep wire spools sealed in plastic bags when not in use. Moisture and shop contaminants can cause porosity and surface defects.
- Don’t touch filler rod with bare hands. Body oils and salts contaminate the surface. Use clean gloves when handling TIG rod.
- Stainless stick electrodes (E308L-16, E316L-16) don’t require rod ovens like carbon steel low-hydrogen rods. The rutile coating is less moisture-sensitive than low-hydrogen coatings. Store in original sealed containers and keep dry.
- Inspect wire before use. Look for rust spots (iron contamination), kinks, and surface discoloration. Contaminated wire produces contaminated welds.
Keep this chart posted near your welding station. The cost difference between the correct filler and the wrong one is a few dollars per pound of wire. The cost of a corrosion failure from the wrong filler is measured in replacement labor and lost production. Get it right the first time.