ER308L is the standard TIG filler rod for welding 304 and 304L stainless steel. The “L” designation means low carbon (0.03% maximum), which prevents sensitization during welding. Sensitization is chromium carbide precipitation at grain boundaries that destroys corrosion resistance in the heat-affected zone. On any 300-series stainless that will see corrosive service, moisture, or food contact, ER308L is the correct filler. Use it with pure argon shielding on DCEN.
The wire is classified under AWS A5.9/A5.9M and deposits a weld with 75,000 PSI minimum tensile strength. Its chemistry contains 19.5-22% chromium and 9-11% nickel to match the austenitic structure of 304 stainless base metal.
AWS A5.9 Classification
The designation tells you exactly what’s in the wire:
- E = Electrode (carries current)
- R = Rod (usable as hand-fed TIG filler)
- 308 = Chemical composition type (18-8 Cr-Ni with modifications)
- L = Low carbon (0.03% max)
The 308 chemistry is designed to match 304 stainless steel. It’s slightly overalloyed in chromium and nickel compared to 304, which compensates for dilution with the base metal during welding and ensures the weld deposit maintains a fully austenitic or slightly ferritic microstructure.
Chemical Composition
| Element | Weight % | Purpose |
|---|---|---|
| Carbon (C) | 0.03 max | Keep low to prevent sensitization |
| Chromium (Cr) | 19.5 - 22.0 | Corrosion resistance, passivation layer |
| Nickel (Ni) | 9.0 - 11.0 | Austenite stabilizer, toughness |
| Manganese (Mn) | 1.0 - 2.5 | Deoxidizer, austenite stabilizer |
| Silicon (Si) | 0.30 - 0.65 | Deoxidizer, fluidity |
| Phosphorus (P) | 0.03 max | Residual (keep low) |
| Sulfur (S) | 0.03 max | Residual (keep low) |
| Molybdenum (Mo) | 0.75 max | Residual |
| Copper (Cu) | 0.75 max | Residual |
The critical spec is carbon at 0.03% max. Standard ER308 allows up to 0.08% carbon, which is enough to cause sensitization in the 800-1500F (425-815C) temperature range. During multi-pass welding, the heat-affected zone spends time in this range repeatedly. Low carbon prevents chromium from bonding with carbon to form carbides, keeping the chromium in solid solution where it provides corrosion resistance.
Mechanical Properties
| Property | AWS Minimum | Typical Value |
|---|---|---|
| Tensile Strength | 75,000 PSI (515 MPa) | 80,000 - 90,000 PSI |
| Yield Strength | Not specified | 45,000 - 60,000 PSI |
| Elongation (2") | 35% min | 38 - 45% |
| Ferrite Number (FN) | 5 - 20 FN (typical) | 8 - 12 FN |
The ferrite number matters. A deposit with 5-20 FN contains enough delta ferrite to resist hot cracking during solidification but stays predominantly austenitic for corrosion resistance. Deposits below 3 FN are crack-sensitive. Deposits above 20 FN may have reduced corrosion resistance and toughness. Most quality ER308L wire produces 8-12 FN, which sits right in the sweet spot.
Sensitization and Why the “L” Matters
Sensitization is the single most common failure mode in stainless steel welds. Here’s what happens: when austenitic stainless sits in the 800-1500F range, carbon atoms migrate to grain boundaries and combine with chromium to form chromium carbides (Cr23C6). This depletes chromium from the area immediately surrounding each grain boundary, dropping the local chromium content below the 10.5% minimum needed for the passive oxide layer that gives stainless its corrosion resistance.
The result is intergranular corrosion. The weld looks fine, but the heat-affected zone corrodes along grain boundaries when exposed to acids, chlorides, or even plain water over time. Knife-line attack on the fusion line is especially common.
ER308L limits carbon to 0.03% max, which leaves too few carbon atoms to form significant carbides even during extended time in the sensitization range. That’s the whole reason low-carbon grades exist.
When you don’t need the L: Non-corrosive service, dry indoor applications, or parts that will be solution-annealed after welding (heating to 1900-2100F and water quenching). In these cases, standard ER308 works fine and actually provides slightly higher tensile strength.
Interpass Temperature Control
Stainless steel retains heat longer than carbon steel, and the austenitic grades don’t transform on cooling to release internal stresses. Controlling interpass temperature prevents overheating the weld zone and the base metal.
Maximum interpass temperature: 350F (175C). Measure with a contact thermometer or temp-indicating crayon before starting each pass. If the joint exceeds 350F, stop and let it cool. Forced air cooling is acceptable. Water quenching on austenitic stainless is fine metallurgically (it doesn’t harden like carbon steel), but thermal shock on thin sections can cause distortion.
Running hot interpass temps on stainless causes:
- Increased sensitization risk (more time in the 800-1500F range)
- Greater distortion from thermal expansion
- Wider heat-affected zone
- Loss of mechanical properties from grain growth
Diameter Selection
| Rod Diameter | Base Metal Thickness | Amperage Range (DCEN) | Applications |
|---|---|---|---|
| 0.045" (1.2 mm) | 22 ga - 18 ga | 15 - 50A | Thin-wall tubing, food equipment |
| 1/16" (1.6 mm) | 18 ga - 1/8" | 40 - 100A | Kitchen equipment, light fab |
| 3/32" (2.4 mm) | 1/8" - 3/16" | 80 - 160A | Pipe, structural, pressure vessels |
| 1/8" (3.2 mm) | 3/16" - 3/8"+ | 130 - 230A | Heavy plate, multi-pass builds |
Stainless steel TIG welding typically runs 10-15% lower amperage than carbon steel of the same thickness. Stainless conducts heat poorly, so the heat concentrates in the weld zone. Running carbon steel settings on stainless causes excessive penetration, carbide precipitation, and warpage.
Base Metal Compatibility
ER308L is matched to specific grades. Using it on the wrong base metal is a common and costly mistake.
Correct base metals for ER308L:
- 304 stainless steel (UNS S30400)
- 304L stainless steel (UNS S30403)
- 302 stainless steel
- 301 stainless steel
- 304H (for non-high-temp service only)
Not appropriate for:
- 316/316L (use ER316L for molybdenum matching)
- 321/347 (use matching ER321 or ER347, or ER308L if the stabilized grades are in solution-annealed condition)
- 309/310 (use ER309L or ER310)
- Dissimilar joints to carbon steel (use ER309L)
The dissimilar metal rule catches people. When welding 304 stainless to carbon steel, ER309L is required because it has higher chromium and nickel to compensate for dilution with the carbon steel. ER308L on a dissimilar joint produces a deposit too low in Cr and Ni where it mixes with the carbon steel side, resulting in a martensitic zone that’s hard and crack-prone.
Back-Purging Requirements
Any stainless steel TIG weld that has an exposed root side needs argon back-purging. The backside of the weld reaches temperatures where chromium oxidizes rapidly in air, forming black, crusty “sugar” (heavy oxide scale). This oxidized root isn’t just ugly; it’s a corrosion initiation point.
Pipe and tube joints: Seal the pipe on both sides of the joint with foam purge dams or aluminum tape. Flow argon at 5-10 CFH through the enclosed space until oxygen levels drop below 1% (below 0.1% for critical work). Maintain purge flow until the root pass and at least the first hot pass are complete.
Open joints and lap welds: Back-purge isn’t practical on some joint configurations. For food-grade and pharmaceutical work, design joints that allow purging or plan for post-weld passivation treatment.
Trail shields: On the cap pass, a trailing gas shield helps prevent surface oxidation (the straw/blue/purple colors that indicate heat tint). While light heat tint on the face isn’t always a structural concern, it reduces corrosion resistance and fails cleanliness standards in food, dairy, and pharmaceutical applications.
Welding Technique Tips
Stainless steel TIG welding with ER308L requires more discipline than carbon steel work. The material punishes sloppy technique.
Travel speed: Keep moving. Lingering in one spot overheats the base metal, causes excessive penetration, and increases sensitization risk. Faster travel with a slightly lower amperage produces better results than slow travel at high amps.
Arc length: Keep it tight, about 1/16 to 3/32 inch. Stainless is sensitive to atmospheric contamination, and a long arc pulls in air past the gas shielding. Contaminated welds turn gray or black and lose corrosion resistance.
Filler addition: Dab the rod into the leading edge of the puddle without pulling it out of the gas envelope. If the rod tip oxidizes (turns black), you’re pulling it too far from the shielding gas. Oxidized rod contaminates the next dab.
Tacking: Keep tacks small and grind the ends to a taper. Stainless tacks that are too large create local hot spots that cause distortion, and heavy tack craters can trap slag that causes porosity when you weld over them.
Storage and Handling
ER308L TIG rod comes in 36-inch cut lengths in sealed plastic tubes. The rod surface should appear clean and uniformly bright. Any discoloration or dark spots indicate oxidation or contamination.
Handle stainless filler rod with clean gloves. Oil from bare hands contaminates the rod surface and introduces carbon into the weld deposit. Don’t mix stainless rod storage with carbon steel rod; carbon steel dust on a stainless rod surface causes carbon contamination and carbide formation in the weld.
Dedicated stainless-only tools apply to more than just filler rod. Use stainless wire brushes, stainless grinding discs (never ones used on carbon steel), and clean gloves to avoid cross-contamination that ruins corrosion resistance.
Common Brands
All major filler metal manufacturers produce ER308L TIG rod:
- Lincoln Electric: Blue Max ER308L
- ESAB: OK Autrod ER308L
- Weldcote: ER308L stainless rod
- Blue Demon: ER308LSI (silicon-enhanced variant for improved wetting)
Price at time of writing runs $20-35 per pound for 36-inch cut lengths, significantly more than carbon steel filler rod. The cost reflects the chromium and nickel content. Buy from suppliers that provide heat lot traceability if you’re welding to code.
For more on TIG filler rod selection across all base metals, see the TIG filler rod selection guide. If you’re welding aluminum instead, the ER4043 aluminum filler guide and ER5356 aluminum filler guide cover those alloys.