304 and 316 are the two most common austenitic stainless steels you’ll weld. Both are 18% chromium, both are non-magnetic, and both weld with similar techniques. The critical difference is 316 contains 2-3% molybdenum, which gives it substantially better resistance to pitting corrosion from chlorides and acids. That difference dictates filler metal selection: 308L for 304 base, 316L for 316 base. Mix them up and you compromise the corrosion resistance you’re paying for.
Both grades contain 8-12% nickel, which stabilizes the austenitic structure and gives these steels their ductility, toughness, and non-magnetic character. Welding doesn’t change the basic austenitic structure, but excessive heat input creates two problems: sensitization (chromium carbide precipitation at grain boundaries) and distortion from the high thermal expansion coefficient.
Composition Comparison
| Element | 304 / 304L | 316 / 316L | Why It Matters |
|---|---|---|---|
| Chromium | 18-20% | 16-18% | Corrosion resistance (passive film) |
| Nickel | 8-10.5% | 10-14% | Austenite stability, toughness |
| Molybdenum | None | 2-3% | Pitting/crevice corrosion resistance |
| Carbon (standard) | 0.08% max | 0.08% max | Sensitization risk |
| Carbon (L grade) | 0.03% max | 0.03% max | Reduced sensitization risk |
| Tensile Strength | 75 ksi (515 MPa) | 75 ksi (515 MPa) | Nearly identical strength |
| Yield Strength | 30 ksi (205 MPa) | 30 ksi (205 MPa) | Lower than carbon steel |
The “L” designation (304L, 316L) means low carbon: 0.03% max instead of the standard 0.08%. Low carbon dramatically reduces the risk of sensitization during welding. For any application involving corrosive environments, specify L-grade base metal and L-grade filler. The small additional cost of L-grade material is insignificant compared to the cost of premature corrosion failure.
Filler Metal Selection
This is where the 304 vs 316 distinction matters most. Using the wrong filler creates a weld zone that doesn’t match the corrosion resistance of the base metal.
304 Stainless: Use 308L Filler
308L filler has slightly higher chromium and nickel than 304 base metal. This compensates for dilution (mixing of filler and base metal in the weld pool) that would otherwise reduce the alloying content below minimum levels. The result is a weld deposit that matches or exceeds 304’s corrosion resistance.
316 Stainless: Use 316L Filler
316L filler contains molybdenum to match the base metal’s composition. Without that molybdenum, the weld becomes the weak point for pitting corrosion in chloride environments, which defeats the purpose of specifying 316 in the first place.
304 to 316 Dissimilar Joint
When joining 304 to 316, use 316L filler. The molybdenum in the filler compensates for dilution from the 304 side and ensures the weld has adequate pitting resistance. Some engineers specify 309LMo for this combination to further control dilution chemistry.
| Base Metal | TIG Filler | MIG Wire | Stick Electrode |
|---|---|---|---|
| 304 to 304 | ER308L | ER308LSi | E308L-16 |
| 304L to 304L | ER308L | ER308LSi | E308L-16 |
| 316 to 316 | ER316L | ER316LSi | E316L-16 |
| 316L to 316L | ER316L | ER316LSi | E316L-16 |
| 304 to 316 | ER316L | ER316LSi | E316L-16 |
| 304 to carbon steel | ER309L | ER309LSi | E309L-16 |
| 316 to carbon steel | ER309LMo | ER309LMo | E309LMo-16 |
Note on “Si” designation: ER308LSi and ER316LSi have added silicon for better wetting and fluidity in MIG welding. The silicon improves bead appearance but doesn’t significantly change corrosion resistance. Use the Si version for MIG, either version for TIG.
TIG Welding 304 and 316
TIG (GTAW) is the preferred process for stainless steel under 1/4 inch thick. It provides the precise heat control that stainless demands.
Polarity: DCEN (straight polarity) Shielding gas: 100% argon, 15-20 CFH Tungsten: 2% lanthanated, ground to a point
| Material Thickness | Tungsten Dia. | Filler Dia. | Amperage | Argon (CFH) |
|---|---|---|---|---|
| 0.030" (22 ga) | 1/16" | 1/16" | 20-40 | 12-15 |
| 0.060" (16 ga) | 1/16" | 1/16" | 40-70 | 15-18 |
| 0.090" (13 ga) | 3/32" | 1/16" | 60-100 | 15-18 |
| 1/8" (3.2 mm) | 3/32" | 3/32" | 80-130 | 15-20 |
| 3/16" (4.8 mm) | 3/32" | 3/32" | 120-170 | 15-20 |
| 1/4" (6.4 mm) | 1/8" | 1/8" | 160-220 | 18-22 |
Back purging is mandatory for any stainless application where the root side will contact corrosive media (food service, chemical processing, pharmaceutical, marine). Purge the back side with argon at 10-15 CFH until oxygen levels drop below 100 ppm. Without purge, the root side oxidizes heavily (sugaring), destroying corrosion resistance. For non-critical applications where the back side won’t see corrosion, you can skip the purge.
MIG Welding 304 and 316
MIG handles stainless production work and thicker sections where TIG would be too slow.
Shielding gas options:
- 98% Ar / 2% CO2: Good all-around, easy setup, acceptable for most work
- 90% He / 7.5% Ar / 2.5% CO2 (tri-mix): Best bead profile and penetration
- 98% Ar / 2% O2: Used for spray transfer on thicker material
Transfer mode: Pulse MIG reduces heat input and is preferred for stainless. Short-circuit works for thin material but can cause cold lapping. Spray transfer handles thick sections in flat position.
Run lower wire feed speeds than you’d use on carbon steel at the same thickness. Stainless holds heat longer, so the puddle stays fluid with less energy input.
Heat Management
Stainless steel has 40% higher thermal expansion and 30% lower thermal conductivity than carbon steel. That combination means it warps aggressively and holds heat longer in the weld zone. Two rules apply:
Interpass temperature: 350F maximum for 304 and 316. Measure between passes with a contact thermometer. Don’t weld the next pass until the previous one cools below 350F. Exceeding this limit increases sensitization risk and distortion.
Heat input: as low as practical. Stringer beads only, no weaving. Move fast. Use the minimum amperage that maintains a stable arc and good fusion. On thin sheet, skip-weld and use chill bars (copper backing) to pull heat away from the joint.
Stainless distortion is addressed by aggressive clamping, balanced welding sequences (alternate sides on double-sided joints), skip welding, and backstep technique. Plan the sequence before you start welding because once stainless distorts, straightening it is extremely difficult.
When to Specify 316 Over 304
304 is adequate for most indoor, mild-environment applications: food equipment (when properly passivated), architectural trim, handrails, machine guards, furniture, and general fabrication.
316 is required when the service environment includes:
- Chlorides (saltwater, road salt, swimming pool chemicals)
- Acidic chemicals (sulfuric, hydrochloric, phosphoric acid)
- Marine environments (coastal structures, boat hardware)
- Pharmaceutical and bioprocessing (316L is the standard)
- High-temperature corrosive environments (better creep resistance above 1000F)
The PREN (Pitting Resistance Equivalent Number) quantifies the difference. 304 has a PREN around 18-20. 316 runs 24-28 because of the molybdenum contribution (PREN = %Cr + 3.3(%Mo) + 16(%N)). Higher PREN means better pitting resistance.
Post-Weld Cleanup
Both 304 and 316 require proper post-weld cleaning to restore full corrosion resistance.
- Remove all heat tint (discoloration) using pickling paste or a pickling solution (nitric/hydrofluoric acid blend). Heat tint indicates a depleted chromium oxide layer that won’t protect against corrosion.
- Passivate the weld zone using a passivation solution (citric acid or nitric acid) to rebuild the protective chromium oxide film.
- Use stainless-only tools. Carbon steel wire brushes, grinding discs, and clamps contaminate the surface with iron particles that rust. Maintain separate, dedicated tools for stainless.
- Remove all iron contamination using a copper sulfate test if the application requires verified cleanliness. Any blue spots indicate free iron on the surface.
Skip the cleanup on non-critical, non-corrosive applications. For anything food-grade, chemical, marine, or pharmaceutical, post-weld cleaning isn’t optional.
Common Defects on 304 and 316
Sugaring (root side oxidation). Heavy black or brown oxidation on the root side of the weld from inadequate back purging. The oxide is porous, rough, and has no corrosion resistance. Fix: purge with argon until oxygen is below 100 ppm. For pipe and tube, seal both ends and flow argon at 10-15 CFH for several minutes before welding.
Hot cracking (solidification cracking). Centerline or crater cracks that form during solidification. Caused by pure austenitic weld metal (insufficient ferrite) or excessive restraint. Fix: use proper L-grade filler (designed to produce 3-10 FN ferrite), fill craters at the end of each pass, and reduce joint restraint where possible.
Porosity. Caused by contaminated base metal (oil, shop dirt, cutting fluid), inadequate gas coverage, or using CO2-containing gas on TIG. Fix: clean the joint thoroughly, check gas flow and cup condition, verify you’re running pure argon for TIG.
Warping and distortion. Stainless has 50% more thermal expansion than carbon steel and lower thermal conductivity. It warps more and faster. Fix: aggressive clamping, balanced welding sequence, skip welding, backstep technique, and minimal heat input.
Contamination from carbon steel tools. Carbon steel particles embedded in the stainless surface from grinding wheels, wire brushes, or clamps cause rust spots. Fix: maintain dedicated stainless-only tools. Wire brushes, grinding discs, and clamps that touch stainless should never touch carbon steel.
Cost Comparison: 304 vs 316
316 costs 15-25% more than 304 for base material, and the filler metals (316L vs 308L) run about 10-15% higher as well. That premium is worth it in chloride, marine, and chemical environments where 304 would fail. In dry, indoor, non-corrosive applications, 304 does the same job for less money.
When a customer asks for 316 “just to be safe” in an indoor, non-corrosive application, the cost premium buys nothing. When they ask for 304 on a seawater application, the money saved disappears in the first year when the 304 pits through.
The bottom line: 304 and 316 weld almost identically in terms of technique and parameters. The differences that matter are filler metal selection (308L vs 316L) and understanding which grade the application actually needs. Get the filler right, control your heat, purge the back side when it matters, and clean up the finished joint. The metallurgy does the rest.