321 vs 316L
Two stainless steels optimized for completely different threats: 316L for chlorides (corrosion), 321 for temperature (creep and sensitization). Choosing the wrong one leads to premature failure — here is how to decide.
| Property | 321 / 321H | 316L |
|---|---|---|
| UNS | S32100 | S31603 |
| Stabilization | Titanium (5×C min) | None (low C only) |
| Molybdenum | — | 2.0 – 3.0% |
| Tensile Strength | ≥ 515 MPa | ≥ 485 MPa |
| Yield Strength | ≥ 205 MPa | ≥ 170 MPa |
| Max Service Temp | 900°C (321H) | 800°C |
| Creep Strength at 600°C | Excellent | Moderate |
| PREN | 18 – 20 | 24 – 28 |
| Cl⁻ Resistance | Poor (like 304) | Good (Mo addition) |
| Sensitization (IGC) | Immune (Ti-stabilized) | Resistant (low C) |
| Relative Cost | 1.0× (base) | 1.3 – 1.5× |
The Core Difference: Ti Stabilization vs Mo Addition
321 adds titanium to "lock up" carbon — preventing chromium carbide formation at grain boundaries during welding and high-temperature exposure. This makes 321 immune to intergranular corrosion (sensitization) even after prolonged service above 425°C. 316L relies on ultra-low carbon (≤ 0.03%) to slow sensitization — effective for typical welding, but insufficient above 425°C long-term. Meanwhile, 316L's molybdenum provides chloride pitting resistance that 321 completely lacks.
Choose 321 When:
- Continuous service above 540°C
- Boiler superheater and reheater tubes
- Refinery fired heater and reformer tubes
- Exhaust systems and manifolds
- Repeated welding where PWHT is impossible
- Thermal cycling between ambient and 800°C+
Choose 316L When:
- Chloride present in any concentration
- Offshore, marine, coastal environments
- Chemical processing with acids
- Temperature below 400°C continuously
- Desalination and water treatment
- Any application where corrosion (not heat) is the threat
The Bottom Line
321 is a heat-resistant stainless steel — optimized for the refinery, power plant, and furnace applications where 316L would creep, sensitize, and fail. 316L is a corrosion-resistant stainless steel — optimized for the chloride, acid, and saltwater environments where 321 would pit within weeks. They are not substitutes; they complement each other. If your application involves BOTH high temperature AND chlorides, skip both and go directly to duplex or nickel alloy.