Duplex: twice the strength, and a hard temperature limit
A roughly 50/50 austenite and ferrite microstructure gives high strength and high chloride resistance at once — with real constraints.
Duplex stainless steels contain roughly equal proportions of austenite and ferrite. That mixed microstructure is not a compromise — it delivers something neither phase achieves alone: roughly double the yield strength of 316L, together with substantially better resistance to chloride stress corrosion cracking.
The grades
| Grade | UNS | Typical PREN | Used for |
|---|---|---|---|
| 2205 | S31803 / S32205 | 34–36 | The workhorse duplex |
| 2507 | S32750 | 42–43 | Seawater, sour service |
| Zeron-type | S32760 | 42–43 | Offshore, severe chloride |
| 2304 | S32304 | 24–26 | Lean duplex, mild service |
Scroll the table horizontally to see all columns.
Why the strength matters commercially
Duplex costs around two and a half times 304 per tonne, which sounds prohibitive until the wall thickness is recalculated. Because the allowable stress is roughly double that of 316L, a pressure-retaining component can often be specified thinner. On a large project that recovers much of the premium, and it reduces weight — which matters enormously offshore.
The constraints, stated plainly
- Hard upper service limit around 300 °C — above that, 475 °C embrittlement degrades the ferrite phase permanently
- Welding must be procedure-controlled to maintain the phase balance; excessive heat input produces too much ferrite and destroys the toughness
- Ferromagnetic, which rules it out where magnetic permeability is a requirement
- Demanding to machine, with high tool wear and a need for rigid setups
- Availability is narrower than the 300 series, particularly in fittings
Products covered
Specifying for a live project?
Send the service conditions and we will tell you what we would specify, and what we hold.