GRADE FAMILY
Duplex and super duplex stainless steels
Duplex grades combine a two-phase ferrite-austenite structure that gives them notably higher mechanical strength than standard austenitics, along with a high PREN. 2205 covers most brackish water and petrochemical needs; super duplex 2507 raises the PREN further for seawater and offshore service.
What is a duplex stainless steel?
A duplex stainless steel has a mixed structure, roughly half ferrite and half austenite, achieved through a precise balance of chromium, nickel, molybdenum and nitrogen. This two-phase structure combines the mechanical strength of ferrite with the toughness of austenite, with stress-corrosion resistance superior to standard austenitics at a comparable PREN.
When to choose this family — and when not to?
Choose it for brackish water, desalination, offshore and petrochemical service, where high mechanical strength also allows a reduction in wall thickness at equal pressure. Avoid it above roughly 300°C, where 475°C embrittlement becomes a risk, and for deep cryogenic service — see the table below for each grade's PREN.
Grades in this family
The grades listed are examples, never a closed catalogue — every grade belongs to one of the nine families, which is what makes this claim reliable.
| Grade (example) | EN | Indicative PREN |
|---|---|---|
| 2101 | 1.4162 | 24–29 |
| 2304 | 1.4362 | 24–30 |
| 2205 | 1.4462 | 34–36 ¹ |
| 2507 | 1.4410 | 42–43 ¹ |
| Zeron 100 | — | — |
¹ inherited value, not yet cross-checked against a product standard.
PREN notes
- 2101
- Comparable to a 316L, with nickel reduced to 1.35–1.70% versus 10–14%. That's the whole logic of lean duplex: performance comes from nitrogen and manganese, not nickel.
- 2304
- Equivalent to 2101 in PREN, but via a different route: 2304 relies on chromium at 21.5–24.5% where 2101 relies on nitrogen and manganese.
Typical applications for this family: Brackish water · Seawater · Offshore · Desalination.
See also: Grade equivalents → · PREN calculator →
Available forms and dimensions
Welded and seamless tube, fittings, flanges and bars available in 2205 and 2507 through our partner mills. U-tubes and non-standard lengths on request — duplex welding requires specific process control, so lead times should be planned for. Outer diameter from 10 to 610 mm — structured data available for 2205 and 2507 only. The other grades in this family don't yet have this in our structured reference data: send us your requirement via a quote request.
Applicable standards
- EN 10216-5
- EN 10217-7
- ASTM A312
- ASTM A213
- ASTM A249
- ASTM A269
- ASTM A270
- EN 10088-1
Frequently asked questions
- Why choose duplex over a reinforced austenitic?
- For the mechanical strength — nearly double a standard austenitic — which allows a thinner wall at equal pressure, on top of a higher PREN for severe chlorinated environments.
- Is super duplex 2507 always preferable to 2205?
- No: 2507 costs more and is harder to fabricate. It is justified in seawater and severe offshore service; for brackish water and standard petrochemical use, 2205 is sufficient in the large majority of cases.
- Does duplex weld like an austenitic grade?
- Not quite: the process must maintain the ferrite-austenite balance in the heat-affected zone, or corrosion resistance degrades. A duplex-qualified welder and suitable process control are required.
- My duplex grade isn't listed — what should I do?
- Other duplex grades exist beyond 2205 and 2507 — lean duplex 2101, 2304, or specific grades like Zeron 100. Send us your designation via a quote request.