Article
Duplex stainless steels are used where 316L is no longer enough: seawater, hot chlorides, and anywhere stress corrosion cracking is a risk. They cost more per kilogram than 316L, but their higher strength often lets a thinner wall be used, and the cost gap narrows once that is counted.
What "duplex" means
Ordinary stainless steels such as 304 and 316 are austenitic. Duplex grades have a mixed microstructure of roughly half austenite and half ferrite. That combination gives them about twice the yield strength of 316L and much better resistance to chloride stress corrosion cracking, which is the weakness of the austenitic grades.
The two common grades
| Duplex 2205 | Super duplex 2507 | |
|---|---|---|
| UNS | S31803 / S32205 | S32750 |
| EN number | 1.4462 | 1.4410 |
| Chromium | about 22% | about 25% |
| Nickel | about 5% | about 7% |
| Molybdenum | about 3% | about 4% |
| Typical PREN | about 35 | over 40 |
S32205 is a tighter version of S31803, with higher minimum nitrogen and chromium. Most material now produced meets both and is certified to both.
PREN: a way to compare pitting resistance
The pitting resistance equivalent number is calculated from the composition, commonly as PREN = %Cr + 3.3 × %Mo + 16 × %N. The higher the number, the better the resistance to pitting in chlorides. 316L sits around 24, duplex 2205 around 35, and super duplex 2507 above 40. PREN is a guide for ranking grades, not a prediction of service life; temperature and chloride concentration matter as much.
Where each is used
- 2205: chemical process piping, desalination, pulp and paper, heat exchangers, offshore process lines, and marine applications at moderate temperature.
- 2507: seawater systems, offshore topsides, fire water, subsea equipment, and high-chloride service where 2205 is marginal.
Things to know before ordering
- Temperature limit: duplex grades are normally not used above about 250 to 300 °C, because they can become brittle with long exposure.
- Welding: heat input and interpass temperature must be controlled to keep the ferrite and austenite balance right. Ferrite content is often checked on welds.
- Testing: project specifications frequently require ASTM A923 testing for detrimental intermetallic phases.
Product standards
Pipe to ASTM A790, plate to ASTM A240, forged flanges and fittings to ASTM A182 F51 and F53, buttweld fittings to ASTM A815.
Tell us the service conditions when you enquire, and we will confirm whether 2205 or 2507 is the right step up from 316L. Browse pipes and flanges.
Frequently asked questions
What is duplex stainless steel?
A stainless steel with a mixed austenite and ferrite structure, giving high strength and good chloride resistance.
What is the difference between 2205 and 2507?
2507 super duplex has more chromium, molybdenum and nitrogen, with PREN above 40 compared with about 35 for 2205.
Why is ferrite content checked in duplex?
The phase balance controls toughness and corrosion resistance, so ferrite is measured in base metal and welds.
Key takeaways
- Duplex offers about twice the yield strength of 316L.
- 2507 super duplex is used for seawater and severe chlorides.
- Control heat input and verify ferrite when welding.
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