What is the difference between duplex 2205 and super duplex 2507?
Duplex 2205 (ASTM A182 F51) has a PREN of roughly 35 and suits moderate chloride service. Super duplex 2507 (F53) carries higher chromium, molybdenum and nitrogen, giving a PREN above 40 and markedly better pitting resistance in seawater and high-chloride service, at a higher cost.
Key Takeaways
- Duplex is a two-phase microstructure, roughly half ferrite and half austenite, which is where its strength and stress-corrosion resistance come from.
- PREN — pitting resistance equivalent number — is the standard shorthand: chromium plus 3.3 times molybdenum plus 16 times nitrogen.
- Duplex 2205 sits at a PREN of about 35; super duplex 2507 exceeds 40, the threshold usually quoted for seawater service.
- Both have roughly twice the yield strength of 316L, so a duplex line can use thinner walls for the same pressure.
- Both are limited to about 300 °C in service, because prolonged higher temperature causes embrittling intermetallic precipitation.
- Ferrite balance is set during forging and heat treatment, so it is worth verifying on the finished part rather than on the billet.
Why duplex exists at all
Austenitic stainless such as 316L resists general corrosion well but has modest yield strength and is vulnerable to chloride stress-corrosion cracking. Ferritic stainless is stronger and resists chloride cracking but has poor toughness. Duplex is a deliberate mixture of the two phases, roughly fifty-fifty, chosen so the alloy inherits the strength and chloride-cracking resistance of ferrite and the toughness and general corrosion resistance of austenite.
The practical result is an alloy with about twice the yield strength of 316L and much better chloride performance. On a project that translates into thinner walls for the same pressure, which offsets part of the higher price per kilogram.
Duplex 2205 against super duplex 2507
The two are used for the same reasons; the difference is how much chloride and how much pressure the service imposes.
| Property | Duplex 2205 (F51) | Super duplex 2507 (F53) |
|---|---|---|
| UNS number | S31803 / S32205 | S32750 |
| ASTM forging grade | A182 F51 | A182 F53 |
| Chromium | about 22 per cent | about 25 per cent |
| Molybdenum | about 3 per cent | about 4 per cent |
| Nitrogen | about 0.17 per cent | about 0.27 per cent |
| PREN | approximately 35 | above 40 |
| Typical minimum yield | about 450 MPa | about 550 MPa |
| Seawater service | Marginal — depends on temperature | Suitable |
| Relative cost | Lower | Higher |
| Upper service temperature | about 300 °C | about 300 °C |
PREN is the number most specifications turn on. It is calculated as chromium plus 3.3 times molybdenum plus 16 times nitrogen, and a value above 40 is the figure usually quoted as the threshold for seawater service at ambient temperature. That is why 2507 appears on desalination and offshore work while 2205 is more common on process lines with moderate chloride content.
The temperature limit that catches people out
Duplex and super duplex are not high-temperature alloys, and specifying them as if higher strength implies higher temperature capability is a genuine error. Above roughly 300 °C, prolonged exposure causes intermetallic phases — sigma phase in particular — to precipitate at the ferrite-austenite boundaries. The alloy loses toughness and corrosion resistance, and the change is not reversible in service.
The same mechanism explains why heat treatment control during manufacture matters so much. Ferrite balance and freedom from intermetallic precipitation are set by the solution annealing and quenching cycle, not by the melt chemistry. Two forgings with identical composition certificates can behave very differently if one was cooled too slowly.
That is why ferrite content measurement and, where the specification calls for it, ASTM G48 pitting testing are worth requiring on the finished component. We supply both on request — see the mill test certificate page for the full document pack.
What we manufacture in duplex and super duplex
Flanges to ASME B16.5 and B16.47 and to EN 1092-1 in ASTM A182 F51, F53 and F55, in Class 150 to 2500. Buttweld fittings to ASME B16.9 in ASTM A815. Forged fittings to ASME B16.11. Pipe and tube to ASTM A790 and A928. Round bar to ASTM A276 and A479. Sheet and plate to ASTM A240. Fasteners in duplex where the specification requires matching bolting.
Typical destinations are desalination trains, offshore topsides, chloride-bearing chemical process lines and seawater cooling circuits — covered in more detail on the desalination and shipbuilding and offshore pages.
Frequently Asked Questions
Pitting resistance equivalent number, calculated as chromium plus 3.3 times molybdenum plus 16 times nitrogen. It ranks an alloy’s resistance to chloride pitting. Duplex 2205 sits at about 35 and super duplex 2507 above 40, which is the value usually quoted as the threshold for seawater service.
Not always the right choice. Super duplex costs more and is harder to weld. Where chloride content and temperature are moderate, duplex 2205 does the job at lower cost, and specifying 2507 spends money without buying performance the service needs.
About 300 degrees Celsius. Above that, prolonged exposure causes sigma phase and other intermetallics to precipitate, which permanently reduces toughness and corrosion resistance.
Often yes. Duplex has roughly twice the yield strength of 316L, so a thinner wall can carry the same pressure. That offsets part of the higher price per kilogram, and on large-bore lines it can offset all of it.
Ferrite content measurement, and ASTM G48 pitting testing where the specification requires it, both carried out on the finished component. Chemistry alone does not confirm that the heat treatment produced the intended microstructure.
ASTM A182 F51 (2205), F53 (2507) and F55 in flanges and forged fittings, with matching pipe, fittings, bar and plate in the corresponding ASTM specifications.