The difference is chromium content and corrosion resistance: duplex stainless steels are built around 22% chromium, super duplex around 25%. That raises the pitting resistance equivalent number (PREN) from about 34 to above 40. Both share a two-phase austenitic-ferritic microstructure roughly twice as strong as standard austenitic stainless, but super duplex resists pitting in tougher environments like seawater.
What sets super duplex apart from duplex?
The headline difference is chromium. Duplex stainless steels are built around 22% chromium; super duplex grades around 25%. More chromium raises the pitting resistance equivalent number (PREN), calculated as PREN = %Cr + 3.3 x %Mo + 16 x %N, from about 34 for duplex to above 40 for super duplex.
Normally, adding chromium means adding expensive nickel to keep the balanced austenitic-ferritic microstructure. Super duplex uses nitrogen instead, giving high strength and corrosion resistance at a competitive price against more heavily alloyed grades. For more on the metric, see Understanding PREN.
How do their properties compare?
Both share a two-phase (duplex) microstructure of austenitic and ferritic grains, making them around twice as strong as austenitic stainless steels, with toughness and ductility between austenitic and ferritic grades, and good resistance to stress corrosion cracking.
| Property | Duplex | Super duplex |
|---|---|---|
| Chromium | ~22% | ~25% |
| Key alloying | Higher nickel to balance the microstructure | Nitrogen in place of nickel |
| PREN | ~34 | >40 |
| Strength | ~2x austenitic | ~2x austenitic |
| Typical use | general corrosion service | subsea, marine, seawater |
| Representative grades | Alloy 2205 | Ferralium® 255, Alloy 32760, Alloy 32750 |
What about hyper duplex?
Hyper duplex grades, based on 27% to 29% chromium, push corrosion resistance higher still. They are harder to manufacture, available only in tube form, and rarely held in stock.
Which grades does Langley Alloys stock?
Langley Alloys has worked with super duplex for more than 50 years and developed the original super duplex, Ferralium® 255 (UNS S32550, F61, 1.4507). Its 2.0% copper gives a higher yield strength than other super duplex grades and strong resistance to sulphuric, nitric and phosphoric acids. We stock the full range:
| Grade | UNS | Also known as |
|---|---|---|
| Ferralium® 255 | S32550 | F61, 1.4507 |
| Alloy 32760 | S32760 | F55, 1.4501 |
| Alloy 32750 | S32750 | F53, 1.4410 |
| Alloy 2205 | S32205 | F51, 1.4462 (22% Cr duplex) |
Stock covers 40 sizes of super duplex solid bar from 0.5" (12.7mm) to 16" (406.4mm) diameter, plate up to 3" (76.2mm) thickness, plus seamless pipes. For a broader primer, see what is super duplex material?.