The five commercially significant super duplex grades, their UNS / EN / trade designations, and how to choose between them
The super duplex tier of stainless steel is defined by a single chemistry threshold: pitting resistance equivalent number (PREN) ≥ 40, calculated as Cr + 3.3 × Mo + 16 × N. Grades that fall below 40 are standard duplex (the dominant 22Cr grades S31803 / S32205, PREN ~35), grades at or above 40 are super duplex. Five grades hold most of the commercial market by tonnage, distinguished by their alloying-element strategy. The Cu-bearing grade Ferralium 255 (UNS S32550) carries 2 percent copper for reducing-acid service. The 2507 family (UNS S32750) is the most-stocked grade with the widest standards coverage. The W-bearing Zeron 100 (UNS S32760) carries 0.7 percent tungsten that contributes a half-point of PREN equivalence and improves crevice resistance. Two newer grades round out the modern super duplex lineup with slightly different alloying strategies for specific service envelopes.
| Grade name | UNS | EN | Cr | Ni | Mo | N | Cu | W | PREN, min |
|---|---|---|---|---|---|---|---|---|---|
| Ferralium 255 | S32550 | 1.4507 | 25 | 5 | 3 | 0.20 | 2.0 | none | 40 |
| Super Duplex 2507 | S32750 | 1.4410 | 25 | 7 | 4 | 0.27 | none | none | 43 |
| Zeron 100 | S32760 | 1.4501 | 25 | 7 | 3.5 | 0.25 | 0.7 | 0.7 | 40 (PREW ~41) |
| SAF 2906 (lean super duplex) | S32906 | 1.4477 | 29 | 6.5 | 2 | 0.40 | none | none | 42 |
| UR 52N+ | S32520 | 1.4507 / 1.4410 family | 25 | 7 | 3.5 | 0.25 | 1.5 | none | 41 |
PREW (PREN with W) = Cr + 3.3 × (Mo + 0.5 × W) + 16 × N. The W contribution recognises the synergistic effect of tungsten on crevice corrosion resistance.
