The duplex and super duplex stainless steel family covers two-phase austenite-ferrite stainless grades, distinguished from each other by Pitting Resistance Equivalent Number (PREN = %Cr + 3.3·%Mo + 16·%N). The PREN ≥ 40 boundary defines super duplex; standard duplex grades sit at PREN ~30–38. The family combines the high mechanical strength of ferritic stainless steel (yield ≥ 450 MPa for standard duplex, ≥ 550 MPa for super duplex) with corrosion resistance superior to standard austenitic stainless steels (304/316L) in chloride-bearing environments.
The most-specified standard duplex grade is Duplex 2205 (UNS S31803 or modern narrowed S32205). Composition: 22 Cr – 5 Ni – 3 Mo – 0.15 N (no Cu). PREN ~35. EN designation 1.4462 / X2CrNiMoN22-5-3. Used in chemical processing, moderate-chloride seawater, oil & gas low-CO2 environments, pulp & paper. Lower-tier standard-duplex grades (Duplex 2304 UNS S32304 with PREN ~26 and Duplex 2003 UNS S32003 with PREN ~30) cover lighter-duty applications.
The commercially significant super duplex stainless steel grades are: Ferralium 255 (UNS S32550, EN 1.4507, Cu-bearing), Super Duplex 2507 (UNS S32750, EN 1.4410, the most-common grade), and Zeron 100 (UNS S32760, EN 1.4501, W-bearing). All meet PREN ≥ 40. Super duplex extends service to warm seawater, high-chloride brines, sour-gas service per NACE MR0175, and reducing-acid chemical processing (Ferralium 255 specifically for the Cu advantage).
| Element | Duplex 2205 (S31803/S32205) | Ferralium 255 (S32550) | Super Duplex 2507 (S32750) | Zeron 100 (S32760) |
|---|---|---|---|---|
| Chromium (Cr) | 21.0–23.0 | 24.0–27.0 | 24.0–26.0 | 24.0–26.0 |
| Nickel (Ni) | 4.5–6.5 | 4.5–6.5 | 6.0–8.0 | 6.0–8.0 |
| Molybdenum (Mo) | 2.5–3.5 | 2.9–3.9 | 3.0–5.0 | 3.0–4.0 |
| Nitrogen (N) | 0.08–0.20 | 0.10–0.25 | 0.24–0.32 | 0.20–0.30 |
| Copper (Cu) | none | 1.5–2.5 | none | 0.5–1.0 |
| Tungsten (W) | none | none | none | 0.5–1.0 |
| Nominal PREN | ~35 | ~40 | ~42 | ~41 |
The PREN ≥ 40 boundary is not arbitrary, it correlates with Critical Pitting Temperature (CPT) per ASTM G48 method A. Standard duplex (Duplex 2205, PREN ~35) has CPT ~25 °C, marginal for typical seawater service at 20–30 °C. Super duplex (PREN ~40+) has CPT ≥ 35 °C, providing safety margin for warm-seawater offshore service and high-chloride environments. Hyper duplex grades (PREN ≥ 49, e.g. UNS S32707, S33207) sit one tier above super duplex for the most extreme chloride/sour-service environments.
Standard upgrade triggers from Duplex 2205 to a super-duplex grade like Ferralium 255: seawater service above 30 °C; sour-gas H2S partial pressure exceeding NACE MR0175 thresholds for standard duplex; reducing-acid service (HCl, dilute H2SO4), here Ferralium 255 specifically (Cu-bearing) outperforms Cu-free super duplex; high-velocity erosion-corrosion service. See Ferralium 255 vs Duplex 2205 upgrade guide.
