Super duplex stainless steel composition is dominated by chromium (24–27%), molybdenum (3–5%), nitrogen (0.20–0.32%), nickel (5–8%), with the balance iron. Differentiating alloying elements distinguish the three commercial super-duplex grades: Ferralium 255 has 1.5–2.5% copper; Zeron 100 has 0.5–1.0% tungsten plus Cu; Super Duplex 2507 has neither Cu nor W (relying instead on slightly higher Mo and N).
| Element | Ferralium 255 (S32550) | Super Duplex 2507 (S32750) | Zeron 100 (S32760) |
|---|---|---|---|
| Carbon (C), max. | 0.04 | 0.030 | 0.030 |
| Chromium (Cr) | 24.0–27.0 | 24.0–26.0 | 24.0–26.0 |
| Nickel (Ni) | 4.5–6.5 | 6.0–8.0 | 6.0–8.0 |
| Molybdenum (Mo) | 2.9–3.9 | 3.0–5.0 | 3.0–4.0 |
| Nitrogen (N) | 0.10–0.25 | 0.24–0.32 | 0.20–0.30 |
| Copper (Cu) | 1.5–2.5 | max 0.5 | 0.5–1.0 |
| Tungsten (W) | none | none | 0.5–1.0 |
| Manganese (Mn), max. | 1.5 | 1.20 | 1.0 |
| Silicon (Si), max. | 1.0 | 0.80 | 1.0 |
| Sulphur (S), max. | 0.030 | 0.020 | 0.010 |
| Phosphorus (P), max. | 0.04 | 0.035 | 0.030 |
Composition directly determines PREN per the formula PREN = %Cr + 3.3·%Mo + 16·%N. With nominal Ferralium 255 composition (25 Cr, 3 Mo, 0.2 N): PREN = 25 + 9.9 + 3.2 = 38.1 minimum; typical 40–42 per heat. Super Duplex 2507 (25 Cr, 4 Mo, 0.27 N): PREN = 42.5 typical. Zeron 100 PREW ~41–42 typical (using modified W-bearing formula). PREN ≥ 40 is the threshold defining super-duplex grades.
Standard Duplex 2205 chemical composition (Cr 22%, Mo 3%, N 0.15%, no Cu) puts PREN at ~35, below the super-duplex threshold. The composition jump from Duplex 2205 to Ferralium 255 is +3% Cr, +0% Mo (similar), +0.05% N, and +2% Cu added. The composition jump from Duplex 2205 to Super Duplex 2507 is +3% Cr, +1% Mo, +0.12% N. Both jumps push PREN above 40.
Composition verified per heat by certified chemical analysis on Mill Test Certificates (EN 10204 type 3.2). PREN calculated and reported per heat. NORSOK M-650 QTRs additionally include intergranular corrosion test per ASTM A923 method A or C, ferrite content per ASTM E562, and Critical Pitting Temperature per ASTM G48 method A. See Ferralium 255 chemistry for grade-specific composition reference.
