Selection between duplex and super duplex stainless steel grades depends on three factors: (1) chloride concentration and service temperature (PREN requirement); (2) sour-gas service (NACE MR0175 hardness ≤ 32 HRC limit); (3) reducing-acid exposure (Cu-bearing Ferralium 255 advantage). The PREN ≥ 40 boundary distinguishes super-duplex from standard duplex; both classes share the balanced two-phase austenite-ferrite microstructure (35–55% ferrite per NORSOK M-630).
Standard duplex stainless steel category is dominated by Duplex 2205 (UNS S31803 / S32205, PREN ~35). Common applications: chemical processing (organic acids, phosphate fertilizer), pulp & paper digesters, moderate-chloride seawater (≤ 30 °C), oil & gas low-CO2 environments, food & beverage. Lower-tier duplex grades like Duplex 2304 (PREN ~26) and Duplex 2003 (PREN ~30) cover lighter-duty applications. Above Duplex 2205 service limits, super-duplex grades are required.
Three commercially significant super duplex grades: Ferralium 255 (UNS S32550, EN 1.4507, Cu-bearing), Super Duplex 2507 (UNS S32750, EN 1.4410, no Cu), Zeron 100 (UNS S32760, EN 1.4501, W-bearing + Cu). All have PREN ~40–42. Super duplex extends service to warm seawater, high-chloride brines, sour-gas (NACE MR0175 / ISO 15156-3), and reducing-acid chemical processing.
| Service environment | Recommended grade | PREN |
|---|---|---|
| Mild chemical processing, low-CO2 oil & gas, food & beverage | Duplex 2205 (S31803/S32205) | ~35 |
| Moderate-chloride seawater (≤ 30 °C) | Duplex 2205 | ~35 |
| Warm seawater (30–50 °C), offshore platforms | Super Duplex 2507 / Zeron 100 / Ferralium 255 | ~40–42 |
| Sour-gas service (NACE MR0175 hardness ≤ 32 HRC) | Any super duplex; Ferralium 255 if Cu spec | ~40–42 |
| Reducing acids (HCl, dilute H2SO4, brines) | Ferralium 255 (Cu advantage) | ~40 |
| Subsea offshore (NORSOK M-650) | Super Duplex 2507 / Zeron 100 / Ferralium 255 | ~40–42 |
| Most aggressive chloride / sour service (PREN ≥ 49) | Hyper duplex (S32707, S33207) | ~49+ |
Material cost typically scales with PREN: 316L < Duplex 2205 (~1.5×) < Super Duplex 2507/Ferralium 255/Zeron 100 (~2–2.5×) < Hyper duplex (~3–4×) < Inconel 625 (~5×). For applications within Duplex 2205 service limits, super-duplex upgrade is unjustified; for applications beyond, super-duplex prevents in-service pitting failures whose cost vastly exceeds the material premium.
Both standard duplex and super duplex grades are covered by the same ASTM specification family across product forms: ASTM A479 (bar), A789 (tube), A790 (pipe), A815 (fittings), A182 (forgings), A240 (plate / sheet). EN equivalents: EN 10088-3, EN 10222-5, EN 10272 (long products); EN 10088-2 (flat products); EN 10216-5 (seamless pipe).
