Super duplex stainless steel is the family of duplex stainless steel grades with PREN ≥ 40, where PREN (Pitting Resistance Equivalent Number) = %Cr + 3.3·%Mo + 16·%N. The category is defined by a balanced two-phase microstructure of austenite and ferrite (typically 35–55% ferrite per NORSOK M-630), high mechanical strength (yield ≥ 550 MPa for most grades), and resistance to pitting and crevice corrosion in chloride-bearing environments substantially superior to standard duplex (Duplex 2205, PREN ~35) or austenitic (316L, PREN ~25) stainless steels.
The commercially significant super duplex stainless steel grades are Ferralium 255 (UNS S32550, Cu-bearing), Super Duplex 2507 (UNS S32750, the most-common grade), and Zeron 100 (UNS S32760, W-bearing). All three are supplied solution-annealed; cold work and exposure to the 600–1000 °C range are avoided to prevent sigma-phase embrittlement and to keep hardness within NACE MR0175 sour-service limits (≤ 32 HRC). TorqBolt manufactures Ferralium 255 components and fasteners at Mumbai and Rajkot, India, with per-heat MTC to EN 10204 type 3.2 and optional NORSOK M-650 QTR + NACE MR0175 compliance documentation.
The four commercially specified grades in the duplex / super duplex family. Ferralium 255 is one of three grades meeting the PREN ≥ 40 super duplex threshold; Duplex 2205 sits one tier below at PREN ~35.
| Grade | UNS | EN | Distinctive element | Nominal PREN | Class |
|---|---|---|---|---|---|
| Ferralium 255 | S32550 | 1.4507 | Cu (~2%) | ~40 | Super duplex |
| Super Duplex 2507 | S32750 | 1.4410 | none (no Cu, no W) | ~42 | Super duplex |
| Zeron 100 | S32760 | 1.4501 | W (~0.7%) | ~41 | Super duplex |
| Duplex 2205 | S31803 / S32205 | 1.4462 | none (lower Mo/N) | ~35 | Standard duplex |
Super duplex composition is dominated by chromium (24–26%), molybdenum (3–4%), nitrogen (0.20–0.30%), and balanced nickel (5–7%) to stabilise austenite. Cu (Ferralium 255) and W (Zeron 100) are the differentiating alloying elements between super-duplex grades. Higher Cr/Mo/N drives PREN above 40; tighter ferrite-balance control distinguishes super duplex from standard Duplex 2205.
| Element | Ferralium 255 (S32550) | SD 2507 (S32750) | Zeron 100 (S32760) | Duplex 2205 (S31803) |
|---|---|---|---|---|
| Chromium (Cr) | 24.0–27.0 | 24.0–26.0 | 24.0–26.0 | 21.0–23.0 |
| Nickel (Ni) | 4.5–6.5 | 6.0–8.0 | 6.0–8.0 | 4.5–6.5 |
| Molybdenum (Mo) | 2.9–3.9 | 3.0–5.0 | 3.0–4.0 | 2.5–3.5 |
| Nitrogen (N) | 0.10–0.25 | 0.24–0.32 | 0.20–0.30 | 0.08–0.20 |
| Copper (Cu) | 1.5–2.5 | none (max 0.5) | 0.5–1.0 | none |
| Tungsten (W) | none | none | 0.5–1.0 | none |
| Carbon (C), max. | 0.04 | 0.030 | 0.030 | 0.030 |
PREN is the standard scalar predictor of pitting and crevice corrosion resistance for stainless steels. PREN ≥ 40 is the definitional threshold for super duplex stainless steel; standard duplex grades sit at PREN ~35; austenitic 316L at PREN ~25.
PREN = %Cr + 3.3 · %Mo + 16 · %N
For super-duplex grades with W content (Zeron 100), the PREW formula adds tungsten weighted at 1.65: PREW = %Cr + 3.3(%Mo + 0.5·%W) + 16·%N. Per-heat PREN is verified by certified composition on Mill Test Certificates (EN 10204 type 3.2). NORSOK M-650 QTRs additionally include Critical Pitting Temperature (CPT) per ASTM G48 method A, typically ≥ 35 °C for super duplex.
ASTM specifications for super duplex stainless steel cover all standard product forms. UNS designations (S32550, S32750, S32760) are referenced within each spec.
| ASTM Specification | Scope / Form | Notes |
|---|---|---|
| ASTM A479 | Bar, billet, shapes for pressure boundary use | Primary bar spec for super duplex |
| ASTM A789 | Seamless and welded ferritic/austenitic tube | Tube spec for super duplex |
| ASTM A790 | Seamless and welded ferritic/austenitic pipe | Pipe spec for super duplex |
| ASTM A815 | Wrought ferritic/austenitic stainless fittings | Fittings spec for super duplex |
| ASTM A182 | Forged or rolled flanges, fittings, valves | Forgings/flanges spec for super duplex |
| ASTM A276 | Bars and shapes (alternative to A479) | Common for stockholders |
| ASTM A240 | Plate, sheet, strip | Flat-product spec |
The duplex stainless steel category covers all two-phase austenite-ferrite stainless grades. The PREN ≥ 40 cutoff distinguishes super duplex from standard duplex. Standard duplex (Duplex 2205) is the workhorse for moderate chloride service (typical seawater up to 30 °C, oil & gas low-CO2 environments). Super duplex (Ferralium 255 / 2507 / Zeron 100) extends application to: warm seawater > 30 °C; sour-gas service per NACE MR0175; high-chloride brines; reverse-osmosis desalination; high-pressure subsea components.
Hyper duplex grades (UNS S32707, S33207) sit one tier above super duplex at PREN ≥ 49, used in extreme chloride and sour-service applications.
Duplex 2205 (UNS S31803, with the modern narrowed S32205 chemistry) is the most-specified duplex stainless steel grade globally. Composition: 22Cr-5Ni-3Mo-0.15N. PREN ~35 places it BELOW the super duplex threshold. EN designation 1.4462 / X2CrNiMoN22-5-3. Common applications: chemical processing (organic acids, phosphate fertilizer), pulp & paper digesters, moderate-chloride seawater, food & beverage. For higher-chloride or sour service, super-duplex grades like Ferralium 255 are specified instead. See Ferralium 255 vs Duplex 2205 upgrade guide for selection criteria, and Duplex 2205 chemical composition for full element-by-element ranges.
For offshore oil & gas service, super duplex stainless steel components require dual compliance: NORSOK M-650 manufacturer Qualification Test Records (QTR) per material data sheet MDS D55, and NACE MR0175 / ISO 15156-3 sour-service compliance. NORSOK M-650 QTRs document heat treatment, ferrite content (35–55% base, 35–65% weld), Charpy ≥ 45 J at −46 °C, CPT per ASTM G48 method A, and microstructural verification. NACE MR0175 caps hardness at ≤ 32 HRC and ferrite at 35–55% per heat. TorqBolt is a NORSOK M-650 capable manufacturer for Ferralium 255; QTR available on request. See NORSOK M-650 Ferralium 255 QTR and NACE MR0175 Ferralium 255 sour-service compliance for documentation samples.
Super duplex stainless steel is selected when service conditions exceed the limits of standard duplex (Duplex 2205) or austenitic stainless (316L). Primary application clusters:
Ferralium 255 (UNS S32550) is one of the three commercially significant super duplex grades. The defining feature is its 2% copper content, which gives Ferralium 255 substantially better resistance to reducing acids (dilute sulphuric, hydrochloric, brines) than Cu-free super duplex grades like 2507 or Zeron 100. For seawater and chloride pitting alone, all three super duplex grades perform comparably; for chemical-processing service involving reducing acids, Ferralium 255 is the preferred specification. TorqBolt manufactures Ferralium 255 in bar (ASTM A479), plate / sheet (A240), pipe (A790), tube (A789), wrought fittings (A815), forged flanges (A182), forgings (A182), and a full range of fasteners (hex bolts, stud bolts, hex nuts, washers, threaded rod, socket-head cap screws) at our Mumbai and Rajkot, India facilities.
Super duplex stainless steel is the family of two-phase austenite-ferrite stainless grades with PREN ≥ 40, where PREN = %Cr + 3.3·%Mo + 16·%N. Commercially significant grades: Ferralium 255 (UNS S32550, Cu-bearing), Super Duplex 2507 (UNS S32750), Zeron 100 (UNS S32760, W-bearing). All have nominal Cr 24-26%, Mo 3-4%, N 0.2-0.3%.
Three grades dominate global super-duplex specification: Ferralium 255 (UNS S32550, EN 1.4507, Cu-bearing variant for reducing-acid service); Super Duplex 2507 (UNS S32750, EN 1.4410, the most-common grade); Zeron 100 (UNS S32760, EN 1.4501, W-bearing variant). All three meet PREN ≥ 40 and are covered by ASTM A479 (bar), A789 (tube), A790 (pipe), A815 (fittings), A182 (forgings/flanges).
The boundary is PREN ≥ 40. Standard duplex grades (Duplex 2205, UNS S31803/S32205) have PREN ~35 with composition 22Cr-5Ni-3Mo-0.15N. Super duplex grades (Ferralium 255, 2507, Zeron 100) have higher Cr (24-26%), higher Mo (3-4%), higher N (0.2-0.3%), pushing PREN above 40. Super duplex extends service to higher chloride concentrations, warmer seawater, and NACE-MR0175 sour service.
Ferralium 255 (UNS S32550) is supplied to ASTM A479 (bar), A789 (tube), A790 (pipe), A815 (fittings), A182 (forgings/flanges), and A276 (bars/shapes alternative). Plate per ASTM A240. EN equivalents: EN 10088-3 (1.4507 flat/long), EN 10222-5 (forgings), EN 10272 (pressure bar).
PREN = %Cr + 3.3 · %Mo + 16 · %N. For Ferralium 255 (25 Cr, 3 Mo, 0.2 N): PREN = 25 + 9.9 + 3.2 = 38.1 minimum, typically 40-42. For Super Duplex 2507 (25 Cr, 4 Mo, 0.27 N): PREN = 25 + 13.2 + 4.3 = 42.5 typical. For Zeron 100 (25 Cr, 3.6 Mo, 0.7 W, 0.25 N): PREW = 25 + 3.3(3.6 + 0.35) + 4.0 = 42.0 typical.
Yes. Super duplex stainless steel grades (Ferralium 255, 2507, Zeron 100) are qualified under NACE MR0175 / ISO 15156-3 for sour service when supplied solution-annealed with hardness ≤ 32 HRC and ferrite content 35-55% per NORSOK M-630. Per-heat MTC (EN 10204 type 3.2) certifies NACE compliance. Common applications: downhole tubing hangers, wellhead bolting, BOP components, X-mas tree internals.
