Ferralium 255 is a copper-bearing super duplex stainless steel (UNS S32550, EN 1.4507, X2CrNiMoCuN25-6-3) originally developed by Langley Alloys (UK). Nominal composition is 25 Cr – 5 Ni – 3 Mo – 2 Cu – 0.2 N; PREN ≥ 40; balanced austenite-ferrite microstructure (35–55% ferrite per NORSOK M-630); supplied solution-annealed (no precipitation hardening). Critical Pitting Temperature (CPT) ≥ 35 °C per ASTM G48 method A. The 2% copper distinguishes Ferralium 255 from other super duplex grades and gives improved resistance to reducing acids (HCl, H2SO4, brines). Used in seawater service, oil & gas downhole, subsea, desalination and chemical processing. Ferralium is a registered trademark of Langley Alloys.
| Element | Min % | Max % | Role |
|---|---|---|---|
| Carbon (C) | none | 0.04 | Cap to avoid carbide precipitation |
| Chromium (Cr) | 24.0 | 27.0 | Pitting resistance, ferrite stabiliser |
| Nickel (Ni) | 4.5 | 6.5 | Austenite stabiliser |
| Molybdenum (Mo) | 2.9 | 3.9 | Pitting / crevice corrosion (PREN x3.3) |
| Copper (Cu) | 1.5 | 2.5 | Reducing-acid resistance (Ferralium signature) |
| Nitrogen (N) | 0.10 | 0.25 | Austenite stabiliser, strength, PREN x16 |
| Manganese (Mn) | none | 1.5 | Deoxidiser |
| Silicon (Si) | none | 1.0 | Deoxidiser |
| Phosphorus (P) | none | 0.04 | Impurity |
| Sulphur (S) | none | 0.03 | Impurity (lowered for sour service) |
| Property | Minimum | Test standard |
|---|---|---|
| 0.2% Yield strength (Rp0.2) | 550 MPa (80 ksi) | ASTM E8 |
| Ultimate tensile strength (Rm) | 760 MPa (110 ksi) | ASTM E8 |
| Elongation (A5) | 15% | ASTM E8 |
| Hardness | ≤ 32 HRC (NACE MR0175 limit) | ASTM E18 |
| Charpy V-notch at −46 °C | ≥ 45 J (NORSOK M-630) | ASTM E23 / ISO 148-1 |
| Density | 7.81 g/cm³ | none |
Ferralium 255 is supplied solution-annealed at 1100 °C followed by water quench, rapid cooling is essential to retain ferrite within the 35–55% range and to avoid sigma-phase, chi-phase, and Cr2N nitride precipitation in the 600–1000 °C range. Unlike precipitation-hardening alloys, super duplex grades do NOT undergo aging; cold work is also avoided as it can push hardness above the NACE MR0175 32 HRC limit and disturb the ferrite-austenite phase balance.
Ferralium 255 can be welded by GTAW, GMAW, SMAW and SAW. Matching filler metal is typically 25.10.4.L (E/ER 2553 / 2594 type) which adds extra Ni for proper austenite reformation in the weld metal. Pre-heat is not required; interpass temperature limited to 150 °C; no post-weld heat treatment for thin sections. Heat input controlled to 0.5–2.5 kJ/mm to maintain the weld zone ferrite content within 35–65% per NORSOK M-630.
