Ferralium 255 / F61 – UNS S32550
25Cr-6Ni-3.5Mo-2Cu-N super duplex stainless steel
Super duplex with copper (1.5–2.5%), developed by Langley Alloys (United Kingdom); Cu improves resistance in sulfuric acid and to erosion-corrosion. It has ~50% ferrite (35–55%) and PREN ≈ 38–41 depending on chemistry (SD50 version ≥ 40). S32520 (UR 52N+, F59) is a close alloy with more N and is accepted as an alternative in plate. Usual tests: ASTM A923 (A: metallography; B: Charpy; C: FeCl3 at 40 °C), ASTM G48 method A at 50 °C/24 h (NORSOK M-630 criterion), Charpy at −46 °C (45 J avg./35 J ind.) and ferrite per ASTM E562 (35–55%).
Designations and equivalents
| System | Designation |
|---|---|
| UNS | S32550 (S32520 = close variant, F59) |
| W.Nr./EN | 1.4507 – X2CrNiMoCuN25-6-3 |
| DIN | X2CrNiMoCuN25-6-3 |
| Trade names | Ferralium 255, Ferralium 255-SD50 (Langley Alloys); UR 52N+ (S32520) |
| Others | ASTM A182 F61 |
Standards by product form
| Form | Standard |
|---|---|
| Bar | ASTM A479 / A276 – S32550 |
| Forging | ASTM A182 F61 (S32520: F59) |
| Plate | ASTM A240 – S32550 / S32520 |
| Tube | ASTM A790 / A789 |
| Offshore | NORSOK M-630 MDS D54/D57; API 6A |
| H2S | NACE MR0175 / ISO 15156-3 |
Tolerances and HS codes
- Forged bars (round and square): machining allowance from 2 mm (Ø 6–16 mm) to 24 mm (Ø 400–500 mm) on the diameter. E.g. Ø 100–125 mm finished: +7 mm, tolerance +6 / −0 (length up to 3,500 mm).
- Forged discs: allowance of 4 to 9 mm on the diameter and 3 to 11 mm on the height, plus-only tolerance.
- Cutting and machining: band saw −0 / +5 to +15 mm on length; waterjet +10 % of thickness on the diameter (min. 2 mm); machined parts −0 / +2 to +5 mm on the diameter. See the full tables (INF 002 and INF 003, the same for every alloy).
| Form | NCM (Brazil, 8 digits) | HS (international, 6 digits) | HTS (USA, 10 digits)* | CN (European Union, 8 digits)* |
|---|---|---|---|---|
| Forged bar | 7222.30.00 | 7222.30 | 7222.30.0024 | 7222 30 51 |
| Hot-rolled bar | 7222.11.00 | 7222.11 | 7222.11.0059 | 7222 11 11 |
| Plate (> 10 mm, width ≥ 600 mm) | 7219.21.00 | 7219.21 | 7219.21.0020 | 7219 21 10 |
| Seamless pipe | 7304.41.90 | 7304.41 | 7304.41.6045 | 7304 41 00 |
| Welded pipe | 7306.40.00 | 7306.40 | 7306.40.5085 | 7306 40 80 |
| Flange and fitting | 7307.21.00 | 7307.21 | 7307.21.5000 | 7307 21 00 |
Tariff class: stainless steel (by chemical composition, under the Chapter 72 Notes and Section XV Note 5).
*: US and EU codes for reference. NCM per current legislation (General Rules of Interpretation, Notes to Chapters 72, 74 and 75 and Section XV Note 5). Confirm the final code with your customs broker according to the item size and finish. HS = first 6 digits of the NCM.
Chemical composition (%)
ASTM A182 F61 / A479 (UNS S32550)
| Element | Range (%) |
|---|---|
| C | ≤ 0.04 |
| Mn | ≤ 1.50 |
| P | ≤ 0.040 |
| S | ≤ 0.030 |
| Si | ≤ 1.00 |
| Cr | 24.0 – 27.0 |
| Ni | 4.5 – 6.5 |
| Mo | 2.9 – 3.9 |
| N | 0.10 – 0.25 |
| Cu | 1.50 – 2.50 |
| Fe | balance |
Mechanical properties
Solution annealed and water quenched (ASTM A182 F61)
| Property | Value |
|---|---|
| Yield strength (0.2%) | ≥ 550 MPa (SD50: ≥ 586 MPa) |
| Tensile strength | ≥ 750 MPa (SD50: ≥ 790 MPa) |
| Elongation | ≥ 25% |
| Reduction of area | ≥ 50% |
| Hardness | 220 – 270 HB (SD50); NACE ≤ 28 HRC |
| Charpy (SD50, −46 °C) | ≥ 45 J average / 35 J individual |
Typical physical properties
| Property | Value |
|---|---|
| Density | 7.81 g/cm³ |
| Modulus of elasticity | 199 GPa |
| Thermal conductivity (20 °C) | 14.2 W/m·K |
| Thermal expansion (20–100 °C) | 11.1 µm/m·K |
| Ferrite (solution annealed) | 35 – 55% |
Heat treatment, welding and machining
| Item | Usual practice |
|---|---|
| Service temperature | The producer recommends up to 275 °C continuous; above that, 475 °C embrittlement and sigma precipitation (700–1,000 °C). Good toughness down to ~−46 °C. |
| Heat treatment | Solution annealing at 1,050–1,125 °C per ASTM A182 F61 (the producer recommends 1,060–1,120 °C) with immediate water quench. Forging between 1,050 and 1,250 °C, followed by solution annealing. |
| Welding | Matching consumable AWS A5.9 ER2553 / A5.4 E2553 (Ferralium type) or ER2594/E2594. Heat input 0.5–1.5 kJ/mm, interpass ≤ 100 °C, N2 purge; no preheat and no PWHT. |
| Machining | Similar to super duplex grades: requires carbide, low speed and coolant; supplied peeled/polished to ease finishing. |
Filler metal for welding
The wire composition is usually not identical to the material: each page explains why.
Typical applications
- Seawater pumps, valves and shafts
- Wellhead and subsea equipment (API 6A)
- Marine propellers, shafts and rudders
- Sulfuric and (wet-process) phosphoric acid processes
- Gas scrubbers and pollution control fans
- Pulp and paper digesters and bleaching
- Sanitation and desalination
Alternatives: F53 (2507) and F55 (Zeron 100) have a similar PREN and greater availability; prefer Ferralium where H2SO4 or erosion is present; for strong reducing acids, nickel alloys.
Ferralium 255 / F61: complete guide
What Ferralium 255 is
Ferralium 255 belongs to the family of special stainless steels, in the super duplex group: a structure of half ferrite and half austenite, 25% chromium and a typical PREN of 38 to 41 (the SD50 version guarantees PREN ≥ 40). It was the first commercial super duplex and differs from the others by the addition of 1.5 to 2.5% copper. Ferralium is the trade name; the UNS is S32550 and, in ASTM A182, the forging is F61. It is unrelated to F60, which is duplex 2205, even though the site menu groups the two.
Chemical composition and role of the elements
Chromium (24 – 27%), molybdenum (2.9 – 3.9%) and nitrogen (0.10 – 0.25%) give pitting and crevice resistance, with a PREN of 35 to 44; nickel adjusts the phase balance; copper, from 1.5 to 2.5%, improves resistance in sulfuric acid and in crevices and gives the alloy its identity. Iron is the balance, between 57 and 67%.
Corrosion and limits of use
It resists pitting and crevice corrosion in seawater, brines and hot chloride solutions, with excellent resistance to stress corrosion cracking, erosion-corrosion and cavitation, which makes it preferred in seawater pump impellers and shafts. Copper extends resistance to sulfuric acid and mild reducing media, but the alloy is not indicated for hydrochloric acid or hot reducing acids. Continuous use goes up to about 275 °C and the minimum is −50 °C.
Ferralium in pulp and paper
When environmental laws began to require closed water circuits in paper mills, chlorides and other chemicals concentrated in the process. Equipment in ordinary stainless started to corrode. Super duplex grades such as Ferralium came into those points through the combination of resistance to chlorides, abrasion and cavitation with high mechanical strength.
Applications and similar grades
Seawater and brine pump shafts, impellers and casings, valve stems and bodies, offshore bolts and components, gas desulfurization, pulp and sulfuric acid equipment, and parts subject to erosion and cavitation. The super duplex grades F53 and F55 are the alternatives without and with tungsten; 254 SMO is the non-magnetic austenitic option.
Supply forms and Dustre services
Round bars and blocks forged in open die, cut to size on a band saw, with rough or finish machining, solution annealing in our own furnaces and hardness, tensile and impact testing. All material is supplied with a 3.1 inspection certificate and heat traceability.
Frequently asked questions
What is the chemical composition of Ferralium 255 / F61?
Per ASTM A182 F61 / A479 (UNS S32550): C ≤ 0.04; Mn ≤ 1.50; P ≤ 0.040; S ≤ 0.030; Si ≤ 1.00; Cr 24.0 – 27.0.
Which standards cover Ferralium 255 / F61?
Bar: ASTM A479 / A276 – S32550. Forging: ASTM A182 F61 (S32520: F59). Plate: ASTM A240 – S32550 / S32520. Tube: ASTM A790 / A789. Offshore: NORSOK M-630 MDS D54/D57; API 6A. H2S: NACE MR0175 / ISO 15156-3.
Up to what temperature can Ferralium 255 / F61 be used?
The producer recommends up to 275 °C continuous; above that, 475 °C embrittlement and sigma precipitation (700–1,000 °C). Good toughness down to ~−46 °C.
Which materials can replace Ferralium 255 / F61?
F53 (2507) and F55 (Zeron 100) have a similar PREN and greater availability; prefer Ferralium where H2SO4 or erosion is present; for strong reducing acids, nickel alloys.
Is Ferralium stronger than duplex 2205?
Yes. The minimum yield strength of Ferralium 255 is 550 MPa, versus 450 MPa for 2205 (S31803) and 241 MPa for Alloy 20, and the PREN is higher (≈ 38 – 41 versus ≈ 35 typical for 2205).
