AISI F255 and AISI F55 are both super duplex stainless steels. Their chromium, nickel and molybdenum levels are close. Where they part company is two elements: F255’s copper and F55’s tungsten.
Copper or tungsten?
F255 (1.4507) contains nominally about 2 % copper and no tungsten. Copper provides resistance particularly in sulphuric and phosphoric acid environments — Langley Alloys, Rolled Alloys, Industeel and IMOA all agree on this.
F55 (UNS S32760) limits copper to 0.50–1.00 % and adds 0.50–1.00 % tungsten instead. The intent of the tungsten is to raise pitting resistance — but as you will see below, whether that addition actually counts at specification level is disputed.
Composition: three specifications, three sets of limits
This is the biggest trap in this grade. Three different specification bands circulate under the same trade name, and they are not interchangeable. The table places all three side by side (weight %).
| ELEMENT | EN 1.4507 | UNS S32520 (ASTM A240) | UNS S32550 (ASTM) |
|---|---|---|---|
| Carbon (C) | 0.030 max | 0.030 max | 0.040 max |
| Silicon (Si) | 0.70 max | 0.80 max | 1.00 max |
| Manganese (Mn) | 2.00 max | 1.50 max | 1.50 max |
| Phosphorus (P) | 0.035 max | 0.035 max | 0.040 max |
| Sulphur (S) | 0.015 max | 0.020 max | 0.030 max |
| Chromium (Cr) | 24.0 – 26.0 | 24.0 – 26.0 | 24.0 – 27.0 |
| Nickel (Ni) | 6.0 – 8.0 | 5.5 – 8.0 | 4.5 – 6.5 |
| Molybdenum (Mo) | 3.0 – 4.0 | 3.0 – 4.0 | 2.9 – 3.9 |
| Nitrogen (N) | 0.20 – 0.30 | 0.20 – 0.35 | 0.10 – 0.25 |
| Copper (Cu) | 1.00 – 2.50 | 0.50 – 2.00 | 1.50 – 2.50 |
| Tungsten (W) | not specified | not specified | not specified |
Metallurgically EN 1.4507 is close to S32520, not to S32550.
Look at the nickel row: 1.4507 (6.0–8.0 %) and S32520 (5.5–8.0 %) very nearly overlap, while S32550 (4.5–6.5 %) is a separate band. The same is true of the nitrogen and carbon ceilings.
Despite that, the market usually pairs 1.4507 with S32550 / F61 — because that is what is held in stock. Langley Alloys says so openly: S32520 is not commonly stocked; the closest readily available grade is S32550. Defence Metal’s product page uses the 1.4507 / UNS S32520 designation.
And one more warning: ASTM’s own specifications give different limits for S32520. ASTM A182 (forgings, F59) opens molybdenum to 3.0–5.0 % and copper to 3.00 %, while ASTM A240 / A276 / A479 give 3.0–4.0 % and 0.50–2.00 % for the same UNS number. State on your order which ASTM product specification you are working to.
Mechanical properties — solution annealed
| SPECIFICATION / PRODUCT | Rp0.2 min | Rm min | A min | Hardness max |
|---|---|---|---|---|
| ASTM A240 plate — S32550 | 550 MPa (80 ksi) | 760 MPa (110 ksi) | 15 % | 302 HBW / 32 HRC |
| ASTM A240 plate — S32520 | 550 MPa (80 ksi) | 770 MPa (112 ksi) | 25 % | 310 HBW |
| EN 10088-3 bar — 1.4507 | 500 MPa | 700 – 900 MPa | 25 % | 270 HB |
| ASTM A182 forgings — F61 (2 organisations) | 550 MPa (80 ksi) | 750 MPa (109 ksi) | 25 % | not specified |
| ASTM A182 forgings — F59 (3 organisations) | 550 MPa (80 ksi) | 770 MPa (112 ksi) | 25 % | not specified |
| ASTM A479 bar — S32550 (ASTM text) | 550 MPa (80 ksi) | 760 MPa (110 ksi) | 15 % | 297 HBW |
Note the elongation difference: in ASTM A240 it is 15 % for S32550 and 25 % for S32520. Two materials sold under the same trade name are subject to different elongation requirements within the same standard.
The PREN dispute — commercially the most important section
In super duplexes the pitting resistance indicator is PREN. But the industry is split on whether tungsten counts in the formula.
The tungsten-free form (PREN = %Cr + 3.3×%Mo + 16×%N) is used by Outokumpu, Alleima, Industeel, Sandmeyer Steel, ATI, Carpenter Technology, the Smiths group, Columbia Metals and — most importantly — NORSOK M-630.
The tungsten-bearing form (PREN = %Cr + 3.3×(%Mo + 0.5×%W) + 16×%N) is used by Rolled Alloys and IMOA.
What this means for the buyer: in the mandatory PREN ≥ 40 calculation of NORSOK M-630 — the specification under which this material is actually bought — F55’s tungsten does not count. The argument that “tungsten gives a higher PREN” is, at specification level, usually not valid.
NORSOK M-630 requirements
For forgings (MDS D54) and bar (MDS D57) the requirements are as follows (from two separate reproductions of the NORSOK M-630 text):
| REQUIREMENT | VALUE |
|---|---|
| PREN (= %Cr + 3.3×%Mo + 16×%N) | ≥ 40.0 |
| Heat treatment | solution anneal + water quench |
| Proof strength (Rp0.2) min | 550 MPa |
| Tensile strength (Rm) min | 800 MPa |
| Elongation min | 15 % |
| Charpy-V at −46 °C | 45 J average / 35 J single |
| Hardness | below 32 HRC (301 HB or 330 HV10) |
| Ferrite content (ASTM E562) | 35 – 55 % |
| Corrosion test | ASTM G48 Method A, 50 °C, 24 h — no pitting, weight loss < 4.0 g/m² |
NORSOK names only S32550.
Neither UNS S32520 nor EN 1.4507 appears in the MDS D54 and D57 texts. The statement “1.4507 is covered by NORSOK” is true only through S32550.
An arithmetic note: the bottom corner of the ASTM band does not reach PREN 40. The bottom corner of S32550 (Cr 24.0 + Mo 2.9 + N 0.10) gives 38.2, and that of S32520 (24.0 + 3.0 + 0.20) gives 37.1. Meeting ASTM does not guarantee PREN 40 — which is why NORSOK and the branded products impose “PRE ≥ 40” separately and narrow their own chemistry bands.
Solution annealing temperature
S32520 / F59: 1080–1120 °C (the ASTM A182 text, IMOA and the Nickel Institute). S32550: 1040 °C (the ASTM A479 text and IMOA); ASTM A182 grade F61 gives 1050–1125 °C. Whichever figure applies, water quenching is mandatory — no specification permits air cooling.
The sour-service hardness limit comes from two separate places and they do not coincide: NORSOK requires below 32 HRC, while NACE MR0175 / ISO 15156 sets a maximum of 28 HRC (270 HB) (Langley Alloys). Meeting ASTM’s 302 HBW ceiling is not sufficient for sour service.
How to read this table. Rows without a source name have been confirmed by at least four independent organisations. Rows with an organisation named in brackets were found in fewer sources and are therefore given with attribution. No figure we could not verify has been published.
“F255” is not an ASTM designation.
“F255” is market shorthand; no such grade letter exists in ASTM A182. Two separate UNS numbers circulate under this trade name: S32520 (ASTM A182 F59) and S32550 (F61). They are different numbers. State the material number and UNS explicitly on your order and certificate; do not settle for “F255”.
Make sure F55’s tungsten has a lower limit as well. Some distributors publish tungsten only as “1.00 % max”; with no lower limit, an S32760 containing no tungsten can be delivered. Write “W 0.50–1.00” into the order.
Upper temperature limit
Super duplexes are not high-temperature alloys. Published upper service temperatures scatter across 270 to 316 °C (Industeel 270 °C, Langley 275 °C, Rolled Alloys 300 °C, IMOA/ASME 315 °C). The cause is alpha-prime precipitation in the ferrite phase — “475 °C embrittlement” — which reduces toughness.
Which one, where?
F255: fertiliser plants, phosphoric and sulphuric acid processes, flue gas desulphurisation, pulp and paper, desalination, pumps and valves, marine propellers and shafts.
F55: oil and gas process and seawater systems, subsea systems, valves, manifolds, flanges and bolting, mining.
In short: where there is acid you look at the copper (F255); for a seawater system you look at F55.
Designations, standards and AMS equivalents
The same steel is called different things depending on which system you are speaking in. Most ordering and certification confusion starts here, so a short summary:
- AISI — the American Iron and Steel Institute’s grade naming (410, 316L and so on). It is the most widely used name in everyday use, but on its own it is not a purchasing specification: composition and mechanical requirements are defined in the ASTM standards.
- UNS — the Unified Numbering System operated jointly by ASTM and SAE (S41000 and so on). It is the number used for ordering and certification in North America, and it is more precise than the AISI name.
- W.Nr / EN — the European material number (1.4006 and so on) and EN name (X12Cr13 and so on). This is the governing designation in Europe. It is usually only an approximate equivalent of the AISI grade; carbon, sulphur or molybdenum limits frequently differ.
- AMS — SAE’s Aerospace Material Specifications. An AMS specification binds not only the composition but also the melting practice, the product form, the heat treatment condition and the inspection requirements. In aerospace and defence orders this is the designation that governs.
| DESIGNATION | AISI F255 | AISI F55 |
|---|---|---|
| AISI / trade name | AISI F255 | AISI F55 |
| EN material no. (W.Nr) | 1.4507 | 1.4501 |
| UNS number | S32520 | S32760 |
| AMS specifications | — | — |
| Product page | AISI F255 technical page | AISI F55 technical page |
Two reminders. First: an AMS number is specific to a product form and condition. The same grade may have separate AMS specifications for bar, sheet and forgings, so establish which product form you need before calling up an AMS number.
Second: the AISI name, material number, UNS and AMS data in the table above are the designations shown on Defence Metal’s product pages. If you are working to a different edition of a standard or to a different product form, state the material number and the specification edition explicitly on your order.