AISI F55 and AISI F53 are the two most common members of the super duplex family. Their chromium and nickel bands are the same. The difference lies in F55’s copper and tungsten additions and — far more importantly — in the fact that ASTM gives the two grades different elongation minima.
The difference in composition
F53 (UNS S32750) is a pure chromium-nickel-molybdenum-nitrogen duplex: neither copper nor tungsten is present as an alloying element. The 0.50 % maximum given for copper is an impurity ceiling, not an addition.
F55 (UNS S32760) contains both 0.50–1.00 % copper and 0.50–1.00 % tungsten. F53 forgoes both and raises the nitrogen instead.
Compared in figures
| PROPERTY | AISI F55 (S32760) | AISI F53 (S32750) |
|---|---|---|
| EN / W.Nr | 1.4501 | 1.4410 |
| Carbon (C) | 0.030 % max | 0.030 % max |
| Chromium (Cr) | 24.00 – 26.00 % | 24.0 – 26.0 % |
| Nickel (Ni) | 6.00 – 8.00 % | 6.0 – 8.0 % |
| Molybdenum (Mo) | 3.00 – 4.00 % | — (see note) |
| Copper (Cu) | 0.50 – 1.00 % (addition) | 0.50 % max (impurity) |
| Tungsten (W) | 0.50 – 1.00 % | not specified |
| Nitrogen (N) | 0.20 – 0.30 % | — (see note) |
| Manganese (Mn) | 1.00 % max | 1.20 % max |
| Silicon (Si) | — | 0.80 % max |
| Proof strength (Rp0.2) | ≥ 550 MPa | ≥ 550 MPa |
| Tensile strength (Rm) | ≥ 750 MPa | ≥ 795 – 800 MPa |
| Elongation (ASTM A240) | ≥ 25 % | ≥ 15 % |
| Elongation (EN / NORSOK) | ≥ 25 % | ≥ 25 % |
Note on molybdenum and nitrogen: for S32750 the ASTM and EN ranges do not overlap. ASTM gives Mo 3.0–5.0 % and N 0.24–0.32 %, while EN 10088-3 (1.4410) gives Mo 3.0–4.5 % and N 0.24–0.35 %. Because the four independent organisations we require could not be reached, we have left these two rows blank. Dual certification of a heat made to ASTM against EN is not automatic.
The most critical row: elongation
Under ASTM A240 the elongation minimum is 15 % for F53 and 25 % for F55.
Against that, F53’s tensile strength minimum is the higher of the two (795–800 MPa against 750 MPa). In other words these two grades are not direct equivalents, and the difference is given incorrectly on most distributor sites. If a drawing calls for 25 % minimum elongation, F53 does not meet it under ASTM A240.
The claim that “F55 is superior”
It is often said in the market that F55’s tungsten makes it superior to F53. The data do not support that.
First: tungsten does not count in most specifications’ PREN calculation. Ten organisations including NORSOK M-630 use the tungsten-free formula (PREN = %Cr + 3.3×%Mo + 16×%N); only Rolled Alloys and IMOA use the tungsten-bearing one.
Second, and more striking: Rolled Alloys’ own comparison — and Rolled Alloys owns the F55 brand — puts F53 ahead even when the tungsten-bearing formula is used (41.9 for 2507 against 41.5 for ZERON 100).
The difference between them therefore cannot be defended on PREN. It should be sought in F55’s copper for acid service, and in weld metal performance.
Shared properties
Both grades are suitable for sour service under NACE MR0175 / ISO 15156 in the solution annealed condition with a hardness limit of 28 HRC maximum, and both appear in the NORSOK M-630 material data sheets.
The upper service temperature is the same as well: approximately 270 to 316 °C. The limit is metallurgical rather than mechanical — alpha-prime precipitation in the ferrite phase, and sigma phase above it.
What to watch when ordering
Three points
1. State whether the elongation requirement is to be taken from ASTM or from EN/NORSOK — for F53 the difference is between 15 % and 25 %.
2. CPT (critical pitting temperature) values measured to different test standards cannot be compared. Even the same producer may publish a different test standard for the same grade. Ask which test standard was used when comparing quotations.
3. NORSOK MDS numbers belong to the product form, not the grade; the same MDS list is given for both F53 and F55. An MDS number alone does not identify the grade.
Where are they used?
Both are used in offshore oil and gas, seawater cooling, desalination, flue gas desulphurisation, pulp and paper, and chemical process equipment. F55’s copper comes into its own in acid environments, while F53’s higher nitrogen shows in strength and post-weld performance.
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 F55 | AISI F53 |
|---|---|---|
| AISI / trade name | AISI F55 | AISI F53 |
| EN material no. (W.Nr) | 1.4501 | 1.4410 |
| UNS number | S32760 | S32750 |
| AMS specifications | — | — |
| Product page | AISI F55 technical page | AISI F53 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.