AISI 318 (duplex 2205) and AISI F53 (super duplex 2507) share the same metallurgical principle: a half austenite, half ferrite structure. The difference is in how far the alloy contents are raised — and the prefix “super” says exactly that.
Chromium, molybdenum, nitrogen: all up
According to our datasheets, F53 has chromium at 24.00–26.00% (21.00–23.00% in 318), molybdenum at 3.00–5.00% (2.50–3.50% in 318) and nitrogen at max 0.30% (0.10–0.22% in 318). Nickel also rises to 6.00–8.00%.
Those three elements directly determine the PREN value, which measures a stainless steel’s resistance to pitting corrosion (PREN = %Cr + 3.3×%Mo + 16×%N). While 2205 class duplex grades are typically in the 34–38 range, the PREN value of 2507 class super duplex grades is above 40. That threshold of 40 is also where the industry draws the line for the term “super duplex”.
Strength rises too: the tensile strength of 318 is 700–900 MPa against 750–930 MPa for F53. Hardness reaches up to 270 HB. In exchange, elongation rises from 20% to 25% — so F53, despite being stronger, is not less ductile.
Comparison in numbers
| PROPERTY | AISI 318 (Duplex) | AISI F53 (Super Duplex) |
|---|---|---|
| Carbon (C) | max 0.03% | max 0.030% |
| Chromium (Cr) | 21.00 – 23.00% | 24.00 – 26.00% |
| Nickel (Ni) | 4.50 – 6.50% | 6.00 – 8.00% |
| Molybdenum (Mo) | 2.50 – 3.50% | 3.00 – 5.00% |
| Nitrogen (N) | 0.10 – 0.22% | max 0.30% |
| Copper (Cu) | — | max 0.5% |
| Sulphur (S) | max 0.02% | max 0.015% |
| Tensile strength Rm | 700 – 900 MPa | 750 – 930 MPa |
| Yield strength Rp0.2 | 460 – 500 MPa | — |
| Elongation A50 | 20% | 25% |
| Hardness | — | max 260 HB |
| Density | 7.805 g/cm³ | 7.75 g/cm³ |
| Modulus of elasticity | 200 GPa | 210 GPa |
| UNS | S31803 · S32205 | S32750 |
| W.Nr (DIN/EN) | 1.4462 | 1.4410 |
Values are taken from our product datasheets; section thickness and the applied heat treatment regime change the result.
Which one, and when?
AISI 318 is sufficient — chemical and petrochemical process piping, pressure vessels, paper industry equipment, bridge and structural members, marine applications with a moderate chloride load. It is easier to source and markedly lower in cost.
AISI F53 is required — systems in direct contact with seawater, offshore platform pipelines, desalination plants, hot process fluids with a high chloride content, wellhead equipment. If the specification asks for PREN ≥ 40, 318 cannot meet it.
Welding and heat treatment are critical in both materials: if they spend a long time in the 300–1000 °C range, sigma phase forms and the material becomes brittle. Super duplex grades are more sensitive to this than duplex grades.
Neighbouring grades
For alternatives in the same class with copper and tungsten additions there are AISI F55 and AISI F255. For a comparison of the duplex structure with its austenitic equivalents, see our AISI 316L – Duplex 318 article.
Technical datasheets
AISI 318 — chemical composition, mechanical values, standard equivalents
AISI F53 — chemical composition, mechanical values, standard equivalents
Both grades are supplied to order in round bar, flat bar, plate, sheet, pipe and forged forms. Contact us for a quotation.