2205 vs 2507 Duplex Stainless Steel: How to Choose the Right Grade
Two Grades, One Metallurgical Family
2205 (UNS S32205, EN 1.4462) and 2507 (UNS S32750, EN 1.4410) both belong to the duplex family: a two-phase microstructure of roughly equal ferrite and austenite. That structure is what delivers high strength together with resistance to chloride stress corrosion cracking. The difference between the grades is not the structure but the alloying level, and that is what procurement and engineering teams actually pay for.
Chemical Composition and PREN
| Element | 2205 / UNS S32205 | 2507 / UNS S32750 |
|---|---|---|
| Chromium | 22.0-23.0 % | 24.0-26.0 % |
| Nickel | 4.5-6.5 % | 6.0-8.0 % |
| Molybdenum | 3.0-3.5 % | 3.0-5.0 % |
| Nitrogen | 0.14-0.20 % | 0.24-0.32 % |
| PREN = Cr + 3.3 Mo + 16 N | about 34-36 | 40 and above |
These windows are the ones published for the two UNS numbers in ASTM A240/A240M for plate, sheet and strip and in ASTM A790/A790M for seamless and welded pipe. Nitrogen is the least expensive element that raises PREN, and the higher nitrogen plus wider molybdenum range of 2507 is what creates the gap between the two grades in chloride service.
Mechanical Properties
| Property (annealed, minimum) | 2205 | 2507 |
|---|---|---|
| Tensile strength | 620 MPa (90 ksi) | 795 MPa (115 ksi) |
| 0.2 % yield strength | 450 MPa (65 ksi) | 550 MPa (80 ksi) |
| Elongation in 2 in. | 25 % | 15 % |
The strength advantage of 2507 is real but predictable: about 20 % higher yield than 2205. What matters commercially is the other direction - 2205 already gives roughly twice the yield strength of 304 or 316 austenitic grades, so wall thickness and weight are reduced even before considering the super duplex option. Lower elongation in 2507 is a reminder that cold forming limits tighten as the alloying level rises.
Corrosion Performance in Chloride Service
Pitting and crevice corrosion resistance scale with PREN, so 2507 tolerates higher chloride concentrations and higher temperatures than 2205 before pitting initiates. Practical selection is therefore built on three inputs: chloride content, oxygen level and metal temperature. Where 2205 is adequate, moving to 2507 buys no measurable service benefit and adds cost; where 2205 sits at its limit, 2507 is the standard upgrade path rather than a jump to nickel alloys.
Fabrication, Welding and Supply Form
Both grades are supplied as plate, sheet, seamless and welded pipe, tube, bar and fittings. Welding follows the same rules: controlled heat input, no preheat, interpass temperature kept low, and nitrogen-bearing filler metals. Matching consumables are classified as ER2209 for 2205 and ER2594 for 2507 under AWS A5.9. Heat treatment is limited to solution annealing followed by rapid cooling; stress relief in the 300-1000 degrees C range risks sigma phase formation and must be avoided.
Cost and Selection Logic
2507 carries a higher alloy surcharge because of nickel, molybdenum and nitrogen content, and it is usually available in a smaller size range with longer lead times. When 2205 satisfies the design PREN and temperature, the lower-cost grade is technically correct, not a compromise. Reserve 2507 for seawater systems, sour service, high-chloride process streams and heat exchangers where the pitting resistance of 2205 has been shown to be marginal.
Frequently Asked Questions
Q: What is the PREN of 2205 and 2507?
A: Using PREN = Cr + 3.3 Mo + 16 N, 2205 typically calculates to about 34-36 and 2507 to 40 or higher, based on the composition windows in ASTM A240/A240M.
Q: Is 2507 stronger than 2205?
A: Yes. Minimum 0.2 % yield strength is 550 MPa for 2507 against 450 MPa for 2205, with minimum tensile strength of 795 MPa against 620 MPa, both taken from the annealed requirements in ASTM A240/A240M.
Q: Can 2205 and 2507 be welded to each other?
A: Yes, with a matching higher-alloy filler metal classified as ER2594 under AWS A5.9 and the same low heat input, low interpass temperature procedure used for either grade alone.
Q: Why is 2507 called super duplex?
A: The term is applied to duplex grades with PREN above 40, achieved through higher chromium, molybdenum and nitrogen than standard duplex such as 2205.
Q: Does 2507 replace 316L in seawater?
A: In most seawater piping and heat exchanger duty, yes. 316L is prone to pitting and crevice attack in chlorides, while 2507 is designed for that environment.
Q: Are duplex grades magnetic?
A: Yes. The ferrite phase makes both 2205 and 2507 magnetic, which is a useful field check when separating duplex material from austenitic 304 or 316 stock.
