Surface Treatments
Certifications
- ISO 9001 - 2015 Certified
- PED 2014/68/EC
- NACE MR0175 / ISO 15156-2
- NORSOK M-650 Qualified
- API 6A Certified
- DFAR
- MERKBLATT AD 2000 W2/W7/W10
Super Duplex vs Duplex Stainless Steel
The key difference between super duplex and duplex stainless steel is the Pitting Resistance Equivalent Number (PREN). Standard duplex grades (2205, 2304) sit at PREN between 30 and 40. Super duplex grades (2507, Zeron 100, Ferralium 255) sit at PREN of 40 or higher. This single number drives the choice between the two classes for any given chloride service.
Both classes share the same balanced 50 percent austenite, 50 percent ferrite microstructure. Both deliver yield strength roughly twice that of austenitic 316L. The functional difference is chloride pitting and crevice corrosion resistance, which scales directly with PREN. The most heavily traded super duplex grade is 2507 (UNS S32750); the most heavily traded standard duplex grade is 2205 (UNS S32205).
Side-by-Side Comparison: 2205 vs 2507
| Property | 2205 (Duplex) | 2507 (Super Duplex) |
|---|---|---|
| UNS | S32205 | S32750 |
| EN / Werkstoff | 1.4462 | 1.4410 |
| Chromium (Cr) | 22.0 to 23.0 | 24.0 to 26.0 |
| Nickel (Ni) | 4.5 to 6.5 | 6.0 to 8.0 |
| Molybdenum (Mo) | 3.0 to 3.5 | 3.0 to 5.0 |
| Nitrogen (N) | 0.14 to 0.20 | 0.24 to 0.32 |
| PREN (typical) | 34 to 38 | 41 to 43 |
| Yield strength (MPa min) | 450 | 550 |
| Tensile strength (MPa min) | 655 | 795 |
| Elongation (%) | 25 | 15 |
| CPT, ASTM G48 Method E (degrees Celsius) | 30 to 40 | 50 to 70 |
| CCT, ASTM G48 Method F (degrees Celsius) | 20 to 30 | 35 to 50 |
| Forging spec | ASTM A182 F60 | ASTM A182 F53 |
Composition Difference
Super duplex chemistry adds approximately 2 to 3 percent more chromium, 1 to 2 percent more nickel, 0.5 to 1.5 percent more molybdenum and 0.10 to 0.12 percent more nitrogen than standard duplex 2205. The effect is a PREN lift from 34 to 38 (2205) up to 41 to 43 (2507). Each PREN point above 40 adds roughly 5 to 8 degrees Celsius to the Critical Pitting Temperature.
Mechanical Property Difference
Super duplex carries roughly 22 percent higher yield strength (550 vs 450 MPa minimum) and 21 percent higher tensile strength (795 vs 655 MPa minimum) than 2205 in the solution-annealed condition. Required elongation is lower for 2507 (15 percent vs 25 percent for 2205), reflecting the higher overall strength level. Charpy V-notch toughness at minus 46 degrees Celsius is the same minimum (45 J) for both classes.
Corrosion Resistance Difference
The Critical Pitting Temperature gap is the practical reason to specify super duplex over duplex. 2205 begins to pit in seawater at approximately 30 to 40 degrees Celsius. 2507 stretches the ceiling to 50 to 70 degrees Celsius. For seawater piping, oil and gas surface and subsea hardware, and aggressive chloride service, super duplex is the design default.
Cost Difference
Super duplex is more expensive than duplex per kilogram, driven by higher molybdenum and nickel content. The price gap typically runs 30 to 50 percent above 2205 for equivalent product forms. For project economics, the higher allowable design stress (and consequently thinner-wall designs) often offsets a substantial portion of the material cost premium, particularly for large-diameter pipe and pressure vessels.
When to Use Each
Use standard duplex 2205 when: chloride concentration is below 200 ppm, service temperature is below approximately 35 degrees Celsius, the application is brackish water, ambient chemical tank service, pulp and paper non-bleach service, or architectural cladding. 2205 is the workhorse for moderate-chloride duty.
Use super duplex 2507 when: the service is seawater (around 19,000 ppm chloride), oil and gas surface or subsea hardware, desalination plant high-pressure piping, chlorine dioxide bleach plant equipment, flue gas desulfurization absorber service, or any chloride-bearing duty above approximately 35 degrees Celsius. 2507 is the design default for chloride-aggressive media.
Common Standards
Both standard duplex 2205 and super duplex 2507 share the same family of international standards. The PREN threshold and the specific UNS grade are the variables.
| Standard | 2205 | 2507 |
|---|---|---|
| ASTM A479 (bar) | UNS S32205 | UNS S32750 |
| ASTM A790 (pipe) | UNS S32205 | UNS S32750 |
| ASTM A815 (fittings) | WP-S32205 | WP-S32750 |
| ASTM A182 (forgings) | F60 | F53 |
| EN 10088-3 | 1.4462 | 1.4410 |
| NORSOK M-630 MDS | D45, D47 | D55, D57 |
Super Duplex vs Duplex Frequently Asked Questions
What is the difference between super duplex and duplex stainless steel?
The PREN threshold. Standard duplex (2205, 2304) has PREN between 30 and 40. Super duplex (2507, Zeron 100, Ferralium 255) has PREN of 40 or higher, achieved through elevated chromium (around 25 percent), molybdenum (3.5 to 4 percent) and nitrogen (0.24 to 0.32 percent). Super duplex offers roughly twice the chloride pitting resistance and around 22 percent higher yield strength.
Is 2205 the same as super duplex?
No. 2205 (UNS S32205) is a standard duplex grade with PREN 34 to 38, below the super duplex threshold of 40. The corresponding super duplex grade is 2507 (UNS S32750), with PREN 41 to 43. Project specifications that name super duplex are not satisfied by 2205, and vice versa.
Can super duplex 2507 replace duplex 2205?
Functionally, yes. 2507 has higher PREN, higher yield, higher tensile and the same minimum Charpy toughness as 2205. The substitution is technically conservative. Commercially, the cost premium (30 to 50 percent above 2205 per kilogram) usually limits the substitution to cases where chloride aggressiveness, temperature, or sour-service conditions require the higher PREN.
Is super duplex more difficult to weld than duplex?
Marginally. Both classes require controlled heat input, interpass temperature below 150 degrees Celsius, and 98 percent argon plus 2 percent nitrogen shielding gas. Super duplex is more sensitive to sigma-phase precipitation due to its higher chromium and molybdenum content. The standard fillers are ER2209 for 2205 and ER2594 for 2507 per AWS A5.9.
Is super duplex stronger than duplex?
Yes. Super duplex 2507 has minimum yield strength 550 MPa vs 450 MPa for 2205 (roughly 22 percent higher), and minimum tensile strength 795 MPa vs 655 MPa (roughly 21 percent higher). The higher strength allows thinner-wall designs and lower fastener counts in pressure equipment.
Which is better for seawater service?
Super duplex 2507. The Critical Pitting Temperature in the standard ASTM G48 Method E ferric chloride test is approximately 50 to 70 degrees Celsius for 2507 vs 30 to 40 degrees Celsius for 2205. Seawater (around 19,000 ppm chloride) generally requires super duplex above 35 degrees Celsius.
Which standards distinguish duplex from super duplex?
NORSOK M-630 sets the PREN 40 threshold for super duplex classification. ASTM A182 lists separate forging grades for each (F51 / F60 for 2205, F53 for 2507). NORSOK MDS classes D45 and D47 cover 2205; D55 and D57 cover 2507. Project specifications routinely cite both the UNS number and the PREN window.
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