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
Duplex and Super Duplex Stainless Steel, When to Use Each
Duplex and super duplex stainless steels share the same balanced 50 percent austenite, 50 percent ferrite microstructure. They differ in chromium, molybdenum and nitrogen content, which sets the Pitting Resistance Equivalent Number (PREN). Duplex grades like 2205 sit at PREN 30 to 40. Super duplex grades like 2507 sit at PREN 40 or higher. The right choice in any given project depends on chloride concentration, temperature, pressure, and presence of H2S.
As a working rule, standard duplex 2205 handles brackish water, chemical tanks and moderate-chloride service. Super duplex 2507 handles seawater, oil and gas surface and subsea hardware, and aggressive chloride media where 2205 would pit or crack. The grade comparison and selection guidance below covers the four most common alloys: 2205 (S32205), 2507 (S32750), Zeron 100 (S32760) and Ferralium 255 (S32550).
Definitions
A duplex stainless steel is one whose microstructure is intentionally split between austenite (FCC) and ferrite (BCC) phases in roughly equal proportions. A super duplex stainless steel is a duplex grade with 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). NORSOK M-630 sets the PREN 40 threshold for super duplex classification and is followed by major project specifications worldwide.
Side-by-Side Comparison
The four grades below cover the bulk of duplex and super duplex commercial trade. They are not interchangeable; project specifications call out the alloy by UNS number, and the forging specs differ.
| Property | 2205 (S32205) | 2507 (S32750) | Zeron 100 (S32760) | Ferralium 255 (S32550) |
|---|---|---|---|---|
| Class | Standard duplex | Super duplex | Super duplex | Super duplex |
| EN / Werkstoff | 1.4462 | 1.4410 | 1.4501 | 1.4507 |
| Chromium (Cr) | 22.0 to 23.0 | 24.0 to 26.0 | 24.0 to 26.0 | 24.0 to 27.0 |
| Nickel (Ni) | 4.5 to 6.5 | 6.0 to 8.0 | 6.0 to 8.0 | 4.5 to 6.5 |
| Molybdenum (Mo) | 3.0 to 3.5 | 3.0 to 5.0 | 3.0 to 4.0 | 2.9 to 3.9 |
| Nitrogen (N) | 0.14 to 0.20 | 0.24 to 0.32 | 0.20 to 0.30 | 0.10 to 0.25 |
| Copper (Cu) | (none) | 0.50 max (residual, not intentional) | 0.50 to 1.00 | 1.50 to 2.50 |
| Tungsten (W) | (none) | (none) | 0.50 to 1.00 | (none) |
| PREN (typical) | 34 to 38 | 41 to 43 | 41 to 44 | 38 to 42 |
| Yield (MPa min) | 450 | 550 | 550 | 550 |
| Forging spec | ASTM A182 F60 | ASTM A182 F53 | ASTM A182 F55 | ASTM A182 F61 |
Property Comparison
Mechanical-property minima per ASTM A479 in the solution-annealed condition. Super duplex grades carry roughly 20 percent higher yield strength than 2205 and substantially higher pitting resistance.
| Property | 2205 | 2507 | Zeron 100 |
|---|---|---|---|
| Yield strength (MPa min) | 450 | 550 | 550 |
| Tensile strength (MPa min) | 655 | 795 | 750 |
| Elongation (%) | 25 | 15 | 25 |
| Hardness (HRC max, sour) | 28 | 28 | 28 |
| Charpy at minus 46 C (J min) | 45 | 45 | 45 |
| CPT (degrees Celsius) | 30 to 40 | 50 to 70 | 55 to 75 |
When to Use Standard Duplex 2205
2205 (UNS S32205, EN 1.4462) is the workhorse of the duplex family. PREN 34 to 38 covers the bulk of moderate-chloride service. Typical use cases:
- Brackish water and produced water piping and tanks
- Chemical processing tanks for dilute acids and chlorides at ambient temperatures
- Pulp and paper digesters and washers (non-bleach)
- Heat exchanger tubing in chloride service up to roughly 35 degrees Celsius
- Storage tanks for organic acids, fertilisers and brines
- Bridge components and architectural cladding in coastal environments
The Critical Pitting Temperature for 2205 in the standard ASTM G48 Method E ferric chloride test is approximately 30 to 40 degrees Celsius. Above that, 2205 begins to pit in seawater and similar chloride media, and the project specification typically moves up to a super duplex grade.
When to Use Super Duplex 2507
2507 (UNS S32750, EN 1.4410) is the most heavily traded super duplex grade. PREN 41 to 43 lifts the chloride-pitting ceiling well above 2205. Typical use cases:
- Seawater piping, manifolds, pumps, valves and fasteners (offshore, FPSO, naval)
- Oil and gas surface and subsea hardware: christmas trees, manifolds, jumpers, wellhead bolting
- Desalination plants: MSF brine heater tubing, MED falling-film evaporators, RO high-pressure piping
- Pulp and paper bleach plants: chlorine dioxide stage piping and tanks
- Flue gas desulfurization (WFGD): absorber, quencher and outlet duct
- Chemical processing in dilute HCl and H2SO4 at moderate temperatures
When to Use Zeron 100
Zeron 100 (UNS S32760) is the copper- and tungsten-bearing super duplex variant developed originally by Rolled Alloys. The 0.5 to 1.0 percent copper improves resistance in non-oxidising acid media, and the 0.5 to 1.0 percent tungsten lifts pitting resistance in some chloride environments. Zeron 100 is project-specified by name in many UK North Sea and oil major project specifications, particularly for seawater pumps and subsea wellhead equipment. The forging spec is ASTM A182 F55 (not F53). Project specifications that name Zeron 100 are not satisfied by 2507.
When to Use Ferralium 255
Ferralium 255 (UNS S32550) is the high-copper super duplex variant developed originally by Langley Alloys. The 1.5 to 2.5 percent copper improves resistance in dilute sulfuric acid and other reducing acid media. Ferralium 255 is project-specified for seawater pumps, propeller shafts, and components exposed to dilute H2SO4. The forging spec is ASTM A182 F61. PREN 38 to 42 places it just inside the super duplex window.
Selection Guidance
A practical selection workflow:
- Confirm the chloride concentration, service temperature, and presence of H2S.
- If chloride is below 200 ppm and temperature below 35 degrees Celsius, 2205 typically suffices.
- If chloride is above 200 ppm or temperature above 35 degrees Celsius, move to super duplex.
- If the project specification names 2507, supply 2507 (UNS S32750, ASTM A182 F53).
- If the project specification names Zeron 100, supply UNS S32760 (ASTM A182 F55), not 2507.
- If the project specification names Ferralium 255, supply UNS S32550 (ASTM A182 F61).
- For sour service, confirm hardness 28 HRC max and ferrite 35 to 65 percent per NACE MR0175.
- For Norwegian Continental Shelf or oil major projects, confirm NORSOK M-630 MDS class and NORSOK M-650 manufacturer qualification.
Common Standards
Both duplex and super duplex stainless steels share the same family of international standards. The PREN threshold and the specific grade are the variables.
| Standard | Scope |
|---|---|
| ASTM A479 | Stainless steel bars and shapes |
| ASTM A789 / A790 | Tube and pipe |
| ASTM A815 | Wrought duplex and super duplex piping fittings |
| ASTM A182 F51 / F60 / F53 / F55 / F61 | Forgings, flanges and forged fittings (graded by UNS) |
| EN 10088-3 | Semi-finished products, bars, rods, wire, sections |
| NORSOK M-630 | Material data sheet (MDS classes D45, D47, D55, D57, etc.) |
| NORSOK M-650 | Manufacturer qualification (QTR) |
| NACE MR0175 / ISO 15156 | Sour service compliance |
Product Forms
TorqBolt holds super duplex 2507 (UNS S32750) stock at the Mumbai warehouse across the form and fastener range. Standard duplex 2205 and Zeron 100 are stocked to project requirement. Type 3.1 mill test certificates per EN 10204 are standard; Type 3.2 third-party-witnessed certificates are available on request.
- Bar (round, hex, flat, square, forged, hollow, cold-finished)
- Sheet, plate, pipe, tube, welded pipe, wire and welding wire
- Buttweld fittings, flanges, forgings
- Fasteners: hex bolts, heavy hex bolts, stud bolts, eye bolts, U-bolts, hex nuts, heavy hex nuts, lock nuts, washers, flat washers, lock washers, socket head cap screws
Duplex and Super Duplex Frequently Asked Questions
What is the difference between duplex and super duplex stainless steel?
Duplex stainless steels have PREN between 30 and 40. Super duplex stainless steels have 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 of 2205 and around 20 percent higher yield strength.
Is 2205 a duplex or super duplex stainless steel?
2205 is a standard duplex stainless steel. PREN of 34 to 38 is below the super duplex threshold of 40. UNS S32205 is the current specification (replacing the older S31803, which had a wider PREN window).
Is 2507 a duplex or super duplex stainless steel?
2507 is a super duplex stainless steel. UNS S32750 has PREN 41 to 43, comfortably above the super duplex threshold of 40 set by NORSOK M-630. The same chemistry is registered as EN 1.4410 and X2CrNiMoN25-7-4 in EN 10088-3.
When should I use 2205 instead of 2507?
Use 2205 for moderate-chloride service where chloride is below 200 ppm and temperature below approximately 35 degrees Celsius. 2205 covers brackish water, chemical tanks, pulp and paper non-bleach service, and architectural cladding. Move up to 2507 for seawater service, oil and gas, desalination, bleach plants and aggressive chloride media.
Can I substitute 2507 for Zeron 100?
No, not when the project specification names Zeron 100 (UNS S32760). 2507 (UNS S32750) and Zeron 100 (UNS S32760) are both super duplex grades but the chemistry differs: Zeron 100 contains 0.5 to 1.0 percent each of copper and tungsten; 2507 contains neither. The forging specs also differ (F53 for 2507, F55 for Zeron 100). A project that calls out S32760 will not accept S32750.
Are duplex and super duplex stainless steels weldable?
Yes, with controlled heat input. ER2209 is the matching filler for 2205, ER2594 is the overmatching filler for 2507, Zeron 100X for Zeron 100. Heat input held in 0.5 to 2.5 kJ per millimetre, interpass below 150 degrees Celsius, shielding gas typically 98 percent argon plus 2 percent nitrogen. NORSOK M-601 covers welding qualification.
Which standards cover duplex and super duplex?
ASTM A479 (bar), A789 (tube), A790 (pipe), A815 (fittings), A182 grades F51 / F60 / F53 / F55 / F61 (forgings, by UNS), EN 10088-3, NORSOK M-630 (material data sheet), NORSOK M-650 (manufacturer qualification), NACE MR0175 / ISO 15156 (sour service), API 6A and API 17D (wellhead and subsea).
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