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 Stainless Steel, Grades, Properties, Applications
Super duplex stainless steel is a class of duplex (austenite plus ferrite) stainless steels with a Pitting Resistance Equivalent Number (PREN) of 40 or higher. The high PREN comes from elevated chromium (typically 25 percent), molybdenum (3.5 to 4 percent) and nitrogen (0.24 to 0.32 percent). Compared with standard duplex grades like 2205, super duplex offers roughly twice the pitting resistance and around 30 percent higher yield strength, while keeping the toughness and weldability of a balanced 50/50 ferrite-austenite microstructure. The most common super duplex grade in commercial use is 2507 (UNS S32750 / Werkstoff 1.4410), which is the primary alloy this site covers in depth.
Super duplex stainless steels are specified for chloride and seawater service, oil and gas surface and subsea hardware, desalination plants, chemical processing, pulp and paper bleach plants, and flue-gas desulfurization systems. The four production grades commonly traded are UNS S32750 (2507), UNS S32760 (Zeron 100), UNS S32550 (Ferralium 255) and UNS S39274 (DP3W). Each carries its own forging spec under ASTM A182 (F53 for 2507, F55 for Zeron 100, F61 for Ferralium 255).
What is Super Duplex Stainless Steel
A super duplex stainless steel is a duplex stainless steel with PREN at or above 40, where PREN equals Cr + 3.3 times Mo + 16 times N. The "super" designation is set by NORSOK M-630 and is followed by major project specifications worldwide. The microstructure is intentionally balanced at approximately 50 percent ferrite and 50 percent austenite, and the alloy is supplied in the solution-annealed and water-quenched condition. There is no aging step, no precipitation hardening, and no temper.
The defining elemental ranges for the super duplex family are 24 to 27 percent chromium, 6 to 8 percent nickel, 3 to 5 percent molybdenum and 0.20 to 0.32 percent nitrogen. Some grades add copper or tungsten (or both) to refine corrosion resistance in specific media. Composition and property data for each grade are documented on dedicated pages.
Super Duplex Grades
Four super duplex stainless steels see routine commercial trade. They are not interchangeable: project specifications call out the alloy by UNS number, and the forging specs differ.
| UNS | Common Name | EN / Werkstoff | Cu | W | PREN | Forging Spec |
|---|---|---|---|---|---|---|
| S32750 | 2507 | 1.4410 | None | None | 41 to 43 | ASTM A182 F53 |
| S32760 | Zeron 100 | 1.4501 | 0.5 to 1.0 | 0.5 to 1.0 | 41 to 44 | ASTM A182 F55 |
| S32550 | Ferralium 255 | 1.4507 | 1.5 to 2.5 | None | 38 to 42 | ASTM A182 F61 |
| S39274 | DP3W | (no EN) | 0.20 to 0.80 | 1.5 to 2.5 | 40 to 43 | ASTM A182 F54 |
Grade-by-grade detail is in the super duplex grades reference. The most commonly stocked grade worldwide, and the focus of TorqBolt's super duplex inventory, is 2507 (UNS S32750).
Composition Window
All four super duplex grades share the broad chromium, nickel, molybdenum and nitrogen window. Differences sit in copper, tungsten, and minor adjustments to manganese, silicon and carbon. The chemistry of 2507 (UNS S32750) per ASTM A479 is the canonical super duplex window, with no intentional copper or tungsten.
| Element | UNS S32750 (2507) | UNS S32760 (Zeron 100) | UNS S32550 (Ferralium 255) |
|---|---|---|---|
| Carbon (C) | 0.030 max | 0.030 max | 0.040 max |
| Chromium (Cr) | 24.0 to 26.0 | 24.0 to 26.0 | 24.0 to 27.0 |
| Nickel (Ni) | 6.0 to 8.0 | 6.0 to 8.0 | 4.5 to 6.5 |
| Molybdenum (Mo) | 3.0 to 5.0 | 3.0 to 4.0 | 2.9 to 3.9 |
| Nitrogen (N) | 0.24 to 0.32 | 0.20 to 0.30 | 0.10 to 0.25 |
| Copper (Cu) | 0.50 max (residual, not intentional) | 0.50 to 1.00 | 1.50 to 2.50 |
| Tungsten (W) | (none) | 0.50 to 1.00 | (none) |
| PREN (typical) | 41 to 43 | 41 to 44 | 38 to 42 |
Mechanical Properties
Super duplex stainless steels deliver yield strength roughly twice that of austenitic 316L while retaining acceptable toughness for low-temperature service. Required minima per ASTM A479 in the solution-annealed condition are tabulated below.
| Property | Minimum | Typical | Test Standard |
|---|---|---|---|
| Yield strength (0.2 percent offset) | 550 MPa (80 ksi) | 620 to 720 MPa | ASTM A370 |
| Tensile strength | 795 MPa (115 ksi) | 800 to 1000 MPa | ASTM A370 |
| Elongation in 50 mm | 15 percent | 25 to 35 percent | ASTM A370 |
| Hardness (sour-service limit) | 28 HRC max | 25 to 28 HRC | ASTM E18, NACE MR0175 |
| Charpy V-notch at minus 46 degrees Celsius | 45 J min | 60 to 100 J | NORSOK M-630, ASTM E23 |
| Critical pitting temperature (CPT) | 50 degrees Celsius | 55 to 70 degrees Celsius | ASTM G48 Method E |
| Ferrite content (base metal) | 35 to 55 percent | 45 to 50 percent | NORSOK M-630, ASTM E562 |
Corrosion Resistance
A PREN above 40 places the super duplex family at a Critical Pitting Temperature (CPT) typically between 50 and 70 degrees Celsius in the standard ASTM G48 Method E ferric chloride test. That is well above 6 percent molybdenum austenitic steels (904L, AL-6XN), and roughly equivalent to nickel-based alloys 825 and 625 in chloride pitting service. In addition to pitting and crevice corrosion resistance, super duplex grades deliver:
- High resistance to chloride stress corrosion cracking, where standard austenitic steels fail rapidly above 60 degrees Celsius
- Acceptable performance in dilute hydrochloric and sulfuric acid at ambient temperatures
- Good resistance in chlorine dioxide bleach service for pulp and paper
- Resistance to erosion-corrosion in seawater up to flow velocities around 6 metres per second
PREN Calculation
The Pitting Resistance Equivalent Number is calculated from the chemistry on the mill test certificate. The formula is Cr + 3.3 times Mo + 16 times N. For tungsten-bearing grades like Zeron 100 and DP3W, PREW (which adds tungsten) is sometimes used: Cr + 3.3 times (Mo + 0.5 times W) + 16 times N. Detail and worked examples sit on the PREN reference page.
Heat Treatment
All super duplex grades are supplied in the solution-annealed condition. The cycle is heating to 1040 to 1100 degrees Celsius, holding to fully dissolve sigma phase and any other intermetallic phases, then water quenching at a rate sufficient to drive the material rapidly through the 600 to 1000 degrees Celsius band. Slow cooling causes sigma-phase precipitation, which collapses Charpy toughness and pitting resistance. There is no precipitation hardening, no aging step, no temper. NORSOK M-630 specifies post-weld solution annealing for material thicker than approximately 6 millimetres in critical service.
Welding
Super duplex stainless steels are weldable with care. The standard filler for 2507 is ER2594 per AWS A5.9 (overmatching, slightly higher PREN than the base). ER2553 is the matching filler for less aggressive service. For Zeron 100, the matching filler is Zeron 100X. Heat input is controlled to 0.5 to 2.5 kJ per millimetre, interpass temperature is held below 150 degrees Celsius, and shielding gas typically runs 98 percent argon plus 2 percent nitrogen to maintain the austenite balance in the weld metal. NORSOK M-601 covers welding qualification.
ASTM, EN, NORSOK, NACE Standards
Super duplex stainless steels appear in the following international standards. Each row links to a dedicated reference page.
| Standard | Scope |
|---|---|
| ASTM A479 | Stainless steel bars and shapes for boiler and pressure vessel use |
| ASTM A789 | Seamless and welded ferritic / austenitic stainless tube |
| ASTM A790 | Seamless and welded ferritic / austenitic stainless pipe |
| ASTM A815 | Wrought ferritic / austenitic / martensitic stainless piping fittings |
| ASTM A182 F53 | Forgings, flanges and forged fittings (UNS S32750) |
| ASTM A182 F55 | Forgings, flanges and forged fittings (UNS S32760) |
| EN 10088-3 | Semi-finished products, bars, rods, wire, sections (1.4410, 1.4501) |
| NORSOK M-630 | Material data sheet (Norwegian Continental Shelf) |
| NORSOK M-650 | Manufacturer qualification (QTR) |
| NACE MR0175 / ISO 15156 | Sour service compliance, hardness 28 HRC max |
| API 6A | Wellhead and christmas tree equipment |
| API 17D | Subsea wellhead and tree equipment |
Applications
Super duplex stainless steel is specified where chloride pitting, seawater service, sour gas, or chloride stress-corrosion cracking would defeat standard austenitic grades. Common service environments:
- 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
- Chemical processing: dilute HCl and H2SO4 service, chlorine dioxide handling
- Pulp and paper bleach plants: chlorine dioxide stage piping and tanks
- Flue gas desulfurization (WFGD): absorber, quencher and outlet duct
- Marine and naval architecture: deck hardware, ballast piping, firewater systems
Product Forms
TorqBolt holds super duplex 2507 (UNS S32750) stock at the Mumbai warehouse across the form and fastener range. Type 3.1 mill test certificates per EN 10204 are standard; Type 3.2 third-party-witnessed certificates are available on request.
- Round bar, hex bar, flat bar, square bar, forged bar, hollow bar, cold-finished bar
- 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
Super Duplex Stainless Steel Frequently Asked Questions
What is super duplex stainless steel?
Super duplex stainless steel is a duplex (austenite plus ferrite) stainless steel with a PREN of 40 or higher, achieved through high chromium (around 25 percent), molybdenum (3.5 to 4 percent) and nitrogen (0.24 to 0.32 percent). The most common super duplex grade is 2507 (UNS S32750 / EN 1.4410). The microstructure is balanced at roughly 50 percent ferrite and 50 percent austenite.
What is the difference between duplex and super duplex stainless steel?
Duplex stainless steels (2205, 2304) have PREN between 30 and 40. Super duplex stainless steels (2507, Zeron 100, Ferralium 255) have PREN of 40 or higher. The higher PREN comes from increased chromium, molybdenum, and nitrogen, giving roughly twice the pitting resistance and around 30 percent higher yield strength. See the duplex and super duplex comparison page.
Which super duplex grade is most commonly used?
UNS S32750 (commercially known as 2507, EN 1.4410, X2CrNiMoN25-7-4) is the most widely traded super duplex grade worldwide. It is the focus of TorqBolt's super duplex inventory and the primary alloy covered on this site. Mill trademarks for the same chemistry include SAF 2507 (Sandvik), UR 52N+ (Industeel ArcelorMittal) and Avesta 2507 (Outokumpu).
What is the PREN of super duplex stainless steel?
PREN of 40 or higher is the super duplex threshold per NORSOK M-630. UNS S32750 has typical PREN 41 to 43; UNS S32760 (Zeron 100) has PREN 41 to 44; UNS S32550 (Ferralium 255) has PREN 38 to 42. The formula is Cr + 3.3 times Mo + 16 times N. See the PREN page for worked examples.
What are the main applications of super duplex stainless steel?
Seawater piping, oil and gas surface and subsea hardware, desalination plants (MSF, MED, RO), chemical processing in dilute acids, pulp and paper bleach plants (chlorine dioxide service), flue gas desulfurization (WFGD), and chloride-bearing service generally. Service temperature is roughly minus 50 to 250 degrees Celsius continuous; sigma-phase risk above 300 degrees Celsius limits long-term high-temperature use.
Is super duplex stainless steel weldable?
Yes, with care. Standard filler for 2507 is ER2594 per AWS A5.9 (slightly overmatching). Heat input is controlled to 0.5 to 2.5 kJ per millimetre, interpass temperature stays below 150 degrees Celsius, and shielding gas typically runs 98 percent argon plus 2 percent nitrogen. Sigma-phase precipitation in the heat-affected zone is the main risk and is controlled by heat input and cooling rate.
Which standards cover super duplex stainless steel?
ASTM A479 (bar), A789 (tube), A790 (pipe), A815 (fittings), A182 grades F53 (S32750) and F55 (S32760) for forgings, A276 (shapes), EN 10088-3 (1.4410, 1.4501), NORSOK M-630 (material data sheet), NORSOK M-650 (manufacturer qualification), NACE MR0175 / ISO 15156 (sour service), API 6A (wellhead) and API 17D (subsea).
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