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 Stainless Steel, Family, Grades, PREN, Selection
Duplex stainless steel is a class of stainless steels with a balanced two-phase microstructure, typically 50 percent austenite and 50 percent ferrite. The duplex family covers everything from lean duplex grades (LDX 2101) through standard duplex (2205) and up into super duplex (2507, Zeron 100, Ferralium 255) and hyper duplex (UNS S32707, UNS S33207). Across the family, duplex steels deliver yield strength roughly twice that of austenitic 304 or 316L while resisting chloride stress-corrosion cracking, the main failure mode that limits austenitic stainless steels in chloride service above 60 degrees Celsius.
The family is classified by Pitting Resistance Equivalent Number (PREN), where PREN equals Cr + 3.3 times Mo + 16 times N. Lean duplex sits at PREN 22 to 27, standard duplex at PREN 30 to 40, super duplex at PREN 40 or higher, and hyper duplex above 48. The most heavily traded super duplex grade is 2507 (UNS S32750 / Werkstoff 1.4410), which is the principal alloy this site documents.
What is Duplex Stainless Steel
A duplex stainless steel is one in which the microstructure is intentionally split between austenite (FCC, face-centred cubic) and ferrite (BCC, body-centred cubic) phases in roughly equal proportions. The two phases are produced by balancing chromium, nickel, molybdenum and nitrogen, then solution annealing and water quenching from approximately 1050 degrees Celsius. Ferrite delivers strength and chloride pitting resistance; austenite delivers ductility and toughness. The combination outperforms either phase alone in chloride service.
Duplex stainless steels are not heat-treatable in the precipitation-hardening sense. There is no aging step. Mechanical properties are set by the rolled and solution-annealed condition, and the only heat treatment after welding is, where required, post-weld solution annealing.
The Duplex Family
The duplex stainless steel family covers four PREN tiers. The grades below are the ones most commonly encountered in commercial trade.
| Tier | UNS | Common Name | Cr | Mo | N | PREN |
|---|---|---|---|---|---|---|
| Lean duplex | S32101 | LDX 2101 | 21.0 to 22.0 | 0.10 to 0.80 | 0.20 to 0.25 | 22 to 26 |
| Lean duplex | S32304 | 2304 | 21.5 to 24.5 | 0.05 to 0.60 | 0.05 to 0.20 | 23 to 28 |
| Standard duplex | S31803 | 2205 (older) | 21.0 to 23.0 | 2.5 to 3.5 | 0.08 to 0.20 | 30 to 36 |
| Standard duplex | S32205 | 2205 (current) | 22.0 to 23.0 | 3.0 to 3.5 | 0.14 to 0.20 | 34 to 38 |
| Super duplex | S32750 | 2507 | 24.0 to 26.0 | 3.0 to 5.0 | 0.24 to 0.32 | 41 to 43 |
| Super duplex | S32760 | Zeron 100 | 24.0 to 26.0 | 3.0 to 4.0 | 0.20 to 0.30 | 41 to 44 |
| Super duplex | S32550 | Ferralium 255 | 24.0 to 27.0 | 2.9 to 3.9 | 0.10 to 0.25 | 38 to 42 |
| Hyper duplex | S32707 | SAF 2707 HD | 26.0 to 29.0 | 4.0 to 5.0 | 0.30 to 0.50 | 49 to 50 |
| Hyper duplex | S33207 | SAF 3207 HD | 29.0 to 33.0 | 3.0 to 5.0 | 0.40 to 0.60 | 52 to 53 |
Composition Window
The defining elements across the duplex family are chromium, nickel, molybdenum and nitrogen. Lean grades (2101, 2304) sit at low molybdenum to control cost. Standard duplex (2205) sits at moderate molybdenum. Super duplex (2507) sits at high molybdenum and elevated nitrogen. Some grades add copper or tungsten for specific media: Zeron 100 adds approximately 0.7 percent each of Cu and W; Ferralium 255 adds 1.5 to 2.5 percent Cu; UNS S32750 (2507) keeps copper as residual only (0.50 max, not intentional) and contains no tungsten.
Mechanical Properties
Yield strength of duplex stainless steels is roughly twice that of austenitic 316L, allowing thinner-wall designs and lower fastener counts in pressure equipment. Required minimum properties per ASTM A240 (plate) and A479 (bar) in the solution-annealed condition:
| Grade | Yield (MPa min) | Tensile (MPa min) | Elongation (%) | Hardness (HRC max) |
|---|---|---|---|---|
| 2304 | 400 | 600 | 25 | 32 |
| 2205 (S32205) | 450 | 655 | 25 | 30.5 |
| 2507 (S32750) | 550 | 795 | 15 | 32 |
| Zeron 100 (S32760) | 550 | 750 | 25 | 32 |
For sour service per NACE MR0175, hardness is restricted to 28 HRC maximum. Charpy V-notch toughness at minus 46 degrees Celsius is typically 60 to 100 J for super duplex base metal.
Corrosion Resistance
Duplex stainless steel resistance scales with PREN. The Critical Pitting Temperature (CPT) per ASTM G48 Method E is approximately 35 degrees Celsius for 2205, 50 to 70 degrees Celsius for 2507, and similar for Zeron 100. Beyond pitting and crevice corrosion, the duplex family resists chloride stress-corrosion cracking up to roughly 200 degrees Celsius (vs around 60 degrees Celsius for austenitic 316L), erosion-corrosion in seawater up to about 6 metres per second, and dilute reducing acids (HCl, H2SO4) at moderate temperatures.
PREN Selection
PREN is the primary number used to select duplex grades for chloride service. The formula is Cr + 3.3 times Mo + 16 times N. NORSOK M-630 sets PREN 40 as the threshold for super duplex classification. For tungsten-bearing grades like Zeron 100, 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 duplex stainless steels are supplied solution-annealed and water-quenched. Solution annealing temperatures climb with PREN: roughly 1020 to 1100 degrees Celsius for 2205 and approximately 1040 to 1100 degrees Celsius for 2507. Water quench must be rapid through the 600 to 1000 degrees Celsius band to avoid sigma-phase precipitation, which collapses Charpy toughness and pitting resistance. There is no aging step. Post-weld solution annealing is required by NORSOK M-630 for thicker super duplex sections in critical service.
Welding
Duplex stainless steels are weldable with controlled heat input. Standard fillers under AWS A5.9 are ER2209 (matching 2205), ER2594 (overmatching 2507), and Zeron 100X (matching Zeron 100). Heat input is held in the 0.5 to 2.5 kJ per millimetre window. Interpass temperature stays below 150 degrees Celsius. Shielding gas commonly runs 98 percent argon plus 2 percent nitrogen to maintain austenite balance in the weld metal. NORSOK M-601 covers welding qualification.
ASTM, EN, NORSOK Standards
Duplex stainless steels appear in the following international standards.
| Standard | Scope |
|---|---|
| ASTM A240 | Plate, sheet and strip for pressure vessels |
| 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 stainless piping fittings |
| ASTM A182 F53 / F55 / F61 / F51 | Forgings, flanges, forged fittings (super duplex and duplex grades) |
| EN 10088-3 | Semi-finished products, bars, rods, wire, sections |
| 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, API 17D | Wellhead and subsea equipment |
Applications
Duplex stainless steel applications scale with PREN. Standard duplex (2205) handles brackish water, chemical tanks and moderate-chloride service. Super duplex (2507, Zeron 100) handles seawater, oil and gas, desalination, and aggressive chloride media. Common service environments:
- Seawater piping, manifolds, pumps, valves and fasteners (offshore, FPSO, naval)
- Oil and gas surface and subsea hardware, including christmas trees and wellhead bolting
- Desalination plants: MSF, MED, RO high-pressure piping
- Chemical processing: dilute HCl and H2SO4, chlorine dioxide bleach
- Pulp and paper bleach plants and digester piping
- Flue gas desulfurization (WFGD) absorbers, quenchers and outlet ducts
- Storage tanks for chloride-bearing brines
- Bridge components and architectural cladding in coastal environments
Selection Guidance
Selecting the right duplex grade depends on chloride concentration, temperature, and presence of H2S. As a rule of thumb, the Critical Pitting Temperature (CPT) per ASTM G48 Method E is the practical service-temperature ceiling in chloride-bearing media. For seawater (around 19,000 ppm chloride), 2205 is acceptable to roughly 35 degrees Celsius, while 2507 stretches the ceiling to 50 to 70 degrees Celsius. For sour service, NACE MR0175 limits hardness to 28 HRC maximum and ferrite to 35 to 65 percent. The detailed comparison page duplex and super duplex sets out when to use each.
Product Forms
TorqBolt holds super duplex 2507 (UNS S32750) stock at the Mumbai warehouse. 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: bolts, hex bolts, heavy hex bolts, stud bolts, eye bolts, U-bolts, nuts, hex nuts, heavy hex nuts, lock nuts, washers, flat washers, lock washers, socket head cap screws, threaded rod
Duplex Stainless Steel Frequently Asked Questions
What is duplex stainless steel?
Duplex stainless steel has a two-phase microstructure that is roughly 50 percent austenite and 50 percent ferrite. The combination delivers yield strength roughly twice that of austenitic 316L plus high resistance to chloride stress-corrosion cracking. The family ranges from lean duplex (PREN 22 to 27) through standard duplex 2205 (PREN 34 to 38) and into super duplex 2507 (PREN 41 to 43).
What is the difference between duplex and super duplex?
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 and around 30 percent higher yield strength than 2205. See the duplex and super duplex comparison.
Which duplex grades are most common?
2205 (UNS S32205, EN 1.4462) is the most heavily traded standard duplex grade. 2507 (UNS S32750, EN 1.4410) is the most heavily traded super duplex grade. Zeron 100 (UNS S32760) and Ferralium 255 (UNS S32550) are project-specified alternatives to 2507 in particular service environments. Lean duplex grades (2304, LDX 2101) trade in lower volumes for tank and architectural use.
What is the PREN of duplex stainless steel?
PREN ranges across the family. Lean duplex sits at 22 to 28, standard duplex 2205 at 34 to 38, super duplex 2507 at 41 to 43, Zeron 100 at 41 to 44, and hyper duplex above 48. The formula is Cr + 3.3 times Mo + 16 times N. See the PREN reference page.
Is duplex stainless steel weldable?
Yes, with controlled heat input. ER2209 is the matching filler for 2205 per AWS A5.9; ER2594 is the overmatching filler for 2507. Heat input is held in 0.5 to 2.5 kJ per millimetre, interpass temperature below 150 degrees Celsius, shielding gas typically 98 percent argon plus 2 percent nitrogen. NORSOK M-601 covers welding qualification.
What are the main applications of duplex stainless steel?
Seawater piping, oil and gas surface and subsea hardware, desalination, chemical processing in dilute acids, pulp and paper bleach plants, flue gas desulfurization, brackish-water tanks, 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.
Which standards cover duplex stainless steel?
ASTM A240 (plate), A479 (bar), A789 (tube), A790 (pipe), A815 (fittings), A182 grades F51 (S31803), F60 (S32205), F53 (S32750), F55 (S32760), F61 (S32550) for forgings, 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).
Satisfaction Survey
