Quick Answer
Quick Answer
Super Duplex 2507 (EN 1.4410, UNS S32750) contains approximately 25 % Cr, 7 % Ni, and 4 % Mo with elevated nitrogen. Its PREN of ~43 exceeds the industry threshold of 40 for seawater service, delivering superior pitting and SCC resistance alongside yield strength nearly three times that of austenitic 316L.
Overview
Super duplex stainless steels are defined by a PREN ≥ 40, achieved through higher chromium, molybdenum, and nitrogen contents compared to standard duplex grades like 2205. Grade 2507 (25 % Cr, 7 % Ni, 4 % Mo) is the dominant super duplex grade in offshore, subsea, and seawater cooling applications.
Key advantages over standard duplex 2205:
- PREN ~43 vs ~35 for 2205
- Higher yield strength (~550 MPa vs ~450 MPa)
- Better resistance to localized corrosion in seawater and hot concentrated chloride solutions
The trade-offs are higher cost, more demanding fabrication requirements, and greater sensitivity to improper heat treatment.
Chemical Composition — Super Duplex 2507 / 1.4410
| Element | ASTM A790 S32750 | EN 1.4410 |
|---|---|---|
| Carbon (C) | ≤ 0.030 | ≤ 0.030 |
| Manganese (Mn) | ≤ 1.20 | ≤ 1.20 |
| Silicon (Si) | ≤ 0.80 | ≤ 0.80 |
| Phosphorus (P) | ≤ 0.035 | ≤ 0.035 |
| Sulfur (S) | ≤ 0.020 | ≤ 0.015 |
| Chromium (Cr) | 24.0 – 26.0 | 24.0 – 26.0 |
| Molybdenum (Mo) | 3.0 – 5.0 | 3.0 – 4.5 |
| Nickel (Ni) | 6.0 – 8.0 | 6.0 – 8.0 |
| Nitrogen (N) | 0.24 – 0.32 | 0.24 – 0.32 |
| Copper (Cu) | ≤ 0.50 | ≤ 0.50 |
The high nitrogen content (0.24–0.32 %) is critical. It boosts PREN, stabilizes austenite, and suppresses sigma phase formation. Any MTC reporting nitrogen below 0.24 % should be carefully evaluated against the specification.
PREN Calculation
PREN = %Cr + 3.3×%Mo + 16×%N
At nominal composition (25 % Cr, 4 % Mo, 0.28 % N): PREN ≈ 25 + 3.3×4 + 16×0.28 ≈ 25 + 13.2 + 4.5 ≈ 42.7
This comfortably exceeds the NORSOK M-001 minimum of 40 for seawater-wetted components.
Mechanical Properties — Super Duplex 2507
| Property | ASTM A790 S32750 | EN 1.4410 (+AT) |
|---|---|---|
| Ultimate Tensile Strength (UTS) | 795 MPa (115 ksi) min | 730 – 930 MPa |
| 0.2 % Proof Strength (YS) | 550 MPa (80 ksi) min | 530 MPa min |
| Elongation in 50 mm | 15 % min | 20 % min |
| Hardness (max) | 310 HBW / 32 HRC | 310 HBW |
| Charpy impact (0 °C) | 41 J (27 ft·lbf) | 80 J (longitudinal) |
Standards Coverage
| Standard | Product Form | Designation |
|---|---|---|
| ASTM A790 | Seamless and welded pipe | S32750 |
| ASTM A240 | Plate, sheet, strip | S32750 |
| ASTM A182 | Forgings | F53 |
| ASTM A276 | Bar | S32750 |
| ASME SA-240 | Pressure vessel plate | S32750 |
| EN 10088-2 | Flat products | 1.4410 |
| NORSOK M-650 | Offshore qualification | 25Cr super duplex |
| ISO 15156-3 | Sour service | Permitted with conditions |
Applications
Super duplex 2507 is selected for the most demanding corrosion environments:
- Subsea production systems — flowlines, manifolds, and wellhead components in seawater and chloride-rich produced fluids
- Seawater cooling and fire-fighting systems — where PREN ≥ 40 is mandated
- Desalination plants — brine heaters and high-pressure pump casings in multi-stage flash and reverse osmosis systems
- FPSOs and offshore platforms — topside and subsea piping systems per NORSOK requirements
- Pulp bleaching — highly corrosive chlorine dioxide and hypochlorite environments
- Chemical injection systems — methanol, corrosion inhibitor, and biocide injection lines offshore
Fabrication Considerations
Super duplex requires tighter process control than standard duplex:
| Parameter | Requirement |
|---|---|
| Solution annealing temperature | 1050 – 1125 °C |
| Rapid quenching | Water quench to avoid sigma phase |
| Welding heat input | 0.2 – 1.5 kJ/mm (tighter than 2205) |
| Max interpass temperature | 100 °C |
| Filler wire | ER2594 (over-alloyed 25Cr/9Ni/4Mo) |
Ready to automate your certificate workflow?
Try TestCert freeFrequently Asked Questions
What is the difference between 2205 and 2507?
2507 has higher Cr (~25 % vs ~22 %), Mo (~4 % vs ~3 %), and N (~0.28 % vs ~0.17 %) than 2205. This raises PREN from ~35 to ~43 and yield strength from ~450 MPa to ~550 MPa. 2507 is chosen when 2205's corrosion resistance is insufficient — particularly in seawater or highly concentrated chloride environments. The cost premium for 2507 is typically 30–50 % over 2205.
Can super duplex 2507 be used in sour service (H₂S)?
Yes, with conditions. ISO 15156-3 permits 2507 in sour environments subject to limits on H₂S partial pressure, temperature, pH, and chloride concentration. Maximum hardness and tensile strength limits apply. Always verify the specific service conditions against the ISO 15156-3 acceptance criteria before specifying 2507 for sour gas service.
Why is the maximum service temperature lower for super duplex than standard duplex?
Super duplex grades are more prone to sigma phase precipitation above 280 °C due to their higher Cr and Mo content. Sigma phase causes severe embrittlement and reduced corrosion resistance. Maximum continuous service temperature is typically 250–280 °C for 2507. Short-term excursions may be acceptable — consult the material supplier and relevant code.
How does TestCert verify super duplex PREN against NORSOK M-001 requirements?
TestCert reads the Cr, Mo, and N values from the uploaded MTC, computes PREN using the standard formula, and compares the result against any customer-specified minimum captured in the purchase order. For projects referencing NORSOK M-001, a minimum PREN of 40 is applied automatically when the grade is identified as super duplex.