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Certifications: EN 10204 3.1 / 3.2 material test certificates, ferrite content and ASTM G48 corrosion test reports, Charpy impact test data, PMI verification, and complete export documentation packages.
Super Duplex
2507
(UNS S32750)
A world-class technical reference for EPC contractors, offshore and subsea engineers, procurement heads, and TPI inspection agencies specifying Super Duplex 2507 stainless steel — covering the duplex austenitic-ferritic microstructure, the Pitting Resistance Equivalent Number (PREN) selection framework, the critical intermetallic (sigma) phase embrittlement risk during heat treatment and welding, and the QC and documentation discipline required for critical offshore and subsea equipment supply.
the PREN Framework
& Selection Logic
Super Duplex 2507 (UNS S32750) is a super duplex stainless steel — combining a balanced austenite-ferrite microstructure with high chromium, molybdenum, and nitrogen content to deliver both roughly double the yield strength of standard austenitic stainless steel and among the highest chloride pitting/crevice corrosion resistance of any commercially available stainless alloy family.
1.1 — What “Super Duplex 2507 (UNS S32750)” Means and Where 2507 Sits in the Family
Duplex stainless steel gets its name from its two-phase microstructure — approximately 50% austenite (face-centred cubic) and 50% ferrite (body-centred cubic) in a correctly heat-treated, balanced condition — combining the corrosion resistance advantages of the austenite phase with the higher strength and chloride stress corrosion cracking resistance of the ferrite phase. “Super duplex” designates the highest-alloyed tier of the duplex family, with substantially higher chromium (~25%), molybdenum (~4%), and nitrogen (~0.27%) content than standard duplex grades (e.g., 2205, UNS S31803/S32205), giving super duplex grades like 2507 markedly superior pitting and crevice corrosion resistance in aggressive chloride environments, at correspondingly higher cost and with more demanding heat treatment and welding process control requirements (discussed in detail in Part 3).
1.2 — The Pitting Resistance Equivalent Number (PREN)
%Mo = Molybdenum content (weight %)
%N = Nitrogen content (weight %)
Engineering significance:
PREN is the standard, widely used empirical index for ranking and comparing stainless and duplex steel grades’ resistance to chloride pitting corrosion — a higher PREN value indicates greater resistance to pit initiation in chloride-bearing environments. PREN is a comparative screening tool, not an absolute guarantee of corrosion performance in every specific service condition — actual performance still depends on temperature, chloride concentration, flow velocity, crevice geometry, and oxidizing species present, and project-specific corrosion testing remains advisable for critical or unusual service conditions.
PREN = 25 + (3.3 × 4) + (16 × 0.27) = 25 + 13.2 + 4.32 = ≈ 42.5
This places 2507 well above the generally accepted “super duplex” PREN threshold of 40, and substantially above standard duplex 2205’s typical PREN of approximately 35, and far above 316L stainless steel’s PREN of approximately 24.
1.3 — Key Engineering Properties
Approximately Double the Yield Strength of Austenitic Stainless
Super Duplex 2507’s minimum yield strength (typically 550 MPa in the solution-annealed condition) is roughly double that of standard austenitic stainless steel (316L, typically 170–205 MPa) — allowing thinner-walled, lighter components for a given pressure rating, a significant advantage for offshore weight-critical structures and high-pressure piping.
Exceptional Chloride Pitting and Crevice Corrosion Resistance
2507’s PREN of approximately 42–43 places it among the highest pitting-resistant stainless alloy families commercially available — suitable for seawater and hot chloride-bearing environments at higher temperatures than standard duplex 2205 or austenitic stainless can reliably withstand without pitting or crevice attack.
Excellent Chloride Stress Corrosion Cracking Resistance
The duplex microstructure’s ferrite phase content provides substantially better resistance to chloride stress corrosion cracking than fully austenitic stainless steel — a critical property for hot chloride-bearing process and seawater service where austenitic stainless steel would be at meaningful SCC risk.
Good General Corrosion Resistance
Good resistance to general corrosion across a range of oxidizing and mildly reducing environments, with performance generally comparable to or exceeding austenitic stainless steel of similar molybdenum content, though 2507 is not typically the first choice for the most aggressive reducing acid environments where nickel alloys (Incoloy 825, Hastelloy family, discussed in RR Hydraulic’s dedicated references) provide superior performance.
Product Forms
& Composition Reference
Super Duplex 2507 is manufactured across forged fitting/flange, bar, plate, and tube/pipe product forms, each governed by a specific ASTM/ASME standard. Full detail on related corrosion- resistant alloys is available across our standards reference library.
Submit form, size, and quantity to sales@rrhydraulics.com for a certified offer.
2.1 — Governing Standards
ASTM A182 (Grade F53) — Forged Flanges and Fittings
Governs forged super duplex flanges, fittings, and valve bodies — Grade F53 corresponds to UNS S32750, the standard specification for critical forged pressure-boundary components in this alloy, referenced alongside RR Hydraulic’s ANSI B16 flange dimensional reference.
ASTM A240 — Plate, Sheet, and Strip
Governs flat-rolled super duplex product — plate for pressure vessel fabrication, sheet and strip for general fabrication requiring the alloy’s high strength and corrosion performance.
ASTM A789 / A790 — Tube and Pipe
A789 governs welded and seamless duplex/super duplex tube; A790 governs welded and seamless duplex/super duplex pipe — the primary specifications for tubular products in process piping, subsea, and instrumentation applications.
ASTM A276 / A479 — Bar
Governs stainless and duplex bar stock for machined components, forging billet, and fastener manufacture in super duplex 2507.
ASTM A995 — Duplex Castings
Governs cast duplex and super duplex components (Grade CD4MCuN and related grades) for valve bodies and pump casings where cast, rather than forged or bar-machined, construction is specified.
NACE MR0175 / ISO 15156
Provides the material qualification framework for sour service use of super duplex 2507, including hardness limits (typically 32 HRC maximum for duplex stainless steels in sour service) — a mandatory reference for offshore oil & gas applications with H₂S exposure.
2.2 — Chemical Composition and Mechanical Properties
| Element / Property | Value / Range |
|---|---|
| Chromium | 24.0–26.0% |
| Nickel | 6.0–8.0% |
| Molybdenum | 3.0–5.0% |
| Nitrogen | 0.24–0.32% |
| Copper | 0.5–1.0% |
| Tensile Strength | 795–950 MPa (min. 795 MPa per spec) |
| Yield Strength | 550–650 MPa (min. 550 MPa per spec) |
| Elongation | 15–25% |
| Hardness | ≤ 32 HRC (typical maximum for sour service compliance) |
2.3 — Duplex 2205 vs. Super Duplex 2507 Comparison
| Property | Duplex 2205 (S31803/S32205) | Super Duplex 2507 (S32750) |
|---|---|---|
| PREN | ~33–38 | ~42–43 |
| Min. Yield Strength | 450 MPa | 550 MPa |
| Relative Cost | Moderate | Higher |
| Typical Use | General offshore/chemical process piping, moderate chloride severity | Highest chloride severity, subsea, seawater-critical, highest strength requirement |
& Welding Discipline
Super duplex 2507’s single most important and specification- critical engineering constraint is its susceptibility to intermetallic (sigma and chi) phase precipitation during incorrect heat treatment or welding — a phenomenon that can severely compromise both toughness and corrosion resistance if not rigorously controlled.
3.1 — Sigma Phase and Intermetallic Precipitation
3.2 — Solution Annealing: Temperature and Rapid Quench Requirements
Super Duplex 2507 must be solution annealed at a sufficiently high temperature (typically 1025–1125°C, the specific target selected to achieve the correct balanced austenite-ferrite phase ratio) followed by rapid quenching (water quench for most product forms and section thicknesses) to prevent the material from dwelling within the 650–980°C intermetallic precipitation range during cooling. Slow air cooling or an inadequate quench rate, particularly for thicker sections, risks sigma phase formation even during the initial mill heat treatment — solution annealing furnace loading, quench equipment capacity, and section thickness must all be verified as adequate for the specific component geometry to achieve a rapid enough quench throughout the material’s cross-section, not merely at the surface.
3.3 — Welding Practice and Heat Input Control
Controlled Heat Input
Welding procedures for super duplex 2507 require carefully controlled heat input (typically lower than would be used for austenitic stainless steel of similar thickness) and controlled interpass temperature to minimise the time spent within the intermetallic precipitation range during multi-pass welding — welding procedure qualification specifically for 2507, rather than assumed from duplex 2205 practice, is essential given 2507’s faster precipitation kinetics.
Matching Filler Metal and Overalloying
Welding consumables for super duplex are frequently slightly “overalloyed” in nickel content relative to the base metal (e.g., ER2594/E2594-type filler) specifically to promote adequate austenite reformation in the weld metal during the rapid cooling associated with welding, compensating for the weld metal’s tendency toward excess ferrite content if a precisely matching (non-overalloyed) filler were used.
Ferrite Content Verification in the Weld
Post-weld ferrite content measurement (per ASTM E562 or magnetic ferrite measurement, typically targeting 35–65% ferrite in the weld metal and heat-affected zone) is a standard, mandatory QC checkpoint for super duplex welds — both excessive ferrite (increased brittleness and reduced corrosion resistance) and excessive austenite (reduced strength and SCC resistance) outside the target balance indicate a welding procedure or heat input control problem requiring correction.
Industry Applications
& Documentation
RR Hydraulic maintains full traceability from certified super duplex heat to finished, tested, and packed 2507 component shipment, with ferrite content, impact testing, and corrosion resistance verification standard on all project-grade supply.
4.1 — Inspection & QC Protocol
4.2 — EN 10204 / Documentation Requirements
| Certificate | Content | EPC Requirement | When Mandatory |
|---|---|---|---|
| 2.1 / 2.2 | Declaration / non-specific | Not acceptable for critical offshore/subsea supply | Never for critical offshore or subsea equipment supply |
| 3.1 (EN 10204) | Heat-traceable chemical + mechanical test report | Mandatory — all EPC supply | All offshore, subsea, and chemical process component supply |
| 3.2 (EN 10204) | 3.1 + TPI countersign | Critical / owner-specified critical items | Subsea, high-consequence offshore pressure equipment |
4.3 — Applications by Industry
Offshore and Subsea Process Piping
Super Duplex 2507 flanges, fittings, and piping for offshore platform and subsea process systems handling seawater, produced water, and sour hydrocarbon fluids — the alloy’s combination of high strength (reducing wall thickness and weight, a critical offshore design consideration) and exceptional chloride resistance make it the standard specification for the most demanding offshore piping applications.
Seawater and Firewater Systems
2507 piping, valves, and pump components for seawater lift, cooling, and firewater deluge systems on offshore platforms and marine vessels — leveraging the alloy’s superior pitting/crevice resistance compared to standard duplex or austenitic stainless steel across the demanding combination of ambient-to-elevated seawater temperature and continuous chloride exposure.
Desalination Plant Equipment
Super duplex 2507 components for reverse osmosis and thermal desalination plant piping and pressure vessels — the hot, concentrated brine environment in desalination processes represents one of the more demanding chloride corrosion environments where 2507’s high PREN provides meaningful reliability advantage over lower-alloyed duplex or austenitic alternatives.
4.4 — Export Packaging Specification
- Flanges, fittings, and bar stock individually protected (bore caps, face protectors) to prevent contamination and mechanical damage during transit, particularly important given the alloy’s typical use in critical offshore and subsea applications
- Heat/lot number stamped or tagged on each item, cross-referenced to the accompanying material test certificate including ferrite content, impact test, and corrosion test results
- Components segregated from carbon steel and other dissimilar materials during packing to avoid surface contamination affecting the alloy’s corrosion performance
- Documentation in a waterproof pocket: EN 10204 3.1/3.2 MTC, chemical composition report, ferrite content report, Charpy impact test report, ASTM G48 corrosion test report, mechanical properties report, hardness report, PMI report, and packing list with form/size breakdown per item
- ISPM-15 timber or export cartons for international shipment, with country of origin and HS tariff code documentation matched to the super duplex product category
Submit your form, size, and quantity to RR Hydraulic for a complete, certified commercial offer.