ASTM A240 is the specification for chromium and chromium-nickel stainless steel plate, sheet, and strip for pressure vessels and general applications. Type 2507 within that specification refers to a specific grade: a super duplex stainless steel registered as UNS S32750 in the American system and EN 1.4410 (X2CrNiMoN25-7-4) in the European system. The trade name most people recognize is SAF 2507, originally developed by Sandvik (now Alleima) in Sweden in the early 1970s.
The "super" in super duplex means the alloy sits above a PREN (Pitting Resistance Equivalent Number) threshold of 40. Standard duplex grades like 2205 (UNS S32205) sit around PREN 35-36. Super duplex 2507 sits at 42-43. That numerical jump is not academic - it is the difference between a material that survives 20 years in North Sea seawater and one that starts pitting in year three.
One thing that causes real confusion in procurement: the name "2507" is used loosely. Some suppliers sell UNS S32760 (also marketed as Zeron 100 or F55) under the "2507" label. S32760 is a related but different super duplex grade - it contains tungsten (0.5-1.0%) and copper (0.5-1.0%) that S32750 does not have.
Product Forms
Bar & Rod
Plate & Sheet
Coil & Strip
Pipe & Tube
Fitting: Flange, Tee, Elbow, Reducer etc.
Forging: Ring, Shaft, Circle, Block etc.
ASTM A240 Type 2507 Chemical Composition
| Element | Spec Min (%) | Spec Max (%) | Function in the Alloy |
|---|---|---|---|
| Carbon (C) | - | 0.030 | Kept low to prevent sensitization and intergranular corrosion |
| Chromium (Cr) | 24.00 | 26.00 | Primary corrosion resistance element; stabilizes ferrite phase; forms passive Cr₂O₃ film |
| Nickel (Ni) | 6.00 | 8.00 | Stabilizes austenite phase; provides toughness; balances ferrite-austenite ratio |
| Molybdenum (Mo) | 3.00 | 5.00 | Resistance to pitting and crevice corrosion in chloride environments; key contributor to PREN |
| Nitrogen (N) | 0.24 | 0.32 | Doubles as an austenite stabilizer and pitting resistance enhancer; the most cost-effective PREN booster per percentage point |
| Manganese (Mn) | - | 1.20 | Deoxidizer; minor austenite stabilizer |
| Silicon (Si) | - | 0.80 | Deoxidizer |
| Phosphorus (P) | - | 0.035 | Impurity - controlled to avoid embrittlement |
| Sulfur (S) | - | 0.020 | Impurity - kept low for weldability and corrosion resistance |
| Copper (Cu) | - | 0.50 | Residual only in S32750; intentional in S32760 (a key difference between the two grades) |
ASTM A240 Type 2507 Mechanical Property Requirements
| Property | Minimum Required | Test Method | Notes |
|---|---|---|---|
| Tensile Strength (UTS) | 795 MPa (115 ksi) | ASTM E8 / EN ISO 6892-1 | Approximately 1.5× the UTS of 316L |
| 0.2% Yield Strength | 550 MPa (80 ksi) | ASTM E8 / EN ISO 6892-1 | Approximately 2× the yield of 316L; enables thinner wall designs |
| Elongation in 2" (50 mm) | 15% | ASTM E8 | Lower than austenitic grades - ductility is traded for strength |
| Hardness | ≤ 310 HBW | ASTM E10 / EN ISO 6506-1 | Maximum hardness cap prevents brittle material from entering service |
| Charpy Impact (if specified) | Typically 40-70 J at -46°C | ASTM E23 / EN ISO 148-1 | Not mandated by A240 but commonly required by project specs for offshore and cryogenic service |
The Yield Strength Advantage
The 550 MPa minimum yield is the single most economically significant property of Type 2507 plate. Compared to 316L (yield ~205 MPa), it allows designers to reduce wall thickness by roughly 40-50% for the same pressure rating. On a large pressure vessel or seawater piping system, that thickness reduction translates to significant weight and cost savings - even though the per-kilogram price of 2507 is 3-5× higher than 316L. The total installed cost can be lower.
Product Forms, Dimensions, and Tolerances
ASTM A240 covers plate, sheet, and strip. For Type 2507, the practical product form you will encounter most often is hot-rolled plate, because the alloy's primary applications (pressure vessels, offshore structures, heat exchanger tubesheets) demand thicker material.
| Product Form | Thickness Range | Typical Widths | Typical Lengths |
|---|---|---|---|
| Cold-rolled sheet | 0.5 – 4.0 mm | 1000, 1219, 1500 mm | 2000 – 6000 mm |
| Hot-rolled plate (No.1 finish) | 3.0 – 80 mm | 1500 – 2500 mm | 3000 – 12000 mm |
| Quarto plate (cut from rolled plate) | 5 – 100 mm | Up to 2500 mm | Up to 12000 mm |
Thickness tolerances for ASTM A240 plate are defined in ASTM A480 (the general requirements specification). Super duplex plate tends to be rolled to heavier gauges than austenitic grades because the alloy's high strength resists rolling reduction. This means that minus tolerance can bite harder - if you order 10 mm plate and receive material at 9.4 mm (within A480 tolerance), and your design needs a minimum 9.5 mm wall after machining, you have a problem. Specify a tighter tolerance class or order to nominal-plus if your application is thickness-critical.
S32750 vs S32760: The Confusion That Costs Money
This is a topic that existing online content barely covers, but it causes real procurement problems. Two super duplex grades are commonly sold under the "2507" name:
| Property | UNS S32750 (Type 2507) | UNS S32760 (Zeron 100 / F55) |
|---|---|---|
| ASTM A240 Grade Name | Type 2507 | Type 32760 (listed separately) |
| Chromium | 24-26% | 24-26% |
| Nickel | 6-8% | 6-8% |
| Molybdenum | 3-5% | 3-4% |
| Nitrogen | 0.24-0.32% | 0.20-0.30% |
| Tungsten (W) | Not specified | 0.50-1.00% |
| Copper (Cu) | ≤ 0.50% (residual) | 0.50-1.00% (intentional) |
| Typical PREN | 42-43 | 42-43 |
| Developed by | Sandvik (Sweden) | Weir Materials (UK) |
The two grades have similar corrosion resistance and mechanical properties, and in many applications they are functionally interchangeable. But they are not the same material. The tungsten in S32760 improves resistance to crevice corrosion in certain environments but can also accelerate sigma phase precipitation if heat treatment is not controlled. The copper in S32760 improves sulfuric acid resistance but is irrelevant in seawater service.
The Sigma Phase Problem: Why Heat Treatment Records Matter
Sigma phase is an intermetallic compound (FeCr) that precipitates in super duplex stainless steel when the material is exposed to temperatures between 700°C and 1000°C for too long. The precipitation happens during slow cooling from the solution annealing temperature, during prolonged holding at elevated temperature, or during welding with excessive heat input. Once sigma phase forms, it:
- Reduces impact toughness dramatically - Charpy values can drop from 70J to under 10J
- Reduces corrosion resistance - particularly pitting and crevice corrosion resistance
- Cannot be reversed by any heat treatment other than a full re-solution anneal
What this means for procurement: the heat treatment record on the mill test certificate is not just paperwork. It is evidence that the plate was quenched fast enough to avoid sigma phase. The MTR should show:
Solution annealing temperature: Should be 1025-1125°C. If it shows a temperature outside this range, the material may not meet the microstructural requirements.
Cooling method: Should be "water quench" or "rapid water cooling." If it says "air cooled" for plate thicker than 6 mm, that is a red flag - air cooling is too slow for super duplex at that thickness.
ASTM A923 test results: ASTM A923 is the standard test method for detecting detrimental intermetallic phases in duplex stainless steels. Method A (etch metallography), Method B (Charpy impact), or Method C (ferric chloride corrosion test). If the MTR does not include A923 results, request them - particularly for plate thicker than 12 mm, where the risk of sigma phase is highest due to slower cooling at the mid-thickness.
For plate thicker than 25 mm, consider specifying ASTM A923 Method A or C on the purchase order. Some mills include it as standard; others do not unless asked. The cost is minimal - a few hundred dollars per lot - but it verifies that the material you receive is free of intermetallic phases that would compromise its service performance.
Where Type 2507 Plate Actually Ends Up in Service
Desalination Plants
Seawater reverse osmosis (SWRO) and multi-stage flash (MSF) desalination plants use Type 2507 plate for high-pressure piping, pump casings, valve bodies, and heat exchanger components exposed to warm chlorinated seawater. The PREN ≥ 42 threshold is what makes 2507 the preferred material - 316L pits in warm seawater within months, and even 2205 can suffer crevice corrosion at gasket surfaces and under deposits. The Middle East accounts for roughly 48% of global desalination capacity, making it the largest regional market for 2507 plate in desalination service.
Oil and Gas - Upstream and Midstream
Subsea manifolds, flowlines, separators, heat exchangers, and wellhead equipment exposed to produced water containing chlorides and H₂S. Type 2507 plate is qualified for sour service under NACE MR0175 / ISO 15156, with restrictions on hardness (≤ 28 HRC) and heat treatment. For offshore platforms, particularly in the North Sea and the Arabian Gulf, 2507 is a standard material for seawater handling systems, firewater piping, and process equipment.
Chemical and Petrochemical Processing
Pressure vessels, heat exchangers, reactors, and storage tanks handling chloride-bearing process streams, organic acids, and sour fluids. Type 2507 plate is particularly effective in environments where both chloride stress corrosion cracking and uniform acid corrosion are concerns - a combination that destroys standard austenitic stainless steels.
Offshore and Marine Structures
Firewater systems, ballast water piping, riser clamps, structural brackets, and pump column pipes on offshore platforms. The high yield strength allows lighter structural designs, which is economically significant on offshore installations where every kilogram of topside weight has a cost multiplier.
Power Generation
Flue gas desulfurization (FGD) absorber vessels, cooling water systems in coastal power plants, and heat exchangers in geothermal applications where chloride-bearing fluids contact the metal surface.
Quality Verification: MTR Checklist for Super Duplex Plate
Reading a mill test certificate for Type 2507 plate requires attention to details that are specific to super duplex grades. Here is the checklist, in priority order:
Heat number and traceability: Verify the heat number on the MTR matches the stenciled marking on the plate bundle. No match = reject.
UNS designation: Confirm the MTR explicitly states S32750, not just "2507" or "super duplex." If it says S32760, you have the wrong grade.
Chemical composition: Every element must fall within ASTM A240 limits. Verify Cr ≥ 24.0%, Mo ≥ 3.0%, N ≥ 0.24%. Calculate the PREN from the actual heat values - it should be ≥ 41 for seawater service, ≥ 42 for critical service.
Mechanical properties: UTS ≥ 795 MPa, yield ≥ 550 MPa, elongation ≥ 15%. If elongation is exactly 15%, request additional testing - properly annealed 2507 should show 25% or higher.
Heat treatment record: Solution annealed at 1025-1125°C, water quenched. If the cooling method is not stated or is "air cooled" for plate > 6 mm, flag it.
Hardness: If NACE MR0175 compliance is required, verify actual hardness ≤ 28 HRC (272 HBW). The ASTM A240 maximum of 310 HBW is not sufficient for sour service.
Ferrite content (if specified): Target 45 ± 5% (or 40-55 FN). Some project specifications require ferrite measurement on the MTR. If it is missing, request it - out-of-range ferrite indicates chemistry imbalance or improper heat treatment.
ASTM A923 results (if specified): Confirm Method A, B, or C results show no detrimental intermetallic phases. For plate > 12 mm, this test is strongly recommended even if not explicitly required.
NACE MR0175 declaration: The MTR should state "NACE MR0175 / ISO 15156 compliant" or "suitable for sour service" with the hardness value listed.
Authorized signature and stamp: The certificate must be signed and traceable to the manufacturer's quality system. Unsigned or photocopied certificates are unacceptable for critical applications.
Fabrication and Welding: The Discipline Super Duplex Demands
Type 2507 plate is supplied in the solution-annealed condition, ready for forming, welding, and machining. But super duplex is less forgiving than austenitic stainless steel, and fabricators transitioning from 316L need to adjust their practices.
Forming
2507 has roughly twice the yield strength of 316L, which means forming requires more force. Plate bending rolls and press brakes need 40-50% more tonnage for the same thickness. Springback is also greater - expect 20-30% more springback than 316L. For tight-radius bends, warm forming at 200-300°C can reduce cracking risk, but do not exceed 300°C - above that temperature, sigma phase precipitation becomes a concern.
Welding
Super duplex welding is a discipline. Key rules:
Filler metal: Use over-alloyed filler ER2594 (AWS A5.9) or ER2593. The filler is over-alloyed with nickel (9-12% vs. 6-8% in the base metal) to ensure the weld metal achieves the correct ferrite-austenite balance. Do not use 2205 or 316L filler - the weld will be ferrite-rich and brittle.
Backing gas: Use argon with 2-3% nitrogen, or pure nitrogen. The nitrogen in the backing gas promotes austenite formation in the root pass and prevents nitrogen loss from the weld pool. Pure argon is insufficient for super duplex.
Heat input: Control to 0.5-2.5 kJ/mm. Too low causes rapid cooling and excessive ferrite. Too high causes slow cooling through the 700-1000°C range, inviting sigma phase precipitation. Use stringer beads, not weave techniques.
Interpass temperature: Keep below 150°C. Use temperature-indicating crayons or a contact pyrometer between passes.
Post-weld inspection: ASTM A923 testing on production weld test coupons. Ferrite measurement (FerriteScope or metallography). Liquid penetrant inspection (LPI) for surface defects. These are standard requirements for offshore and pressure-containing applications.
Post-weld heat treatment: Generally not required if heat input was controlled. If sigma phase is suspected (e.g., from excessive heat input), a full re-solution anneal at 1050-1100°C followed by water quenching is the only remedy.
Machining
2507 machines like other high-strength, work-hardening stainless grades. Expect 30-40% shorter tool life than 316L. Use coated carbide inserts (TiAlN or AlTiN coating), moderate cutting speeds (40-60 m/min for turning), and high-pressure coolant. Rigidity is critical - the material's high strength means any tool deflection causes chatter, which work-hardens the surface and accelerates tool wear.
Cutting
Plasma cutting is acceptable with proper parameters - use nitrogen or argon-hydrogen plasma gas, not air plasma, to avoid nitrogen contamination of the cut edge. Laser cutting produces excellent edge quality for thin plate. Waterjet cutting is ideal for thick plate and produces no heat-affected zone, but it is slow. For all thermal cutting methods, grind back the cut edge by 1-2 mm before welding to remove the heat-affected zone.
Frequently Asked Questions
What is the difference between ASTM A240 Type 2507 and EN 10088-3 1.4410?
They are the same alloy - UNS S32750 / EN 1.4410. ASTM A240 is the American standard for flat-rolled stainless steel; EN 10088-3 is the European equivalent. Chemistry and mechanical property requirements are essentially identical. The differences are in tolerance systems, surface finish definitions, and certification formats. Order to whichever standard your project specification references.
Is Type 2507 plate magnetic?
Yes, partially. The ferrite phase (approximately 45% of the microstructure) is ferromagnetic, so 2507 plate will attract a magnet - though less strongly than carbon steel. This is actually useful for distinguishing it from austenitic grades like 316L, which are non-magnetic. However, PMI verification should still use XRF equipment to confirm the Mo content.
Can Type 2507 plate be used for sour service (H₂S environments)?
Yes, under specific conditions. NACE MR0175 / ISO 15156 lists UNS S32750 as an acceptable material for sour service, provided the hardness does not exceed 28 HRC and the material is in the solution-annealed condition. The standard also imposes restrictions on temperature and partial pressure of H₂S depending on the application. Verify the specific service conditions against NACE MR0175 Table A.4 before specifying the material.
What is the maximum service temperature for Type 2507 plate?
The recommended upper limit for continuous service is approximately 250-300°C. Above 300°C, sigma phase precipitation becomes a risk over long exposure times. For intermittent service at higher temperatures, the total time at temperature above 300°C should be limited. The material is also not recommended for cryogenic service below -50°C, as the ferrite phase undergoes a ductile-to-brittle transition.
How does Type 2507 compare to 6% molybdenum super-austenitic grades like 254SMO?
Both are "super" corrosion-resistant stainless steels, but they achieve their performance through different mechanisms. 2507 is a duplex grade (45% ferrite, 55% austenite) with PREN ~42 and yield strength ~550 MPa. 254SMO is a fully austenitic grade with PREN ~43 and yield strength ~300 MPa. 2507 offers higher strength (enabling thinner designs) and better stress corrosion cracking resistance. 254SMO offers better weldability and higher toughness at cryogenic temperatures. For most seawater and chloride service, 2507 is the more cost-effective choice due to the strength advantage.
What welding filler metal should I use with Type 2507 plate?
Use over-alloyed duplex filler ER2594 (AWS A5.9 classification, also sold as 2507P or 25.10.4.L). The filler contains approximately 9-12% nickel (vs. 6-8% in the base metal) to ensure the weld metal achieves the correct 45-55% ferrite balance after rapid solidification. For dissimilar welds to carbon steel or austenitic stainless, ER309LMo or ERNiCrMo-3 (Inconel 625) filler can be used. Always use a nitrogen-containing backing gas (argon + 2-3% N₂ or pure N₂).
How do I verify that my supplier's 2507 plate is free of sigma phase?
Request ASTM A923 testing on the purchase order. Method A (etch metallography) is the most direct - it visually identifies intermetallic phases in the microstructure. Method B (Charpy impact at -40°C) is an indirect indicator - low impact energy suggests sigma phase. Method C (ferric chloride corrosion test) measures corrosion rate, which increases when sigma phase is present. For plate thicker than 12 mm, Method A or C is recommended because sigma precipitation is most likely at the mid-thickness where cooling is slowest.
What is the minimum order quantity for ASTM A240 Type 2507 plate?
For stock material at a service center or distributor, cut-to-size pieces are often available with no minimum order. For mill-direct production of non-stock sizes, the minimum is typically 1-3 metric tons for plate. Small-quantity orders from stock carry a cutting and handling surcharge of 20-40%. For project quantities (5+ tons), direct mill pricing is significantly more economical.
