Super Duplex 2507: What UNS S32750 Actually Buys You Over Standard Duplex

Updated August 2026

Super Duplex 2507 is a 25Cr-7Ni-4Mo-N duplex stainless steel (UNS S32750, sometimes written simply as 2507 stainless steel) built for highly corrosive conditions and chloride-heavy service that standard duplex grades cannot reliably hold. Before you specify it, though, the grade name alone settles almost nothing: the PREN number that defines “super” duplex varies by source, the filler metal for welding it is not the same as 2205’s, and at least one documented failure shows that alloy upgrade alone does not guarantee immunity from corrosion. This guide works through the identity, the metallurgy, the standards, and the honest limits — not a sales sheet, but a reference for the engineer or buyer who has to get the specification right the first time.

What Is Super Duplex 2507? (UNS S32750 / EN 1.4410 / SAF 2507)

Super Duplex 2507 is the alloy identified by UNS S32750 under a 50/50 ferrite-austenite microstructure, with EN 1.4410 as its European material number and SAF 2507 as Alleima’s registered producer trade name for the same composition. None of these three labels is a purchase specification on its own — product form, delivery condition, and acceptance testing come from the governing ASTM/ASME standard, covered next.

A UNS number identifies the alloy family, an EN number identifies the same alloy under a different standards body, and a producer trade name identifies one company’s commercial datasheet for it — three labels for one composition, none of them substituting for the other two.

A grade name is not a purchase specification — product form, standard, unit system, inspection method, and delivery documentation all have to agree before a 2507 order is genuinely ready to place, a discipline this guide returns to in the RFQ section below.

Typical demanding applications span offshore platforms and subsea equipment, desalination and seawater intake systems, chemical and petrochemical processing vessels, heat exchangers in the power industry, and pulp-and-paper digesters — service environments where standard austenitic grades show high molybdenum and chromium demand just to approach adequate resistance to pitting, and duplex or super duplex becomes the more economical long-term choice despite the higher upfront alloy cost. Reliability in these settings depends on matching the full specification, not the alloy name alone, which is the theme this guide returns to throughout.

One related grade worth disambiguating up front: CD4MCu is a cast duplex stainless steel, not a wrought grade like 2507, and it is not automatically interchangeable with wrought S32750 stock even though both fall under the general “duplex stainless” umbrella.

What Makes It “Super”? The PREN Threshold, Precisely

A duplex stainless steel earns the “super” designation once its Pitting Resistance Equivalent Number (PREN) crosses roughly 40 — the exact minimum cited varies by source, running from 40 to 43 depending on which producer datasheet or standards reference is doing the citing. None of the figures below is wrong; they reflect slightly different composition assumptions and rounding conventions, so treat “roughly 40 and up” as the honest answer, not any single number presented as exact.

Alleima’s SAF 2507 figures as carried on producer-datasheet-derived listings state a minimum PREN of 41; several distributor pages state “PREN>42”; Langley Alloys states “PREN>40” for its equivalent Alloy 32750; and a weldfabworld.com grade-comparison table lists a typical PREN near 43 for S32750.

What Is the Difference Between Duplex and Super Duplex Stainless Steel?

The practical difference is alloy content and the resulting PREN band, not a separate microstructure family — both are roughly 50/50 ferrite-austenite. That roughly five-to-eight-point PREN gap is what separates “adequate for general oil-and-gas service” from “built for aggressive brine and stagnant chloride exposure,” a difference of degree along one property axis, not a different class of material.

Standard duplex (2205/S32205) runs about 22% Cr, 3% Mo, 0.14-0.20% N for a PREN near 35; super duplex (2507/S32750) runs about 25% Cr, 4% Mo, 0.27-0.3% N for a PREN in the 40-43 range — offshore injection water and stagnant chloride exposure are where that gap typically earns its keep, while non-stagnant seawater often stays within 2205’s window.

PREN worked example (2507’s own composition)

PREN = %Cr + 3.3×%Mo + 16×%N. Using Alleima’s published nominal SAF 2507 composition (25% Cr, 4% Mo, 0.3% N): PREN = 25 + (3.3×4) + (16×0.3) = 25 + 13.2 + 4.8 = 43. That lands at the top of the reported range and matches the weldfabworld.com PREN comparison table‘s figure, a useful sanity check when a supplier’s certificate quotes a lower PREN: it likely reflects a leaner nitrogen or molybdenum content within the standard’s allowable range, not an error.

Chemical Composition and Mechanical Properties

This section covers Super Duplex 2507 chemical composition and the ASTM A240 mechanical minimums in one place — pull the exact figures from Alleima’s Super Duplex 2507 datasheet before finalizing a purchase order, since the numbers below are producer-reference values, not order minimums.

Mechanical minimums under ASTM A240 for this grade sit well above both austenitic 316L and standard Duplex 2205, driven by the same higher alloy content that raises its PREN. Buyers reading a mill test certificate should expect these minimums as a floor, not a target — actual heats commonly report higher, and a certificate reading close to the floor is not itself a red flag, just a reminder to check the specific standard edition cited.

Composition figures are producer-reference nominal values (Alleima, weldfabworld.com comparison data) not ASTM A240 order minimums or maximums – request the actual mill certificate against the specific standard edition cited on your purchase order.

The combination that gives 2507 its reputation for exceptional strength and corrosion resistance is the balanced austenite and ferrite phase structure working alongside the elevated chromium and nitrogen content – the ferrite phase contributes high strength and good resistance to chloride stress corrosion cracking, while the austenite phase contributes toughness and ductility that a fully ferritic structure would lack. Physical properties worth noting beyond the headline mechanical numbers: thermal expansion and thermal conductivity both run closer to carbon steel than to austenitic stainless, which matters for mixed-material piping design, and the alloy’s high mechanical strength holds up well through the solution-annealed heat treated condition specified by the governing ASTM standard.

On general corrosion and pitting resistance specifically: 2507 shows resistance to uniform corrosion and excellent resistance to chloride pitting and crevice corrosion attack in the great majority of chloride-containing service, and its resistance to chloride stress corrosion is a primary reason it gets specified over austenitic grades like 316 or 316L in demanding applications. On a super duplex 2507 vs 316 comparison, the gap is not a close call for aggressive chloride duty — 2507’s PREN sits well above austenitic 316’s typical range — even though 316 remains adequate and far cheaper for milder service. It isn’t universally immune, however – resistance to uniform corrosion by organic acids such as formic and acetic acid is good but not unlimited, and acid concentration, temperature, and aeration all shift where the practical limit sits; treat any acidic-service claim with the same source-and-condition scrutiny applied to chloride service in Section 8.

Which ASTM/ASME Standard Governs Your Product Form?

The governing ASTM standard for Super Duplex 2507 changes by product form, and citing the wrong one is one of the more common RFQ errors. That distinction sits on top of the composition and mechanical minimums covered above — meeting the alloy chemistry is necessary but not sufficient if the certificate is written against the wrong product-form standard.

Plate, sheet, and coil fall under ASTM A240/A240M; pipe under A790/A790M (current edition A790/A790M-24, confirmed active via direct ASTM store record); tube under A789/A789M; round bar and shapes under A276/A276M or A479/A479M; and forged piping components under A182/A182M when the F53 grade designation applies; wrought fittings under A815/A815M; and cast valves, flanges, and other pressure-containing parts under A995/A995M.

This is not an exhaustive list of every possible S32750 product form — it covers the seven most commonly ordered. Any form not listed here still warrants a direct check against the current ASTM catalog rather than defaulting to the nearest-sounding entry above. TiAlloy’s 2507 Standard Selector maps product form and unit system to the confirmed purchase-standard route for exactly this table.

For microstructure quality control specifically – sampling, acceptance criteria, and documentation for the duplex phase balance itself, separate from the product-form dimensional standard – ISO 17781:2017 is the relevant reference where a project invokes it, and it applies across all the product forms above rather than replacing any one of them.

When 2205 Is Enough — and When It Isn’t

Reflexively specifying the higher-alloy grade is not automatically the correct call – Super Duplex 2507 typically costs meaningfully more than Duplex 2205, and grade substitution should be justified by the actual service condition, not by “the strongest option available.” That same standards discipline applies here too: the Nickel Institute makes the same broader point about over-specifying corrosion-resistant grades — matching alloy to condition beats defaulting to the highest PREN on the shelf.

Should You Choose Super Duplex 2507 Over Duplex 2205?

Only if your service condition specifically demands it — high chloride concentration combined with elevated temperature, stagnant flow, or crevice-prone geometry (gaskets, threaded joints, deposit-forming service) is where 2205’s lower PREN margin runs out and 2507’s higher alloy content earns its cost premium. If your service is general oil-and-gas piping, moderate-chloride process fluid, or non-stagnant seawater at ambient-to-moderate temperature, 2205 frequently holds and 2507 is over-specification.

The honest answer requires naming your actual chloride level, temperature, and geometry — not the grade name alone.

For a full grade-selection deep dive on the 2205 side of this comparison, see our Duplex 2205 guide.

2507 vs Other Super Duplex Grades (Zeron 100 / S32760)

UNS S32750 (2507) is not the only super duplex grade on the market — Zeron 100, sold under UNS S32760, is a related but chemically distinct alloy with its own PREN>40 and NACE MR0175 listing, and the two are not automatically substitutable on a design that names one specifically. The same upgrade-only-when-justified logic from the 2205 comparison above applies to weighing 2507 against its super duplex peers, too. The specific difference is small but real: S32760 adds copper and tungsten to a composition otherwise close to S32750, additions suggested to improve corrosion resistance in certain conditions, and S32760 tends toward higher elongation while S32750 tends toward slightly higher tensile strength. Treat “super duplex” as a performance class covering several related but non-identical trade names, the same way “SAF 2507” and “UNS S32750” both point at the same alloy without being interchangeable purchase specifications — substitution between S32750 and S32760 needs the same design-authority approval as any other grade change.

Welding and Fabrication, Getting the Filler Metal Right

ER2209/E2209 — the filler metal commonly specified for Duplex 2205 — is the wrong choice for Super Duplex 2507, and the reason is a straightforward PREN mismatch, not a minor technicality. Per ASME SFA-5.9 Annex A8.55 as cited by a specialist welding-code reference, ER2209 has a nominal composition of 22.5Cr-8.5Ni-3Mo-0.15N and a PREN near 35, matched to 2205 base metal (UNS S31803/S32205). Super Duplex 2507 base metal runs PREN 40 and above; welding it with ER2209 creates a weld zone with lower corrosion resistance than the base metal on both sides of it — effectively a built-in anodic pit-initiation site in chloride or seawater service.

The correct filler class per SFA-5.9 Annex A8.58 is ER2594 (GTAW/GMAW) or its covered-electrode equivalent E2594, nominally 25.5Cr-9.2Ni-3.5Mo-0.25N with a PREN of 40 and above, matched specifically to S32750 and S32760 base metal. Both Lincoln Electric and Washington Alloy list ER2594 commercially for exactly this purpose. With the right filler and heat control, 2507 is generally described as having good weldability for a high-alloy grade — the practical challenge is procedure discipline (interpass temperature, shielding gas nitrogen content) rather than the base metal itself resisting fusion, and the payoff for getting it right is a weld zone that preserves the base metal’s toughness and corrosion resistance rather than degrading it.

Beyond filler selection, welded 2507 requires post-weld screening under ASTM A923, which covers Methods A, B, and C for detecting detrimental intermetallic phases that chemistry and tensile testing alone will not catch. The purchase order should state which method applies, the sample location, and whether the test is mandatory or informative for that specific project — no single method is universally required for every order.

Where It Fails: Real-World Limitations

Higher PREN does not guarantee immunity from corrosion failure, and two independent documented cases make that point more convincingly than any datasheet claim. Engineers discussing a documented case in an ASM International community thread report that an ASTM A790 Super Duplex 2507 pipe-and-flange assembly in real service failed after roughly two years in stagnant hot condensate at approximately 12% salinity and 57-75°C; alloy upgrade alone did not resolve the geometry- and stagnation-driven crevice risk in that specific application.

Separately, a peer-reviewed 2024 study of a 2507 circulation-pump impeller found the failure was driven by a combination of surface wear, cavitation, and chloride/fluoride ion corrosion working together, not a single “the alloy was not strong enough” cause.

One more finding cuts even closer to the point: PREN ranking does not always predict which alloy actually pits less in a given test. Under high-pressure, high-temperature sour-service conditions (2.55% H2S, 50% CO2), a peer-reviewed C-ring study found that Super Duplex 2507 (PREN>40) developed more pitting density than a 28Cr austenitic stainless steel with a lower PREN (below 40) — though the 2507’s pits were shallower and the austenitic grade’s fewer but deeper. Both materials passed sulfide stress cracking resistance in that same test; the divergence was specifically in pitting behavior. This is not a reason to distrust PREN as a screening tool generally — it is a reason to treat PREN as a starting-point ranking, not a guaranteed performance ordering, and to validate against the specific service chemistry (H2S/CO2 partial pressures, temperature, static vs. flowing conditions) rather than the alloy composition alone.

2507 is generally resistant to uniform corrosion across its rated service temperature range, and its high thermal conductivity relative to austenitic grades helps in heat-exchanger duty specifically — but neither property is unconditional. Above the 316° C (600°F) service temperature ceiling, embrittlement risk changes the calculus regardless of how favorable the room-temperature corrosion data looks.

The practical takeaway: PREN and mechanical minimums describe the alloy’s resistance to uniform and localized chemical attack under defined test conditions. They do not describe what happens when geometry traps stagnant fluid, when cavitation erodes the passive film mechanically, or when a weld is made with the wrong filler metal. Grade selection is one input to a service-life decision, not the whole decision.

RFQ Readiness, What to Send With Your Super Duplex 2507 Inquiry

A grade name and a rough size are not enough information for a supplier to quote accurately or for you to compare two quotes on equal terms. What actually determines price, delivery, and whether the material will pass inspection is the product form, the governing standard and edition, the unit system, the delivery condition, and the testing and documentation scope – not a single number.

One RFQ verification point worth calling out explicitly: a supplier’s marketing page referencing ISO 9001 or EN 9100 does not always mean the entity you are contracting with holds that certificate directly. TiAlloy’s own super-duplex page discloses that no demonstrable legal link ties the ISO 9001/EN 9100 certification on that page to the TiAlloy brand directly — those certificates are issued instead to a separate, named legal entity, Jiangsu Daxun Alloy Co., Ltd. Ask any supplier to name the certificate holder by legal entity, not just by brand, before that certification goes into your own documentation chain.

Once your service condition, product form, and testing scope are defined, TiAlloy’s 2507 RFQ Readiness Checker validates your inputs against these same fields, and TiAlloy’s Super Duplex 2507 order-ready supply program covers plate, pipe, tube, bar, and forged components against the fields listed above.

Frequently Asked Questions