Views: 1 Author: Monica Publish Time: 2026-10-23 Origin: Site
Table of Contents
Super Duplex S32750 — known commercially as 2507 — is a stainless steel engineered for some of the most corrosive environments in industry: seawater, chlorinated brines and aggressive oilfield fluids. Its performance is not an accident; it is built into its chemical composition. Every percentage point of chromium, molybdenum, nitrogen and nickel in the alloy has a specific job, and the specification limits that define the grade are tight enough that a batch of molten steel either qualifies as S32750 or it does not.
This article explains the composition of S32750 element by element: the specification limits, what each element contributes, how the chemistry produces the alloy's famous two-phase microstructure and PREN score, how S32750 differs from Duplex 2205 and S32760, and how buyers can verify the chemistry on a mill test certificate. It is a companion piece to our complete guide to Super Duplex S32750, which covers properties, welding and applications in depth.
S32750 is a super duplex stainless steel built on roughly 25% chromium, 7% nickel, 4% molybdenum and 0.3% nitrogen, with iron making up the balance — the full specification limits are listed element by element below.
The values below are the specification limits for UNS S32750 as published in ASTM A240/A240M, the product specification for stainless steel plate, sheet and strip; the numbers are reproduced in published industry references, and for contractual purposes the current edition of the standard itself governs. The nominal column shows one producer's typical aim chemistry (Alleima SAF 2507) — real heats report actual values that sit somewhere inside those limits.
Element | ASTM A240 limit (wt %) | Producer nominal (wt %) | Role in the alloy |
Carbon (C) | 0.030 max | ≤ 0.030 | Kept low to protect weldability and corrosion resistance |
Manganese (Mn) | 1.20 max | ≤ 1.2 | Deoxidizes the steel during melting |
Phosphorus (P) | 0.035 max | ≤ 0.035 | Impurity, held to a ceiling |
Sulfur (S) | 0.020 max | ≤ 0.015 | Impurity, held to a ceiling |
Silicon (Si) | 0.80 max | ≤ 0.8 | Deoxidizes the steel during melting |
Chromium (Cr) | 24.0 – 26.0 | 25 | Builds the passive film; backbone of corrosion resistance |
Nickel (Ni) | 6.0 – 8.0 | 7 | Stabilizes austenite; toughness and formability |
Molybdenum (Mo) | 3.0 – 5.0 | 4 | Pitting and crevice corrosion resistance in chlorides |
Nitrogen (N) | 0.24 – 0.32 | 0.3 | Strength, austenite stability and pitting resistance |
Copper (Cu) | 0.50 max | ≤ 0.5 | 0.50% maximum — not a required addition range |
Iron (Fe) | Balance | Balance | The matrix that holds everything together |
Three conventions in this table are worth understanding. "Max" means a ceiling: carbon at 0.030 max can be anything from zero to 0.030. A range means both ends are enforced: chromium must be at least 24.0 and at most 26.0 — a heat at 23.9 does not qualify. "Balance" means iron is everything left over, roughly two-thirds of the alloy.
The same S32750 chemistry defines the grade across the common product specifications — A240 plate, A790 pipe, A789 tube and A182 F53 forgings — so this table is the reference point whatever the product form; buyers should still confirm the current edition of each product standard in the contract.
Chromium and molybdenum build the alloy's protective film and its pitting resistance, nitrogen and nickel create and stabilize the austenite phase, and the remaining elements are either processing aids or impurities held to low ceilings.
A useful way to picture the alloy is as a team in which every player has a defined position. The four starters — chromium, nickel, molybdenum and nitrogen — do nearly all the work, and the International Molybdenum Association's duplex fabrication guidelines describe how these elements divide their responsibilities in the duplex family of grades.
• Chromium (24.0–26.0%). Chromium is the classic stainless steel ingredient. It reacts with oxygen to form an invisible, self-healing skin — the passive film — that separates the metal from the corrosive liquid around it. The 24–26% band is what separates super duplex from standard duplex: more chromium in solid solution means a tougher, more stable film.
• Molybdenum (3.0–5.0%). Molybdenum strengthens that film exactly where chlorides attack it. Pitting and crevice corrosion — the pinholes and hidden attacks that destroy stainless steel in seawater — are the problems molybdenum is best at solving, which is why molybdenum-bearing grades are the usual choice when chloride pitting and crevice attack drive material selection.
• Nitrogen (0.24–0.32%). Nitrogen is the signature element of super duplex. It strengthens the alloy, stabilizes the austenite phase, and contributes to pitting resistance with the highest weight of any element in the PREN formula — sixteen times its own percentage. In the S32750 specification used here, nitrogen must fall within a deliberate, narrow band of 0.24–0.32%.
• Nickel (6.0–8.0%). Nickel's job is balance: it stabilizes the austenite half of the dual structure and gives the alloy its toughness and formability. It is kept moderate — enough to do the job, not so much that the alloy becomes expensive or loses its duplex character.
• Manganese and silicon (1.20 and 0.80 max). These are processing aids: they remove oxygen from the steel during melting and are simply held below ceilings rather than targeted.
• Carbon, phosphorus and sulfur (0.030, 0.035 and 0.020 max). These are the controlled impurities. Carbon is kept low because excess carbon combines with chromium at weld temperatures and disarms the passive film exactly where a weld meets corrosive fluid. Phosphorus and sulfur are kept low to protect toughness.
• Copper (0.50 max). The S32750 specification sets a copper maximum of 0.50% — a tolerance ceiling, not a required addition range. By contrast, the S32760 specification requires copper at 0.50–1.00% and tungsten at 0.50–1.00%, which is the clearest chemical line between the two super duplex grades.
The chemistry is engineered so that, after solution annealing and rapid quenching, the metal contains roughly equal amounts of two different crystal structures — ferrite and austenite — but chemistry only makes that balance possible; heat treatment and fabrication are what actually set it.
"Duplex" means dual-phase, and it is the physical source of the grade's advantages. Ferrite and austenite are two different ways the same iron atoms can stack together: ferrite is chromium-rich, strong and magnetic; austenite is nickel- and nitrogen-rich, tough and non-magnetic. A metal that is roughly half of each inherits the best of both — the strength and chloride-stress-cracking resistance of ferrite, and the toughness and workability of austenite.
That is why a 50:50 target balance is the design intent: published guidance for duplex grades generally points to ferrite within about 35–60% in parent metal (and a wider 30–70% in welds) — though the acceptance criteria that actually govern are those in the relevant product or project specification.
The critical caveat for buyers: a compliant chemistry table does not by itself prove that balance. Slow cooling or a bad heat-treatment cycle can push the structure toward brittle intermetallic phases such as sigma, which consume chromium and molybdenum from the surrounding metal and quietly destroy corrosion resistance — which is why the industry tests for them, most commonly through the three methods of ASTM A923 (a metallographic etch screen, a Charpy impact test and a ferric chloride corrosion test). Chemistry is the starting condition; verified microstructure is the finished product.
PREN is calculated as %Cr + 3.3 × %Mo + 16 × %N, and with S32750's nominal 25/4/0.3 chemistry the score is about 43 — a ranking of pitting resistance in chloride solutions, not a guarantee of performance.
PREN — the Pitting Resistance Equivalent Number — is a scoring system, nothing more mysterious than a weighted sum of the elements that resist pitting. For duplex and austenitic grades the accepted formula is PREN = Cr + 3.3 × (Mo + 0.5 × W) + 16 × N, in weight percent. Each element is multiplied by a factor that reflects how much pitting resistance one unit of it buys: molybdenum is worth 3.3 times its weight, and nitrogen sixteen times its weight.
An illustrative calculation with the nominal chemistry makes this concrete. Taking 25% chromium, 4% molybdenum and 0.3% nitrogen: PREN = 25 + 3.3 × 4 + 16 × 0.3 = 25 + 13.2 + 4.8 = 43. Two cautions follow. Super duplex grades are conventionally described as those scoring around 40 or above — but conventions are not specifications, and the contract is what counts.
Equally important is what PREN cannot do. It is an empirical ranking, not a corrosion rate, not a service lifetime, and not a seawater guarantee. It does not know about temperature, crevices, flow velocity, fabrication quality or the specific chemistry of your process fluid — and it is only as good as the actual heat analysis behind the numbers used in the calculation.
Compared with Duplex 2205, S32750 raises chromium, molybdenum and nitrogen into the super duplex band; compared with S32760, it achieves high performance without deliberate tungsten and copper additions.
Element | Duplex 2205 (S32205) | Super Duplex S32750 | Super Duplex S32760 |
Chromium | 22.0 – 23.0 | 24.0 – 26.0 | 24.0 – 26.0 |
Nickel | 4.5 – 6.5 | 6.0 – 8.0 | 6.0 – 8.0 |
Molybdenum | 3.0 – 3.5 | 3.0 – 5.0 | 3.0 – 4.0 |
Nitrogen | 0.14 – 0.20 | 0.24 – 0.32 | 0.20 – 0.30 |
Tungsten | — | Not specified | 0.50 – 1.00 |
Copper | — | 0.50 max (tolerance ceiling) | 0.50 – 1.00 (required range) |
2205 is the workhorse standard duplex: strong, chloride-resistant and economical. S32750 takes the same architecture and raises the three pitting-resistance elements — chromium, molybdenum and nitrogen — which is the chemical signature that distinguishes super duplex from standard duplex grades. S32760 then takes a different route to a similar destination: it keeps the 25% chromium base but adds deliberate tungsten and copper, additions that further strengthen pitting resistance and improve performance in reducing acids such as dilute sulfuric.
For buyers, the practical conclusion is that more alloying is not automatically better — the choice among these grades is a match-to-service decision for the corrosion engineer and the buyer, in which environment, availability, cost and supplier qualification usually decide.
We cover these trade-offs in more detail in our Duplex 2205 vs Super Duplex 2507 comparison, the Duplex 2205 product pages, and the comparison sections of the S32750 guide.
Request the manufacturer's material test report or the inspection certificate required by the order, reporting the actual heat analysis traceable to a heat number — and remember that a handheld XRF analyzer cannot verify carbon, and nitrogen requires laboratory methods.
Each "heat" — one batch of molten steel — is sampled and analyzed at the ladle before casting; the resulting heat analysis is what appears on the mill test certificate; and every product made from that heat carries the heat number. Request the certificate type the order requires — where EN 10204 applies, specify it — and check that the certificate reports actual results for the elements required by the governing specification (iron is typically reported as balance), each inside its limit, with the heat number on the certificate matching the marking on the delivered material.
Handheld X-ray fluorescence — PMI's most common tool — reads most alloy families reliably and will confirm the chromium, nickel and molybdenum of S32750 in seconds, but it has known limitations with low-atomic-number elements: the cited guidance notes that carbon cannot be detected by XRF at all. Nitrogen, another light element, is likewise outside the scope of routine field XRF verification, so confirming nitrogen and carbon requires laboratory analysis of a sample or another method agreed in advance. A supplier who offers "PMI verification" of a super duplex grade without addressing nitrogen is offering to verify half the story.
Finally, keep chemistry in its place: it is necessary but not sufficient. A compliant heat analysis says nothing about the phase balance, the absence of detrimental phases, the mechanical properties or the weld quality of the delivered product — those are established by the heat-treatment condition, the tests in the product specification and, for duplex integrity, methods such as the ASTM A923 tests described earlier. Verify the chemistry, then verify the rest.
Put the chemistry requirements in the purchase order itself: the exact grade designation, the product specification and edition, the document type, heat-number traceability, and any PREN minimum the application genuinely requires.
A purchase order for S32750 products should state, at minimum:
• The grade designation explicitly: UNS S32750 (or EN 1.4410 / X2CrNiMoN25-7-4), not just the trade name "2507".
• The product specification matched to the form — ASTM A240 for plate, sheet and strip; A790 for pipe; A789 for tube; A276/A479 for bar; A182 Grade F53 for forgings — together with the edition.
• The certificate requirement: the material test report or inspection certificate the order is built on — where EN 10204 applies, specify 3.1 as standard or 3.2 where the project requires independent validation.
• Heat-number traceability from certificate to product marking on every item delivered.
• A PREN minimum only if the service justifies it — for seawater duties, engineers commonly require PREN ≥ 40, but the threshold and its basis should be agreed with the corrosion engineer — calculated from the actual heat analysis.
• Verification rights: agreement in advance on PMI scope, any laboratory nitrogen verification, and third-party inspection where the risk warrants it.
These six lines cost nothing to write and convert "the supplier seemed good" into "the material is proven." If you are evaluating S32750 for a chloride-rich application, ask candidate suppliers for a recent EN 10204 3.1 certificate for a comparable order and check it against this article — how a supplier responds to that request tells you as much as the certificate itself.
S32750 and 2507 are the same alloy family; copper is not a required addition in S32750; nitrogen is the grade's defining element; and compliant chemistry alone does not prove performance — the details follow.
Are S32750 and 2507 the same material?
Yes. "2507" is the commercial name — a 25-chromium, 7-nickel naming convention — while S32750 is the UNS designation, 1.4410 / X2CrNiMoN25-7-4 is the European designation, F53 is the ASTM forging grade, and 022Cr25Ni7Mo4N is the corresponding Chinese designation. They describe the same alloy family; contracts should name the UNS or EN designation explicitly so the specification limits are unambiguous.
Is copper deliberately added to S32750?
No. The cited S32750 specification sets a copper maximum of 0.50% — copper is tolerated up to that level, not required within it. By contrast, the S32760 specification requires copper at 0.50–1.00% and tungsten at 0.50–1.00%, which is the clearest chemical line between the two super duplex grades; in S32750, tungsten is simply not specified.
Why is nitrogen so important in S32750?
Nitrogen does three jobs at once: it strengthens the alloy, stabilizes the austenite phase, and carries the highest weight — sixteen times its percentage — in the PREN formula. The 0.24% minimum is the single clearest line between standard duplex and super duplex chemistry.
Does a compliant chemistry prove the material will perform?
No — it is the entry ticket, not the finish line. Compliant chemistry says the raw material was right; performance also requires a correct phase balance, freedom from detrimental phases, proper mechanical properties and sound fabrication. That is why the certificate should be read together with the product specification's tests, not instead of them.
Is EN 1.4410 exactly identical to S32750?
Very close, but not word-for-word identical: the EN 10088 ranges for 1.4410 differ slightly from the ASTM limits — for example, molybdenum is capped at 4.5% against ASTM's 5.0%, and nitrogen at 0.35% against ASTM's 0.32%, with small differences in sulfur and manganese allowances as well. Most heats satisfy both, but the governing system should be named in the purchase order so the certificate is checkable against one set of limits.
