{"id":1491,"date":"2026-06-30T08:29:00","date_gmt":"2026-06-30T02:59:00","guid":{"rendered":"https:\/\/www.xtd-ss.com\/blog\/?p=1491"},"modified":"2026-07-06T21:15:43","modified_gmt":"2026-07-06T15:45:43","slug":"super-duplex-s32750-vs-s32760-vs-s32550","status":"publish","type":"post","link":"https:\/\/www.xtd-ss.com\/blog\/super-duplex-s32750-vs-s32760-vs-s32550\/","title":{"rendered":"Super-Duplex S32750 vs S32760 vs S32550 Comparison"},"content":{"rendered":"
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Specification engineers turn to super-duplex when 316L cannot withstand the chloride load, selecting among S32750 vs S32760 and S32550 becomes considerably more difficult once the datasheets are placed side by side. All three grades claim a PREN above 40, yet tungsten in S32760 and copper in S32550 direct their real-world performance along different paths. A grade that appears superior on paper can still underperform in service if its alloying philosophy does not align with the corrosive environment it is specified for. This comparison examines chemistry, ASTM A790 mechanical properties, corrosion behavior, embrittlement risk, and a selection matrix grounded in actual service conditions rather than headline PREN figures.<\/p>\n\n\n\n

Why Three Super-Duplex Grades Exist<\/strong><\/h2>\n\n\n\n

Three super-duplex grades exist because metallurgists addressed the same corrosion challenge through different elements, and each approach carries its own cost and availability trade-off. Every grade targets a PREN of 40 or higher, though the route to that threshold follows distinct alloying strategies. <\/p>\n\n\n\n

S32750 established itself as the reference super-duplex grade, molybdenum-rich and the most widely stocked within the super-duplex family, a position that keeps lead times shorter and mill availability more consistent across most regions. S32760 follows a different approach, adding 0.5 to 1.0 % tungsten alongside 0.5 to 1.0 % copper to strengthen crevice and mild-sulfuric resistance beyond what molybdenum alone provides. S32550 predates both grades in design intent and relies on 1.5 to 2.5 % copper rather than tungsten. Certain specifications classify it as super duplex comparison grade rather than super-duplex, since its PREN can fall to 38 depending on heat chemistry, placing it below the threshold most engineers apply.<\/p>\n\n\n\n

Composition Compared<\/strong><\/h2>\n\n\n\n

Chemistry governs every mechanical and corrosion outcome that follows. The table below presents verified UNS specification ranges rather than typical or average figures. Molybdenum, nitrogen, copper, and tungsten affect duplex content, distinguishing these three grades far more than chromium or nickel content does, and modest shifts in any single element can move the calculated PREN enough to matter on a specification sheet.<\/p>\n\n\n\n

Property<\/strong><\/td>UNS S32750<\/strong><\/td>UNS S32760<\/strong><\/td>UNS S32550<\/strong><\/td><\/tr>
%Cr<\/td>24.0 \u2013 26.0<\/td>24.0 \u2013 26.0<\/td>24.0 \u2013 27.0<\/td><\/tr>
%Ni<\/td>6.0 \u2013 8.0<\/td>6.0 \u2013 8.0<\/td>4.5 \u2013 6.5<\/td><\/tr>
%Mo<\/td>3.0 \u2013 5.0 (ASTM A790)<\/td>3.0 \u2013 4.0<\/td>2.9 \u2013 3.9<\/td><\/tr>
%N<\/td>0.24 \u2013 0.32 (ASTM A790)<\/td>0.20 \u2013 0.30<\/td>0.10 \u2013 0.25<\/td><\/tr>
%Cu<\/td>0.50 max<\/td>0.50 \u2013 1.00<\/td>1.50 \u2013 2.50<\/td><\/tr>
%W<\/td>\u2014<\/td>0.50 \u2013 1.00<\/td>\u2014<\/td><\/tr>
PREN (typical, midpoint chemistry)<\/td>\u2248 42<\/td>\u2248 42 (using PREW)<\/td>\u2248 38 to 40<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

Note: EN 10088-3 narrows S32750 to Mo 3.0-4.5 % and widens N to 0.24-0.35 %. State whether ASTM or EN preference when citing exact ranges.<\/p>\n\n\n\n

Mechanical Properties per ASTM A790<\/strong><\/h2>\n\n\n\n

ASTM A790 establishes the mechanical base against which mills certify their material, and the minimums presented below apply regardless of which grade a project specifies. Yield strength remains closely aligned across all three grades, while elongation and hardness limits reveal where the underlying alloying differences become apparent in finished pipe.<\/p>\n\n\n\n

Property<\/strong><\/td>S32750<\/strong><\/td>S32760<\/strong><\/td>S32550<\/strong><\/td><\/tr>
Min 0.2% yield (MPa)<\/td>550<\/td>550<\/td>550<\/td><\/tr>
Min UTS (MPa)<\/td>800<\/td>750<\/td>approx. 760 (verify against the current A790 edition)<\/td><\/tr>
Min elongation (%)<\/td>15<\/td>25<\/td>15<\/td><\/tr>
Max hardness<\/td>293 HB \/ 31 HRC (per xtd-ss live A790 page)<\/td>32 HRC<\/td>32 HRC<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

All three grades deliver roughly double the yield strength of 316L, a margin that permits meaningful wall-thickness reduction on pressure-governed piping designs. That reduction translates directly into lighter spool assemblies and reduced freight costs on extended offshore tie-back installations, where every kilogram of installed weight carries a corresponding cost.<\/p>\n\n\n\n

Corrosion Performance<\/strong><\/h2>\n\n\n\n

Open-surface pitting resistance generally follows PREN values. On that basis, S32750 and S32760 perform at nearly the same level, while S32550 ranks slightly lower, a pattern confirmed through any standard PREN calculation guide<\/a>.\u00a0<\/p>\n\n\n\n

\"Chart<\/figure>\n\n\n\n

Crevice corrosion under stagnant chloride conditions presents a different picture. Tungsten in S32760 provides a measurable advantage, particularly within gasketed flange joints and heat exchanger tube-to-tubesheet crevices where chloride concentrates over time. The margin over S32750, however, remains modest rather than substantial, and should not be characterized as a significant performance gap without supporting field data. <\/p>\n\n\n\n

Chloride stress corrosion cracking presents little concern for any of the three grades in seawater service, where each substantially outperforms austenitic stainless grades such as 316L and 317L. Dilute sulfuric acid service reverses the ranking once again, favoring the copper content in S32550, with S32760 gaining a smaller benefit from its own copper addition, while S32750 falls behind in this particular service band because it contains no copper.<\/p>\n\n\n\n

Intermetallic Embrittlement: A Common Metallurgical Risk<\/strong><\/h2>\n\n\n\n

Every super-duplex grade shares one vulnerability. Precipitation occurs whenever the alloy remains within the 600 to 950 \u00b0C window during welding or heat treatment, a window that can be traversed gradually and unnoticed during multi-pass welds on heavy wall pipe. <\/p>\n\n\n\n

Once this precipitation forms, it compromises toughness and pitting resistance simultaneously, and a heat that fails the acceptance limit can pit more severely than 316L once placed into service, regardless of its nominal PREN. This risk is precisely why mill heats and fabrication weld coupons both require a documented ASTM A923<\/a> test, performed to Method A, B, or C, before material ships to the site. Solution annealing followed by a rapid water quench at the mill remains the primary control measure, though the certificate must reflect the actual heat treatment cycle and cooling rate rather than a checked compliance box.<\/p>\n\n\n\n

Standards, Approvals, and Selection<\/strong><\/h2>\n\n\n\n

ASTM A789 governs tube while A790 governs pipe for all three grades, so material form rarely narrows the selection on its own. NORSOK MDS D55 explicitly names S32750 and S32760, whereas S32550 appears less frequently on modern North Sea material data sheets and tends to surface mainly on older, still-active project specifications.<\/p>\n\n\n\n

Service<\/strong><\/td>Recommended UNS<\/strong><\/td><\/tr>
NORSOK North Sea projects<\/td>S32750 or S32760<\/td><\/tr>
Crevice-dominated heat exchangers<\/td>S32760<\/td><\/tr>
Dilute sulfuric acid service<\/td>S32550 or S32760<\/td><\/tr>
High weld-volume fabrication (cost-driven)<\/td>S32750<\/td><\/tr>
Default super-duplex where MDS is silent<\/td>S32750<\/td><\/tr>
Legacy specifications citing S32550<\/td>S32550 (verify against current project MDS)<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

NACE MR0175 and ISO 15156<\/a> accept all three grades in solution-annealed condition, each listed under its own alloy-specific row rather than a blanket duplex category, so sour service approval still requires verification grade by grade.\u00a0<\/p>\n\n\n\n

\"Super-duplex<\/figure>\n\n\n\n

Buyers sourcing S32750 seamless pipe<\/a> for North Sea service, S32760 seamless pipe<\/a> for crevice-heavy heat exchangers, or S32550 seamless pipe<\/a> for legacy sulfuric-acid duty should confirm the current project MDS before finalizing an order, since S32550 has fallen out of favor on newer specifications even where it remains technically acceptable.<\/p>\n\n\n\n

Conclusion<\/strong><\/h2>\n\n\n\n

S32750 remains the default super-duplex grade, and its stocking volume makes it a practical choice whenever a project MDS remains silent on grade selection. S32760 earns its place in crevice-prone and mildly acidic service, where tungsten and copper both contribute measurable gains. S32550 continues to serve copper-favored acid duties and legacy specifications that cite it by name. Zhejiang Xintongda Special Steel<\/a> manufactures all three grades at its mill in Zhejiang, China. We provide super-duplex products supported by A923 embrittlement test results and EN 10204 3.1 or 3.2 mill certificates.<\/p>\n\n\n\n