Duplex vs Super Duplex Stainless Steel: Composition, PREN, Strength & When to Use Each
Duplex and super duplex stainless steels are both two-phase alloys, built from roughly 50/50 austenite and ferrite. What separates them is chemistry: super duplex carries more chromium, molybdenum and nitrogen. That extra alloying pushes its pitting resistance equivalent number (PREN) to 40 or above, compared with roughly 34–38 for standard duplex. The payoff is higher strength and better resistance to chloride pitting. The cost is a higher price tag and a narrower fabrication window.
"Duplex" here means the metallurgical austenitic-ferritic stainless steel family. It doesn't mean a duplex apartment, duplex communication mode, a duplex nail, or a duplex pump or filter. This page compares metallurgical duplex against super duplex stainless for fluid-system components, then covers the instrumentation forms Crestflo supplies in both grades.
Duplex vs Super Duplex: The Short Answer
Both families count as duplex stainless steels: a balanced microstructure of austenite and ferrite that pairs ferrite's strength with the toughness and corrosion resistance austenite brings. Super duplex is really just duplex pushed further. Add more chromium, molybdenum and nitrogen, and PREN climbs past the 40 mark that defines the "super" tier. Standard duplex 2205 lands around PREN 34-38. Super duplex 2507 sits at 40 or above. Flip the question around and ask super duplex vs duplex, and you land in the same place: the microstructure is shared, and alloy content plus PREN is where they part ways. The table below gets straight to the decision.
| Property | Duplex (2205) | Super Duplex (2507) |
|---|---|---|
| Chromium (Cr) | ~22–23% | ~24–26% |
| Molybdenum + nitrogen | ~3.0–3.5% Mo · ~0.14–0.20% N | ~3.0–5.0% Mo · ~0.24–0.32% N |
| PREN (=Cr+3.3Mo+16N) | ~34–38 | ≥ 40 (typ. 40–43) |
| Min 0.2% yield strength | ~65 ksi (450 MPa) | ~80 ksi (550 MPa) |
| Chloride-pitting resistance | High | Higher (greater CPT/CCT) |
| Relative cost | Lower | Higher |
These figures are standard published ranges for the exemplar grades, per ASTM A789/A790/A182/A479/A240 grade limits. They're meant for engineering comparison only, not as a certified test report. Certified, grade-specific values come with the material test certificate on each order.
What Counts as Duplex vs Super Duplex: Grades, UNS Numbers & Designations
Duplex stainless steels go by plenty of different names and codes. A spec that lists 1.4462, S32205 or F51 is talking about the same standard duplex grade, the one most people just call 2205. Do the same with 2507, S32750, 1.4410 or F53, and you're looking at the super duplex family. Where's the line drawn? PREN. Below roughly 40, it's standard, or lean, duplex; hit 40 or cross it, and you're in super duplex territory.
| Class | Example grade | UNS | Werkstoff | A182 forging | PREN band |
|---|---|---|---|---|---|
| Lean duplex | 2304 | S32304 | 1.4362 | — | ~24–26 |
| Standard duplex | 2205 | S32205 / S31803 | 1.4462 | F51 | ~34–38 |
| Super duplex | 2507 | S32750 | 1.4410 | F53 | ≥ 40 |
| Super duplex | Zeron 100 | S32760 | 1.4501 | F55 | ≥ 40 |
| Super duplex | Ferralium 255 | S32550 | — | — | ~38–41 |
| Hyper duplex | 2707 | S32707 | — | — | > 45 |
2205 (UNS S32205 / S31803) is standard duplex, not super duplex, because its PREN comes in under 40. UNS S32750 (2507) crosses that line and counts as super duplex once PREN hits 40 or higher. Trade names common to the super duplex group include Zeron 100, SAF 2507 and Ferralium 255. Rolled Alloys holds the trademark for Zeron 100, Alleima (formerly Sandvik) for SAF 2507, and Langley Alloys for Ferralium. Crestflo isn't tied to any of these companies; the names appear here only to identify the grades.
Composition Compared: Chromium, Molybdenum, Nickel & Nitrogen
Three elements set the two families apart on corrosion resistance: chromium, molybdenum and nitrogen. Chromium and molybdenum build the passive film that keeps pitting at bay. Nitrogen does double duty. It stabilises the austenite phase and adds a real boost to PREN, while also lifting strength. Super duplex simply carries more of each.
| Element | Duplex 2205 | Super Duplex 2507 |
|---|---|---|
| Chromium (Cr) | ~22–23% | ~24–26% |
| Molybdenum (Mo) | ~3.0–3.5% | ~3.0–5.0% |
| Nickel (Ni) | ~4.5–6.5% | ~6.0–8.0% |
| Nitrogen (N) | ~0.14–0.20% | ~0.24–0.32% |
| Iron (Fe) | Balance | Balance |
Ranges reflect standard published composition limits for the applicable ASTM grade; exact per-heat chemistry shows up on the material test certificate. For the complete spec sheet on each grade, check Crestflo's Duplex 2205 material page and the Super Duplex 2507 material page. Want the two side by side? See 2205 vs 2507.
PREN: The Pitting-Resistance Dividing Line
PREN, the pitting resistance equivalent number, comes from a simple formula: PREN = %Cr + 3.3 × %Mo + 16 × %N. It ranks how well an alloy holds up against chloride pitting. Standard duplex 2205 typically lands around 34–38. Super duplex 2507 pushes past 40, and that's exactly what earns it the "super" label. Don't mistake PREN for an absolute corrosion rating or a guaranteed service limit, though. It's strictly a comparative index. Real pitting behavior still hinges on temperature, chloride concentration, pH, and surface condition. Crestflo's full PREN comparison table, covering the entire alloy range, sits right here: PREN comparison table.
Strength & Hardness: Duplex vs Super Duplex Mechanical Properties
Both duplex families beat austenitic 304 and 316 by a wide margin, roughly double the yield strength. That's down to the ferrite phase and nitrogen alloying, which push the proof stress higher. Super duplex goes further still, outpacing standard duplex. And that extra yield buys something practical: thinner tube walls, or higher pressure ratings if the wall stays the same.
| Property | Duplex 2205 | Super Duplex 2507 |
|---|---|---|
| Min 0.2% yield strength | ~65 ksi (450 MPa) | ~80 ksi (550 MPa) |
| Min tensile strength | ~95 ksi (655 MPa) | ~116 ksi (795 MPa) |
| Typical max hardness | ≤ ~31 HRC | ≤ ~32 HRC |
| Elongation (min) | ~25% | ~15% |
These values are the standard published minimums set out in the applicable ASTM product specification. For the austenitic contrast, see the duplex/super duplex vs 316 comparison below.
Corrosion Resistance: Pitting, Crevice, SCC & Sour Service
Super duplex holds up against chloride pitting and crevice attack at a higher critical pitting temperature (CPT) and critical crevice temperature (CCT) than standard duplex does. That tracks with its higher PREN. Both duplex families beat austenitic 304 and 316 by a wide margin when it comes to chloride stress-corrosion cracking (SCC) in warm chloride environments. It's the main reason specifiers turn to duplex in the first place.
Does duplex 2205 rust? Not easily. It resists corrosion far better than 304/316 do in chlorides. But resistant isn't the same as immune: push it above its CPT and it can pit. Drop it into a strongly reducing acid and it'll corrode too. That's why PREN-based selection matters so much: matching the grade to the actual chloride level and temperature, not just picking the alloy that sounds toughest.
For sour (H2S) service, both grades come in options that meet NACE MR0175 / ISO 15156. Check the sour-service (NACE MR0175) selection guide, along with the standard references for NACE MR0175 and ISO 15156. Need corrosion behavior for specific media beyond seawater chlorides? The corrosion-by-media alloy matrix covers that, and Crestflo keeps it current.
Machinability, Weldability & Temperature Limits
Super duplex work-hardens faster than standard duplex, and it's tougher to machine and weld. Welders need tighter control over heat input and the ferrite-to-austenite balance to keep corrosion resistance and toughness intact. Standard duplex, by contrast, gives you a wider, more forgiving window for fabrication.
Both families run into the same ceiling on temperature. Keep them in the roughly 300–1000 °C range for any length of time and you risk 475 °C embrittlement along with sigma-phase precipitation. That rules out sustained high-heat service, the kind nickel alloys handle without trouble. Duplex is built for chloride and pressure, not heat. Crestflo machines both grades in-house on CNC, VMC and EDM equipment, and that in-house capability counts for the most when you're cutting hard-to-machine super duplex parts.
Cost & Lifecycle
Super duplex carries a higher price tag than standard duplex, and the reason comes down to chemistry: it packs more chromium, molybdenum, nickel and nitrogen. That premium often pays for itself in aggressive chloride or seawater service. There, it avoids two costly missteps: over-specifying nickel alloys, and replacing 316 after an early failure. Milder duty tells a different story. Standard duplex isn't far behind here; it delivers most of the same benefit at a lower purchase price. That's why the choice should track the service conditions, not a blanket rule of "always buy the higher grade."
Where Each Is Used: Duplex & Super Duplex Applications
Standard duplex handles chemical processing, general marine work, pulp and paper, and standard oil-and-gas pipework. Super duplex is the step up for the harshest chloride and seawater conditions: subsea systems, offshore platforms, seawater desalination, and flue-gas service.
| Service / environment | Recommended family | Industry page |
|---|---|---|
| General chemical processing | Standard duplex | Petrochemical & chemical |
| Onshore oil & gas pipework | Standard duplex | Oil & gas |
| General marine / shipboard | Standard duplex | Marine & shipbuilding |
| Offshore platform / topsides | Super duplex | Offshore |
| Subsea & seawater systems | Super duplex | Subsea |
For the full breakdown of chloride-driven alloy choices, see Crestflo's offshore chloride selection guide.
Duplex / Super Duplex vs 316 Stainless Steel
Both duplex families outperform 316/316L by a wide margin: roughly double the yield strength, and a PREN high enough to resist chloride pitting and stress-corrosion cracking far more effectively. That's why they replace 316 in seawater and warm-chloride service. Where chlorides are mild and pressure stays modest, though, 316 remains cheaper and easier to fabricate, so it's still the default choice. For an austenitic upgrade rather than a full jump to duplex, see 904L vs 316 and the 316/316L material page. The super-austenitic and super duplex comparison is covered in 254 SMO vs super duplex vs AL-6XN.
When to Choose Duplex vs Super Duplex: A Decision Guide
It all hinges on how severe the service is. Run through this checklist:
| Decision driver | Points to duplex 2205 | Points to super duplex 2507 |
|---|---|---|
| Chloride level & temperature | Low to moderate | High / seawater / warm chloride |
| Pressure or weight target | Standard | High pressure or thin-wall weight saving |
| Sour (H2S) service severity | Milder sour duty | Severe sour duty (NACE-compliant options) |
| Budget vs lifecycle | Lower first cost | Longer life in aggressive service justifies premium |
As a rule of thumb, match the PREN you need to how tough the service actually is. When chloride levels and temperature climb, or when you need that extra strength-to-weight edge, reach for super duplex. Standard duplex holds up fine for milder chemical, marine, and oil-and-gas pipework. Crestflo lays out a broader, service-based method in its guide on material selection by service.
Get Duplex & Super Duplex Components from Crestflo
Crestflo supplies both grades, Duplex 2205 and Super Duplex 2507 (Zeron 100), as instrumentation product lines: tube and compression fittings, tubing, manifolds, and instrumentation valves. Compression tube fittings are performance-qualified to ASTM F1387 as drop-in interchange fittings. Instrumentation valves are held to a different set of standards: the applicable valve codes, ASME B16.34 and the relevant API standards, with NACE MR0175 / ISO 15156 options where sour service applies. Traceability isn't an afterthought either. Every part carries a heat number and an EN 10204 3.1 material test certificate, 3.2 on request, covering imperial and metric sizes up to 2" (50 mm) OD. Crestflo is certified to ISO 9001 / 14001 / 45001, and to PED as well.
Crestflo is the instrumentation division of an export house that's been in business for four decades and carries recognition from the Government of India. The company builds US drop-in replacements, keeping ready stock on standard items and making specials to order, with delivery running 6–8 weeks. Standard product comes from Indian origin. US or European melt is available on request, complete with full material traceability. See US-origin compliance and material traceability for details.
Each grade has its own instrumentation forms, and they're worth browsing one by one: Super Duplex 2507 tube fittings, Super Duplex 2507 tubing, Super Duplex 2507 valves, Duplex 2205 tube fittings and Duplex 2205 tubing. Standards govern each product form. Tube falls under ASTM A789 and A790; forgings under A182; bar under A479.
To source components in either grade, request a quote. Send along your requirement, and Crestflo's engineering team will get back to you with grades, forms and lead time.
Frequently Asked Questions: Duplex vs Super Duplex
What is the difference between duplex and super duplex stainless steel?
Both are roughly 50/50 austenite-and-ferrite stainless steels, but super duplex pushes further on chromium, molybdenum and nitrogen. That gets you a PREN of 40 or better, versus roughly 34–38 for standard duplex. Strength climbs too, and so does resistance to chloride pitting. The trade-off: you pay more for it, and both machining and welding leave a lot less room for error.
Is 2205 duplex or super duplex?
2205 (UNS S32205 / S31803) counts as standard duplex, not super duplex. Its PREN falls somewhere between 34 and 38, short of the 40 mark that separates it from the super duplex class.
Is UNS S32750 duplex or super duplex?
UNS S32750 (2507) counts as a super duplex grade. Its PREN comes in at 40 or higher, and that's exactly what earns it a spot in the super duplex band.
Does duplex 2205 rust?
2205 resists corrosion far better than 304 or 316 in chloride environments, and that's well established. But resistant isn't the same as immune. Push it above its critical pitting temperature and it can pit. Drop it into a strongly reducing acid and it can corrode too. That's exactly why PREN-based selection matters: it matches the grade to the chloride level and temperature the material will actually face in service.
Is super duplex better than 316 stainless steel?
For chloride exposure, seawater, and higher-pressure service, yes: super duplex carries roughly twice the yield strength of 316, plus a far higher PREN. That combination makes it far more resistant to pitting and stress-corrosion cracking. In milder environments, though, 316 wins on cost and ease of fabrication. It's the sensible default there.
What is PREN and how do the two families compare?
PREN, the pitting resistance equivalent number, comes from the formula %Cr + 3.3 × %Mo + 16 × %N. Standard duplex grades typically run about 34–38, while super duplex hits 40 or higher. The number ranks pitting resistance, nothing more. It isn't an absolute corrosion rating, nor is it a guaranteed service limit.
Which is harder to machine and weld?
Super duplex is different. It work-hardens faster and demands tighter control over heat input and the ferrite-to-austenite balance during welding to hold onto its corrosion resistance and toughness. Standard duplex, by contrast, gives fabricators a wider, more forgiving window to work in.
Is super duplex more expensive than duplex?
Yes, thanks to its higher chromium, molybdenum, nickel and nitrogen content. The trade-off shows up as longer service life in aggressive chloride environments, and that can bring down total lifecycle cost even with the higher purchase price.
When should I choose super duplex over duplex?
Pick super duplex when chloride concentration and temperature run high. Subsea, seawater, desalination and offshore duty all fall into that category. It's also worth the upgrade whenever a higher strength-to-weight ratio is needed. Standard duplex holds up fine for milder chemical exposure, marine environments and onshore oil-and-gas pipework.
Can Crestflo supply fittings, tubing and valves in both grades?
Yes. Duplex 2205 and Super Duplex 2507 (Zeron 100) come in every in-scope instrumentation form we carry, tube and compression fittings, tubing, manifolds and valves. NACE MR0175 options are available, along with EN 10204 3.1 material test certificates (3.2 on request) and full heat-number traceability. Both imperial and metric sizes are stocked.
What ASTM specifications cover these grades?
Product forms track ASTM A789 and A790 for tube, A182 (F51/F53 grades) for forgings, and A479 for bar. Plate falls under A240. Values follow whichever standard applies, and we can adjust them to fit your requirements.
What is the lead time?
Crestflo supplies duplex and super duplex instrumentation components. Standard items sit ready in stock, so those ship quickly. Specials aren't stocked; they're made to order, with delivery typically running six to eight weeks.