Super duplex

The Pitting Resistance Equivalent Number is what separates a super duplex from a standard one. IMOA gives the relationship as:

PREN = Cr + 3.3(Mo + 0.5W) + 16N

Chromium, molybdenum, tungsten and nitrogen in weight per cent. Tungsten counts at half the weight of molybdenum because that is roughly how effective it is. The British Stainless Steel Association states the threshold plainly: grades with a PREN of 40 or more are called super austenitics or super duplex types, according to which family they belong to, and specifications such as NORSOK and NACE often set a minimum PREN of 40 as a requirement.

Put 2507 through it at the middle of its published band, 25% chromium, 4% molybdenum and 0.28% nitrogen, and you get 25 + 13.2 + 4.48, a PREN of 42.7. IMOA's own figure for the grade is 40–43. Standard 2205 comes out at 35–36 on the same basis.

The number sorts grades. It does not certify a part. IMOA is explicit that the relationship assumes ideally processed material, so a casting that met 40 on the ladle analysis and was then heat treated badly is not a super duplex in service. The caveats are set out in full on the family page.

Four wrought grades cover most of what gets sold as super duplex: 2507 (UNS S32750, EN 1.4410), S32760 (EN 1.4501), S32520 (EN 1.4507) and alloy 255 (S32550). None of those names belongs to a producer. The producer's name you will meet is Zeron 100, which IMOA lists as a product name for S32760. They sit close together on IMOA's composition table, and the differences that matter are tungsten and copper rather than chromium.

Two things in that table are worth saying out loud. The tungsten in S32760 is why the formula carries a tungsten term at all. And alloy 255, S32550, is listed at 38–41, so it straddles the line rather than clearing it. A specification that says "super duplex" and a specification that says "PREN 40 minimum" are not the same specification, and 255 is where the difference shows.

Above super duplex sit the hyper duplex grades, S32707 at a PREN of 49–50 and S33207 at 52–53. IMOA puts them against aggressive chloride service such as hot tropical seawater, particularly where crevices are unavoidable.

For a pump the wrought grades are only half the story, because a casing is a casting. The European casting grade in the same composition band is EN 1.4469, GX2CrNiMoN26-7-4, specified to EN 10213 and EN 10283 at 25–27% chromium, 6–8% nickel, 3–5% molybdenum and 0.12–0.22% nitrogen. A quotation naming a wrought grade for a cast part has not been checked properly.

The extra chromium, molybdenum and nitrogen buy chloride pitting and crevice resistance, and that is almost all they buy. IMOA puts the top of the duplex family at or above the 6% molybdenum austenitic grades the trade calls seawater stainless steels, and records the super duplex grades in critical seawater handling where strength and chloride resistance are both needed. It marks where that resistance stops as well. The super duplex grades do not corrode in lower-temperature seawater, but they have limits in higher-temperature service, which is where the hyper duplex grades take over.

It does not buy temperature. The 475°C embrittlement mechanism belongs to the ferrite phase, and super duplex has just as much ferrite as 2205. IMOA reports the German TÜV code holding 2507 in tube form to 250°C, the same figure it gives welded 2205, with ASME at 315°C across the grades listed. Anyone specifying super duplex to get a hotter duty out of it has misunderstood the material.

It also costs you something in the fabrication. The driving force for sigma phase rises with alloy content, and IMOA puts sigma formation in 2507 across 700–1000°C against 700–950°C in 2205, so the window for welding and annealing is tighter than it is on the standard grade. Add the price of the alloy and the lead time on a non-standard casting, and super duplex is a decision with a real cost attached rather than an upgrade you tick.

"Super duplex" gets used loosely, and on a pump the loose usage usually covers a build that is high-alloy without being super duplex. The Grundfos SE1 and SEV wastewater range is a clean example, and a good build rather than a bad one.

Grundfos offers that range in four material variants, and the pH the pump is rated for moves with each. The standard cast iron build takes 6.5–14. The R variant, a complete pump in EN 1.4408/1.4301, takes 1–14. The top variant, D, is given as a stainless steel pump to EN 1.4517/1.4539 and takes 0–14. Look at what those two numbers are. EN 1.4517 is GX2CrNiMoCuN25-6-3-3, a cast ferritic-austenitic grade to EN 10213 and EN 10283 with 24.5–26.5% chromium, 2.5–3.5% molybdenum, 2.75–3.5% copper and 0.12–0.22% nitrogen. EN 1.4539 is 904L, which the British Stainless Steel Association lists as an austenitic at a PREN of 32.2–39.9. One is a copper-bearing cast duplex, the other is not a duplex at all. At nominal chemistry EN 1.4517 works out around 38, and only a heat well into the top of its band clears 40, so the specified range does not guarantee the line. The SE range is a capable pump for aggressive effluent. It is not a super duplex pump, and a specification that assumed it was would be wrong about the chloride limit.

Where super duplex genuinely is on offer, it is normally a quoted option rather than a catalogue build. Roto Pumps will quote duplex, super duplex and higher alloys against the chemistry on a progressing cavity pump, which is the usual pattern for this end of the materials range.

So ask for the designation, not the family name. A UNS or EN number on the material certificate is the thing that means something, and where the specification names a PREN, the certificate has to carry a chemistry that meets it.

Most chloride duty in the UK is covered by standard 2205. Super duplex earns its place when the chloride concentration is higher, when the temperature is higher, or when the geometry gives you a crevice you cannot design out, and the honest headline case is seawater. That is why it turns up in marine and seawater service, on cooling, ballast and firewater duties, more than in general process work.

It is the wrong answer three ways. It will not survive a reducing acid, which is a nickel alloy duty and belongs with Hastelloy. It will not give you a hotter duty, for the reason above. And it is money wasted on any medium a thermoplastic or a lined casing would have held, which is a large share of what arrives described as corrosive. The order to work through is on corrosive duty.

The way to settle it is the chloride figure and the temperature, not the family name. Send the medium with its chloride content, the temperature it runs at and the worst case the process reaches, and say whether anything has already failed and how. Pitting on a wetted surface, cracking at a weld and general thinning point at three different answers, and only one of them ends in super duplex.