Super duplex stainless steels combine very high strength with excellent resistance to pitting, crevice and stress corrosion cracking, which suits them to demanding applications across oil and gas, marine, chemical processing, water treatment, and pulp and paper. The term is typically applied to duplex stainless steels with a chromium content of 25% and a PREN value above 40.
The pitting resistance equivalent number (PREN) is a quick reference to the corrosion resistance of stainless steels and other corrosion resistant alloys (CRA), which are most likely to be affected by pitting and crevice corrosion. For Alloy 316L the PREN is only 25, and for duplex grades it is 34, so the uplift for super duplex stainless steels (SDSS) is significant.
As a very crude rule of thumb, Alloy 316L can be used successfully in rural, urban and estuary environments but is limited in more exposed coastal settings; 2205 duplex can be used comfortably in estuary, marine and offshore environments; but applications with long-term immersion may need a super duplex.
Beyond corrosion resistance
PREN and corrosion resistance are only part of the picture. Super duplex strength levels are twice those of regular duplex grades, and three to four times those of Alloy 316L. That brings more applications within scope and lets end users use less metal, saving cost, reducing component size and decreasing suspended weight.
All duplex grades resist stress corrosion cracking, again making them suitable for aggressive environments. Their higher strength contributes to this, but it is the duplex microstructure of austenite and ferrite grains that helps prevent cracks propagating through the alloy.
For an alloy family with this very favourable combination of physical and mechanical properties, the alternatives are generally more highly alloyed and much more expensive. Super duplex stainless steels benefit from a lower nickel content, reducing their cost and limiting their price volatility.
Applications
Since Langley Alloys patented the first in 1968, super duplex stainless steels are now widely accepted across a variety of applications:
- Oil and gas: downhole tooling, wellhead and subsea equipment, pumps and valves all make use of super duplex alloys. As a family, these alloys are included in NACE MR0175 / EN15156-3 as suitable for use in H2S-containing environments, i.e. sour service wells.
- Bolts and fasteners: a very common application, thanks to the very high starting strength and the ability to work harden to even higher strength levels.
- Pollution control scrubbers: a successful application in the fabrication of precipitators, fans and pumps. These grades strongly resist corrosion in such environments, where seawater is frequently used as a coolant and acids such as sulphuric acid form from the emissions of the burnt fuel.
- Marine: propellers, shafts, rudders and seals are frequently supplied in SDSS where grades such as S32205 and XM-19 are not deemed acceptable on the grounds of corrosion resistance or strength.
- Chemical processing: a large number of processes use sulphuric, nitric and phosphoric acids, such as producing PP, PVC, TiOx, dyes and agrochemicals. SDSS are generally resistant to reducing acids and offer good abrasion and wear resistance. Ferralium® 255 is widely specified in fertiliser production, as mixers, tanks, vessels, pumps and valves, as it strongly resists corrosion and the mechanical impact of phos rock.
- Vegetable processing: a less obvious application has been in equipment for processing grains and vegetables. The severe wear and erosion corrosion of sugar cane processing, mixers and centrifuges have been well served by Ferralium® 255.
- Water treatment: associated applications such as sewage treatment, desalination and swimming pools all use SDSS to resist corrosion from seawater and contaminated or brackish solutions.
- Paper and pulp: most components throughout the production process can make use of super duplex alloys. Duplex and lean duplex grades are typically specified where possible on grounds of cost, with SDSS used in the areas of greatest risk of failure.
- Pump shafts: a sweet spot for SDSS, exploiting the combination of high strength, wear and corrosion resistance.
Marine scrubbers are a recent application that cannot exploit the excellent properties of SDSS, and instead have to use more expensive super austenitic stainless steels. Legislation forces such vessels to employ their scrubber systems selectively in certain locations, and when the scrubber is not in use it sees temperatures beyond the sensible working range for SDSS.
What we stock
We stock a comprehensive range of super duplex stainless steels as solid bars and plate:
- Ferralium® 255 (UNS S32550, 1.4507): from ½" to 14" diameter, plus plate up to 3" thick.
- Alloy 32750 (SAF2507, 1.4410, UNS S32750): from ½" to 16" diameter, plus plate up to 3" thick.
- Alloy 32760 (1.4501, UNS S32760, Zeron 100®): from ½" to 16" diameter, plus plate up to 2 1/2" thick.
| Grade | Bar diameter | Plate thickness |
|---|---|---|
| Ferralium® 255 (UNS S32550, 1.4507) | ½" to 14" | up to 3" |
| Alloy 32750 (SAF2507, 1.4410) | ½" to 16" | up to 3" |
| Alloy 32760 (1.4501, Zeron 100) | ½" to 16" | up to 2½" |