The term super duplex is typically applied to duplex stainless steels with a chromium content of 25% and a PREN value >40. 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 impacted 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 comfortably used in estuary, marine and offshore environments; but for applications with long-term immersion then a super duplex may be necessary.
However, when discussing PREN and corrosion resistance, we are overlooking the other significant benefits of super duplex stainless steels. Firstly, the strength levels are twice those of regular duplex grades, and three to four times those of Alloy 316L. This means that not only can more applications fall within the scope of these alloys, but also end users can exploit this virtue to use less metal, saving cost, reducing component size and decreasing the suspended weight.
All duplex grades resist the effect of stress corrosion cracking, again making them suitable for application in a number of aggressive environments. Their higher strength will contribute to this resistance, but the duplex microstructure of austenite and ferrite grains helps to prevent the propagation of cracks through the alloy.
Finally, for an alloy family with this very favourable combination of physical and mechanical properties, alternative metals are generally more highly-alloyed and significantly more expensive. Super duplex stainless steels benefit from a lower nickel content, reducing their cost and limiting their price volatility.
Since their invention in 1967 by Langley Alloys, super duplex stainless steels are now widely accepted in a variety of applications:
a) Oil & 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 MR1075 / EN15156-3 as being suitable for use in H2S-containing environments i.e. sour service wells.
b) bolts and fasteners – are a very common application of SDSS, due to their very high starting strength and the possibility to work harden them to even higher strength levels.
c) pollution control scrubbers – this has been a successful application for SDSS in the fabrication of precipitators, fans and pumps. These grades strongly resist corrosion in such environments; seawater is frequently used as a coolant, and acids such as sulphuric acid are formed from the emissions of the burnt fuel.
The recent application of marine scrubbers are unable to exploit the excellent properties of SDSS, and are having to use more expensive super austenitic stainless steels instead. Legislation is forcing such vessels to selectively employ their scrubber systems in certain locations. When not in use, the scrubber will see temperatures beyond the sensible working range for SDSS.
d) marine applications – propellers, shafts, rudders and seals are frequently supplied in SDSS when grades such as S32205 and XM-19 are not deemed acceptable on the grounds of either corrosion resistance or strength.
e) chemical process industry – a large number of processes will utilise sulphuric acid, nitric acid and phosphoric acid, such as in the production of PP, PVC, TiOx, dyes and agrochemicals. Fortunately, SDSS are generally resistant to reducing acids, as well as offering good abrasion and wear resistance.
Ferralium 255 is widely specified in the production of fertilisers, as mixers, tanks and vessels, pumps and valves, as it strongly resists corrosion and the mechanical impact of ‘phos rock’.
f) vegetable processing – a less obvious application has been in the construction of equipment for processing grains and vegetables, The severe wear and corrosion (‘erosion corrosion’) conditions involved in sugar cane processing, mixers and centrifuges, have been well-served in Ferralium 255.
g) water treatment – associated applications such as sewage treatment, desalination and swimming pools all use SDSS to resist the threat of corrosion from seawater, contaminated or brackish solutions.
h) paper and pulp – most components throughout the production processes of pulp and paper can make use of super duplex alloys. Duplex and lean duplex grades are typically specified where possible on grounds of cost, but SDSS will be used in areas of greatest risk of failure.
i) pump shafts – this is a ‘sweet spot’ for SDSS, exploiting the combination of high strength, wear and corrosion resistance.
Langley Alloys stocks a comprehensive range of super duplex stainless steels as solid bars and also plate.
Ferralium 255 (UNS S32550, 1.4507) – from ½” to 14” diameter, plus plate up to 3“ thickness.
Alloy 32750 (SAF2507, 1.4410, UNS S32760) – from ½” to 16” diameter, plus plate up to 3“ thickness.
Alloy 32760 (1.4501, UNS S32760, Zeron 100) – from ½” to 16” diameter, plus plate up to 2 1/2“ thickness.