Super duplex stainless steels are highly alloyed, high-performance duplex grades with a pitting resistance equivalent (PREN) greater than 40, based on Cr% + 3.3Mo% + 16N%. Their two-phase austenitic-ferritic microstructure combines high strength with excellent corrosion resistance.
Langley Alloys developed the very first super duplex steel in the late 1960s, promoted as Ferralium® 255, which has given more than 50 years of successful service in aggressive environments. Its popularity grew with the discovery of oil and gas in the UK North Sea, where high strength, corrosion resistance and competitive pricing were all equally critical.
With a high chromium content, super duplex steel resists acids, chlorides, caustic solutions and other aggressive media in the chemical, petrochemical, pulp and paper industries. It often replaces 300 series stainless steel, higher-nickel super austenitic steels and nickel-based alloys.
A cost-competitive composition
The composition exploits nitrogen as a lower-cost addition that improves strength without impairing toughness, while also enhancing corrosion resistance. This makes super duplex grades extremely cost competitive against alternatives that rely on more expensive additions such as nickel or molybdenum.
The two-phase microstructure also gives higher resistance to stress corrosion cracking than austenitic stainless steels. Fabrication and welding can be readily undertaken as long as qualified procedures are followed.
Common uses for super duplex stainless steels
Super duplex stainless steels are used where their increased strength and corrosion resistance can be exploited, including valves, pumps, mixers and agitators, tooling and fasteners. Typical operating environments include:
- Oil and gas
- Chemical processing, including fertiliser production
- Mining
- Water treatment and desalination
- Flue gas desulphurisation and gas scrubbers
- Marine and naval applications