superduplex

Superduplex, or super duplex stainless steel, refers to a subclass of duplex stainless steels with higher alloy content to achieve superior corrosion resistance and strength. Its microstructure is a balanced mixture of ferrite and austenite, optimized by elevated levels of chromium, nickel, molybdenum, and nitrogen. Typical compositions for superduplex alloys include chromium around 25–27%, nickel about 4–8%, molybdenum 2–5%, and nitrogen roughly 0.20–0.30%. Common grades are UNS S32750 and UNS S32760.

Properties of superduplex steels include high strength and excellent toughness, with yield strengths commonly in the 450–650 MPa range and ultimate tensile strengths near 800–1000 MPa. They offer outstanding resistance to pitting and crevice corrosion in chloride environments and improved resistance to stress corrosion cracking compared with standard duplex and austenitic stainless steels. Corrosion resistance is often described by high PREN values, typically in the 40–60 range for these alloys.

Fabrication and welding require careful control of heat input to preserve the balanced ferrite–austenite microstructure. They can be welded with common processes such as TIG, MIG, or SAW using appropriate duplex or super duplex filler metals. Post-weld heat treatment is not generally required for many applications, though some large or critical components may be solution-annealed and quenched to restore microstructure after welding.

Applications include offshore oil and gas, chemical and petrochemical processing, desalination, marine components, heat exchangers, pumps, and valves—any service that combines high corrosion resistance with strong mechanical performance in aggressive chloride environments.

In summary, superduplex alloys extend the corrosion resistance and strength available from standard duplex grades, making them suitable for demanding, corrosive-service applications.