Duplex stainless steels are stainless steels with a microstructure comprising typically 40-50% ferrite and the rest austenite. These steels combine important properties from both ferritic and austenitic stainless steels. They show good stress corrosion resistance and also good ductility and weldability. Most duplex stainless steels have a low carbon content.
Duplex grades with PRE-number (PRE+Pitting Resistance Equivalent = % Cr + 3.3% MO + 16% N) greater than 40 are called super duplex. These steels possess very good corrosion properties, especially in chloride containing environments.
The duplex structure gives high mechanical strength, approximately twice that of austenitics, combined with low thermal expansion, close to that of carbon steel. The low Ni-content is cost saving and high mechanical strenght means lighter constructions, which also gives cost advantages.
The upper service temperature of duplex stainless steels is around 300ºC due to the risk of embrittlement and formation of precipitates.
The weldability of duplex stainless steels is good. Welding of duplex stainless steels with proper welding parameters and matching filler metals gives good corrosion and mechanical properties.
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