Duplex stainless steels have a structure of approximately equal amounts of ferrite and austenite and, therefore, may be referred to as ferritic-austenite stainless steels.
The chromium content varies from 18 to 33%. The nickel content of 4.5 to 8% is insufficient to develop a fully austenitic crystal structure. "Lean" duplex grades substitute some of the nickel with other elements such as manganese.
Most grades contain molybdenum in the range of 2.5 to 4% plus a small nitrogen addition which enhances both strength and pitting resistance.
Basic properties of the duplex stainless steels include:
- A mixed ferritic-austenitic, i.e. duplex, crystal structure that results in high resistance to stress corrosion cracking
- An increased level of passivity due to higher chromium (Cr), molybdenum (Mo) and nitrogen (N)
- Good weldability and formability, although heat control more important than with austenitics
- Higher tensile and yield strengths (compared with austenitic and ferritic stainless steels).
Common uses include applications such as heat exchanger panels and tubes, materials handling equipment, tanks and vessels where high chloride concentrations are present, e.g. sea water cooling, desalination, food pickling baths and aggressive mine waters.