The difference is microstructure. Austenitic stainless steels have a single austenitic structure, are non-magnetic, and are prized for formability and predictable corrosion resistance. Duplex stainless steels combine austenite and ferrite, which makes them roughly twice as strong and more resistant to stress corrosion cracking, at the cost of being slightly harder to fabricate.
Austenitic stainless steels
Thanks to their predictable corrosion resistance and excellent mechanical properties, austenitic stainless steels are the most commonly used grades. The family divides loosely into three groups:
- Common chromium-nickel (300 series)
- Manganese-chromium-nickel-nitrogen (200 series)
- Specialty alloys
Easily recognised by their non-magnetic properties, the fully austenitic grain structure makes them very formable, readily welded, and able to retain ductility down to cryogenic temperatures. They can be prone to sensitization of the weld heat-affected zone and to weld metal hot cracking, which is limited by choosing lower carbon grades, for example 316L instead of 316.
Cost-competitive and widely available, austenitic stainless steel is used across industrial and consumer applications, notably chemical plants, power plants, food processing and dairy equipment.
Duplex stainless steels
Duplex stainless steels combine austenite and ferrite microstructures. This gives them much higher strength and resistance to stress corrosion cracking, though they are slightly harder to fabricate and have lower toughness. They suit applications between -50degC and +250degC.
Their increased strength and corrosion resistance make them suitable for more aggressive environments, such as oil and gas, chemical process industries and marine applications.