Alloys: Duplex stainless steel
Given that stainless steels are loosely classified as those steels which have over ten percent chromium (and thereby increased corrosion resistance), there are hundreds of varieties of stainless steels, all with varying compositions and properties. Duplex stainless steel is one such variety, named for the fact that their microstructure has a mixture of austentic and ferritic grains – a “duplex” structure.
Grades of stainless steel are often classified into five families, based upon their microstructures: ferritic, martensitic, austenitic, duplex, and precipitation hardenable. The transition from ferritic stainless steel to austenitic stainless steel can often be achieved by adding nickel to the composition, so duplex stainless steel is a steel which has more nickel than ferritic stainless steels but less nickel than is required for a fully austenitic stainless steel.
As the structure of a material has a huge influence on its properties, this dual microstructure helps duplex stainless steels differ from the other forms. Both ferritic and duplex stainless steels are magnetic, while austenitic stainless steels are not. Many of their other properties are somewhat between those properties of austenitic and ferritic stanless steels.
Duplex stainless steels are also relatively rare. Most stainless steel produced is austentic, about 25% is ferritic, and only a small fraction of stainless steels produced are either duplex, martensitic, or precipitation hardenable. Duplex stainless steels have the tendency to form brittle intermetallic phases, if heated incorrectly, which makes their production difficult. However they are also more resistant to corrosion than purely austenitic stainless steels and have higher strength than ferritic stainless steels, so their applications tend to take advantage of this combination of properties.