Types of Stainless Steel

What are the types of stainless steel?

Stainless Steels are produced in different grades according to their chemical distribution and content.

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Austenitic Stainless Steels (3XX)

Austenitic stainless steels are non-magnetic. When a sufficient amount of nickel is added to stainless steel, the crystal structure turns into “ostenite”. The basic composition of austenitic stainless steels is 18% chromium and 8% nickel. This increases corrosion resistance and transforms the structure from ferritic to austenitic. Austenitic grades are the most commonly used stainless steels, accounting for more than 70% of production (type 304 is by far the most commonly cited grade). They can not be hardened by heat treatment.

Super austenitic grades have improved pitting and crack corrosion resistance compared to ordinary austenitic or duplex types. This is because additional chromium, molybdenum, and nitrogen are added to these grades (316L)

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Ferritic Stainless Steels (4XX)

Ferritic stainless steels are plain chromium stainless steels with a chromium content and low carbon content ranging from 10.5 to 18%. They are magnetic and cannot be hardened by heat treatment. Ferritic alloys have good ductility and formability, but have relatively low high temperature resistance compared to austenitic grades.

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Martensitic Stainless Steels

Martensitic stainless steels are the first stainless steels developed commercially (as cutlery) and to have a relatively high carbon content (0.1 - 1.2%) compared to other stainless steels. They are plain chromium steels containing between 12% and 18% chromium. Like plain carbon steels, they are magnetic and can be hardened by quenching and annealing, and find their main applications in cutlery, aerospace and general engineering.

A relatively new group of martensitic stainless steels are superartensitic stainless steels. Superartensitic grades combine high strength and low temperature toughness with acceptable corrosion resistance in many applications. (Please also see the section on superartensitics in the “Other Alloys” Fact Sheet.)

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Duplex Stainless Steels

These are stainless steels with relatively high levels of chromium (from 18 to 28%) and a moderate amount of nickel (from 4.5 to 8%). The combination of high corrosion resistance and excellent mechanical properties of duplex stainless steels can be explained by their chemical compositions and the balanced ('duplex') microstructure of ferrite and austenitic in approximately equivalent volume.

Super duplex grades have improved pitting and crack corrosion resistance compared to ordinary austenitic or duplex species. This is due to the addition of chromium, molybdenum and nitrogen to these grades.

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