There are four main families, which are primarily classified by their crystalline structure: austenitic, ferritic, martensitic, and duplex.
Austenitic stainless steel
Austenitic stainless steel is the largest family of stainless steels, making up about two-thirds of all stainless steel production (see production figures below).
They possess an austenitic microstructure, which is a face-centered cubic crystal structure. This microstructure is achieved by alloying steel with sufficient nickel
and/or manganese and nitrogen to maintain an austenitic microstructure at all temperatures, ranging from the cryogenic region to the melting point.
Thus, austenitic stainless steels are not hardenable by heat treatment since they possess the same microstructure at all temperatures.
Ferritic stainless steels
Ferritic stainless steels possess a ferrite microstructure like carbon steel, which is a body-centered cubic crystal structure,
and contain between 10.5% and 27% chromium with very little or no nickel. This microstructure is present at all temperatures due to the chromium addition,
so they are not hardenable by heat treatment. They cannot be strengthened by cold work to the same degree as austenitic stainless steels.
Martensitic stainless steels
Martensitic stainless steels offer a wide range of properties and are used as stainless engineering steels, stainless tool steels, and creep-resistant steels.
They are magnetic, and not as corrosion-resistant as ferritic and austenitic stainless steels due to their low chromium content.
Duplex stainless steel
Duplex stainless steels have a mixed microstructure of austenite and ferrite, the ideal ratio being a 50:50 mix, though commercial alloys may have ratios of 40:60.
They are characterized by higher chromium (19–32%) and molybdenum (up to 5%) and lower nickel contents than austenitic stainless steels.
Duplex stainless steels have roughly twice the yield strength of austenitic stainless steel. Their mixed microstructure provides improved resistance to chloride stress
corrosion cracking in comparison to austenitic stainless steel Types 304 and 316.