There are several families of stainless steel: FERRITIC, MARTENSITIC, AUSTENITIC and DUPLEX. These names are derived from the crystal structure of the steels, which governs their metallurgical behaviour.

FERRITIC stainless steels are magnetic, have a low carbon content and contain chromium as the main alloying element, typically between 13% and 17%.They are not hardenable by heat treatment.

MARTENSITIC stainless steels are magnetic, containing typically 12% chromium with a higher carbon content than the ferritic types. They are hardenable by quenching and tempering like plain carbon steels and find their main application in cutlery, aerospace and general engineering.

AUSTENITIC stainless steels are non-magnetic and, in addition to chromium typically around 18%, contain nickel. This enhances their corrosion resistance and modifies the structure from ferritic to austenitic. They are the most widely used group of stainless steels. They are not hardenable by heat treatment.

DUPLEX stainless steels are used where combinations of higher strength and corrosion resistance are needed. They have a mixed structure of austenite and ferrite, hence the term "duplex". They are not hardenable by heat treatment.

PRECIPITATION HARDENING stainless steels, like the martensitic types, can be strengthened (ie hardened) by heat treatment. The mechanism is metallurgically different to the process in the martensitic types. This means that either martensitic or austenitic precipitation hardening structures can be produced.

"Super" austenitic or "super" duplex grades have enhance pitting and crevice corrosion resistance compared with the ordinary austenitic or duplex types. This is due the further additions of chromium, molybdenum and nitrogen to these grades.