There are different types of stainless steels: when nickel, for instance is added the austenite structure of iron is stabilized. This crystal structure makes such steels non-magnetic and less brittle at low temperatures. For higher hardness and strength, carbon is added.
Significant quantities of manganese have been used in many stainless steel recipes. Manganese preserves an austenitic structure in the steel as does nickel, but at a lower cost.
Stainless steels are also classified by their crystalline structure:
- Austenitic stainless steels comprise over 70% of total stainless steel production. They contain a maximum of 0.15% carbon, a minimum of 16% chromium and sufficient nickel and/or manganese to retain an austenitic structure at all temperatures from the cryogenic region to the melting point of the alloy. A typical composition is 18% chromium and 8% nickel, commonly known as 18/8 stainless. "Superaustenitic" stainless steels, such as alloy AL-6XN, exhibit great resistance to chloride pitting, crevice corrosion and stress-corrosion cracking over the 300 series.
- Ferritic stainless steels are highly corrosion resistant, but far less durable than austenitic grades and cannot be hardened by heat treatment. They contain between 10.5% and 27% chromium and very little nickel, if any. Most recipes include molybdenum; some, aluminium or titanium.
- Martensitic stainless steels are not as corrosion resistant as the other two classes, but are extremely strong and tough as well as highly machineable, and can be hardened by heat treatment. They contain 11.5 to 18% chromium and significant amounts of carbon. Some grades include additional alloying elements in small quantities.
