what is the difference between ferritic and austenitic steels?
Ferritic Stainless Steel
Ferritic steels are made up of ferrite crystals, a form of iron which contains only a very small amount (up to 0.025%) of carbon. Ferrite absorbs such a small amount of carbon because of its body centred cubic crystal structure - one iron atom at each corner, and one in the middle. This central iron atom is what gives ferritic stainless steels their magnetic properties.
Ferritic stainless steels are less widely-used due to their limited corrosion resistance and average strength and hardness.
Austenitic Stainless Steel
Face centred cubic crystalline structure of austenite.
Austenitic stainless steels contain austenite, a form of iron which can absorb more carbon than ferrite. Austenite is created by heating ferrite to 912 degrees C, at which point it transitions from a body centred cubic crystal structure to a face centred cubic crystal structure. Face centred cubic structures can absorb up to 2% carbon.
When austenite cools, it generally reverts back to its ferrite form, which makes austenite difficult to utilise at anything below the extreme temperatures of a smelting furnace. Austenite can be forced to retain its crystal structure at low temperatures with the inclusion of chemical additives, such as the nickel and manganese found in many austenitic stainless steels.
Austenitic stainless steels cannot be significantly hardened by heat treatment, but can be hardened by cold working. Austenitic stainless steels are widely used, particularly in stainless steel screws, due to their excellent resistance to corrosion.
Martensitic Stainless Steel
Martensitic stainless steel is formed by the creation of martensite. Martensite has been a key element of quenched steel for hundreds of years, but was officially named in the 20th century after the metallurgist Adolf Martens (1850 - 1914).
Martensite is a body centred cubic form of crystallised iron which is created when heated austenite is rapidly cooled by quenching. The increased rate at which Martensite crystals are created prevents cementite from being formed, and causes carbon atoms to become unnaturally trapped in crystals which would ordinarily expel excess carbon during gradual cooling.
Martensitic stainless steels can be heat treated and hardened, but have reduced chemical resistance when compared to austenitic stainless steels. Martensitic stainless steel is often used when hardness is critical, such is in knives, where surface hardness creates a sharper blade.