Austenite is a metallic, non-magnetic solid solution of carbon and iron that exists in steel above the critical temperature of 1333˘XF (about 723˘XC). It is named after Sir William Chandler Roberts-Austen (1843-1902). Its face-centred cubic (FCC) structure allows it to hold a high proportion of carbon in solution.

As it cools, this structure either breaks down into a mixture of ferrite and cementite (often in special forms such as pearlite and bainite), or undergoes a slight lattice distortion known as martensitic transformation. The rate of cooling determines the relative proportions of these materials and therefore the mechanical properties (e.g. hardness, tensile strength) of the steel. Quenching (to induce martensitic transformation), followed by tempering (to break down some martensite and retained austenite), is the most common heat treatment for high-performance steels.

The addition of certain other metals, such as manganese and nickel, can stabilize the austenitic structure, facilitating heat-treatment of low-alloy steels. In the extreme case of austenitic stainless steel, much higher alloy content makes this structure stable even at room temperature. On the other hand, such elements as silicon, molybdenum, and chromium tend to de-stabilize austenite, raising the eutectoid temperature.

A Blacksmith causes phase changes in the iron-carbon system in order to control the material's mechanical properties, often using the quenching-and-tempering process described above. In this context, the color of light emitted by the workpiece is an accurate gauge of temperature, with the transition from red to orange corresponding to the formation of Austenite in medium- and high-carbon steel.

Maximum carbon solubility in austenite is 2.03% C at 1147˘XC.