Austenite

Austenite, known as gamma-iron (gamma-Fe), is the face-centered cubic (FCC) solid solution of iron in which carbon and other alloying elements are dissolved. It is the high-temperature phase of iron and steel and can accommodate relatively high carbon contents compared with ferrite.

In the iron–carbon system, austenite forms when steel is heated into the austenite region. For pure iron, gamma-iron is stable roughly from 912°C to its melting point. In carbon-containing steels, the temperature and composition define the austenite stability field; carbon solubility in austenite can reach about 2.0% at around 1147°C.

Austenite is stabilized at room temperature in certain alloy systems, notably austenitic stainless steels, where elements such as nickel and manganese enlarge the austenite stability field and maintain the FCC structure. The phase is generally paramagnetic at typical processing temperatures.

Cooling austenite leads to transformations into other microstructures. Depending on carbon content and cooling rate, it may transform into ferrite and cementite (pearlite) near the eutectoid composition (about 0.76% C) at about 727°C, or form bainite at intermediate temperatures, or transform to martensite with rapid quenching. Some austenite may be retained at room temperature as retained austenite, especially in high-alloy or high-nickel steels.

In practice, the concept of austenite is central to heat treatment: heating steel into the austenitizing range dissolves carbides and homogenizes composition, followed by quenching or controlled cooling to develop desired microstructures like martensite, bainite, or pearlite. The term is named after Sir William Chandler Roberts-Austen.