Austenite Definition

What Austenite and Austenitic Mean

Austenite refers to steel and other iron alloys that have the FCC crystal structure.
Austenite refers to steel and other iron alloys that have the FCC crystal structure. Monty Rakusen, Getty Images

Austenite Definition

Austenite is face-centered cubic iron. The term austenite is also applied to iron and steel alloys that have the FCC structure (austenitic steels). Austenite is a non-magnetic allotrope of iron. It is named for Sir William Chandler Roberts-Austen, an English metallurgist known for his studies of metal physical properties.

Also Known As: gamma-phase iron or γ-Fe or austenitic steel

Example: The most common type of stainless steel used for food service equipment is austenitic steel.

Related Terms:

Austenitization, which means heating iron or an iron alloy, such as steel, to a temperature at which its crystal structure transitions from ferrite to austenite.

Two-phase austenitization, which occurs when undissolved carbides remain following the austenitization step.

Austempering, which is defined as a hardening process used on iron, iron alloys, and steel to improve its mechanical properties. In austempering, metal is heated to the austenite phase, quenched between 300–375 °C (572–707 °F), and then annealed to transition the austenite to ausferrite or bainite.

Common Misspellings: austinite

Austenite Phase Transition

The phase transition to austenite may be mapped out for iron and steel. For iron, alpha iron undergoes a phase transition from 912 to 1,394 °C (1,674 to 2,541 °F) from the body-centered cubic crystal lattice (BCC) to the face-centered cubic crystal lattice (FCC), which is austenite or gamma iron.

Like the alpha phase, the gamma phase is ductile and soft. However, austenite can dissolve over 2% more carbon than alpha iron. Depending on the composition of an alloy and its rate of cooling, austenite may transition into a mixture of ferrite, cementite, and sometimes pearlite. An extremely fast cooling rate may cause a martensitic transformation into a body-centered tetragonal lattice, rather than ferrite and cementite (both cubic lattices).

Thus, the rate of cooling of iron and steel is extremely important because it determines how much ferrite, cementite, pearlite, and martensite form. The proportions of these allotropes determine the hardness, tensile strength, and other mechanical properties of the metal.

Blacksmiths commonly use the color of heated metal or its blackbody radiation as an indication of the metal's temperature. The color transition from cherry red to orange red corresponds to the transition temperature for austenite formation in medium-carbon and high-carbon steel. The cherry red glow is not easily visible, so blacksmiths often work under low-light conditions to better perceive the color of the glow of the metal.

Curie Point and Iron Magnetism

The austenite transformation occurs at or near the same temperature as the Curie point for many magnetic metals, such as iron and steel. The Curie point is the temperature at which a material ceases to be magnetic. The explanation is that the structure of austenite leads it to behave paramagnetically. Ferrite and martensite, on the other hand, are strongly ferromagnetic lattice structures.