Austenite

Austenite, also known as gamma-phase iron (γ-Fe), is a metallic, non-magnetic allotrope of iron or a solid solution of iron, with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of 1000 K (727°C); other alloys of steel have different eutectoid temperatures. The austenite allotrope exists at room temperature in stainless steel. It is named after Sir William Chandler Roberts-Austen (1843–1902).

From 912 to 1,394 °C (1,674 to 2,541 °F) alpha iron undergoes a phase transition from body-centred cubic (BCC) to the face-centred cubic (FCC) configuration of gamma iron, also called austenite. This is similarly soft and ductile but can dissolve considerably more carbon (as much as 2.03% by mass at 1,146 °C (2,095 °F)). This gamma form of iron is exhibited by the most commonly used type of stainless steel for making hospital and food-service equipment.

Austenitization means to heat the iron, iron-based metal, or steel to a temperature at which it changes crystal structure from ferrite to austenite. The more open structure of the austenite is then able to absorb carbon from the iron-carbides in carbon steel. An incomplete initial austenitization can leave undissolved carbides in the matrix.

For some irons, iron-based metals, and steels, the presence of carbides may occur during the austenitization step. The term commonly used for this is two-phase austenitization.

Austempering is a hardening process that is used on iron-based metals to promote better mechanical properties. The metal is heated into the austenite region of the iron-cementite phase diagram and then quenched in a salt bath or other heat extraction medium that is between temperatures of 300–375 °C (572–707 °F). The metal is annealed in this temperature range until the austenite turns to bainite or ausferrite (bainitic ferrite + high-carbon austenite).

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