Austenite

Iron carbon phase diagram

IronAlfa&IronGamma

Austenite grain

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 727 °C (1341 °F); other alloys of steel have different eutectoid temperatures. The austenite allotrope is named after Sir William Chandler Roberts-Austen (1843–1902), an English metallurgist. Its face-centered cubic (FCC) crystal structure distinguishes it from other iron allotropes, such as alpha iron (α-Fe, ferrite) with a body-centered cubic (BCC) structure and delta iron (δ-Fe), also with a BCC structure.

Properties

Austenite is paramagnetic and, unlike ferrite, is non-magnetic. The FCC structure of austenite can dissolve considerably more carbon (up to 2.1% by mass at 1148°C) than ferrite. Its ability to dissolve carbon is one of the features that gives austenite its importance in metallurgy, especially in the manufacturing of steel where controlling the carbon level is essential to the properties of the finished material.

Formation and Decomposition

Austenite forms from ferrite as a result of heating steel above the upper critical temperature, where it becomes a high-temperature phase. Upon slow cooling, austenite transforms into a mixture of ferrite and cementite in a pearlitic structure. Rapid cooling, or quenching, of austenite transforms it into martensite, a hard, brittle form of steel. The exact temperature range for austenite formation and decomposition depends on the composition of the steel, particularly the carbon content.

Stability

The stability of austenite is significantly influenced by the presence of alloying elements. Elements such as nickel and manganese stabilize the austenite phase, expanding its temperature range into lower temperatures. This stabilization is utilized in the production of austenitic stainless steel, which is highly resistant to corrosion and retains its austenitic microstructure at all temperatures.

Applications

Austenitic steels are extensively used in a wide range of applications due to their high corrosion resistance, excellent formability, and high-temperature performance. These steels are found in kitchen utensils, piping, and automotive parts. Additionally, certain grades of austenitic stainless steel are used in cryogenic applications due to their ability to retain toughness at very low temperatures.

See Also

• Ferrite (iron)
• Cementite
• Martensite
• Pearlite
• Stainless steel

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