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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. [1] In plain-carbon steel , austenite exists above the critical eutectoid temperature of 1000 K (727 °C); other alloys of steel have different eutectoid temperatures.
Diffusional transformations like austenite transforming to a cementite and ferrite mixture can be explained using the sigmoidal curve; for example the beginning of pearlitic transformation is represented by the pearlite start (P s) curve. This transformation is complete at P f curve. Nucleation requires an incubation time.
In order for any transformation to take place, the microstructure of the metal must be austenite structure. The exact boundaries of the austenite phase region depend on the chemistry of the alloy being heat treated. However, austenitizing temperatures are typically between 790 and 915 °C (1,454 and 1,679 °F). [5]
The A cm, where austenite is in equilibrium with cementite + γ-Fe, is beyond the right edge in Fig. 1. The α + γ phase field is, technically, the β + γ field above the A 2 . The beta designation maintains continuity of the Greek-letter progression of phases in iron and steel: α-Fe, β-Fe, austenite (γ-Fe), high-temperature δ-Fe, and ...
Convergent beam electron diffraction (CBED) transmission electron micrograph of a [111] zone axis of austenitic stainless steel. Austenitic stainless steel is one of the five classes of stainless steel as defined by crystalline structure (along with ferritic, martensitic, duplex and precipitation hardened). [1]
Enthalpy of transformation. This applies to the transformations from one solid phase to another, such as the transformation from α-Fe (bcc ferrite) to -Fe (fcc austenite). The transformation is designated ΔH tr. Enthalpy of fusion or melting. This applies to the transition of a solid to a liquid and is designated ΔH m.
Austenite has much higher stacking-fault energy than martensite or pearlite, lowering the wear resistance and increasing the chances of galling, although some or most of the retained austenite can be transformed into martensite by cold and cryogenic treatments prior to tempering.
TRIP steels possess a microstructure consisting of austenite with sufficient thermodynamic instability such that transformation to martensite is achieved during loading or deformation. Many automotive TRIP steels possess retained austenite within a ferrite matrix, which may also contain hard phases like bainite and martensite. [2]