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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.
Its primary crystalline structure is austenite (face-centered cubic). Such steels are not hardenable by heat treatment and are essentially non-magnetic. [2] This structure is achieved by adding enough austenite-stabilizing elements such as nickel, manganese and nitrogen.
In steel it produces a bainite microstructure whereas in cast irons it produces a structure of acicular ferrite and high carbon, stabilized austenite known as ausferrite. It is primarily used to improve mechanical properties or reduce / eliminate distortion. Austempering is defined by both the process and the resultant microstructure.
Iron allotropes, showing the differences in structure. The alpha iron (α-Fe) is a body-centered cubic (BCC) and the gamma iron (γ-Fe) is a face-centered cubic (FCC). At atmospheric pressure, three allotropic forms of iron exist, depending on temperature: alpha iron (α-Fe, ferrite), gamma iron (γ-Fe, austenite), and delta iron (δ-Fe).
Microstructures of four kinds of duplex stainless steel in each direction. Duplex stainless steels are usually divided into three groups based on their pitting corrosion resistance, characterised by the pitting resistance equivalence number, PREN = %Cr + 3.3 %Mo + 16 %N.
Rather, the austempering process forms acicular ferrite and austenite. [2] The latter of these has a face-centered cubic (FCC) structure. The FCC has 12 slip systems that allow for dislocations motion, resulting in high ductility in the austenitic phase of ADI. This ductility also translates to relatively high toughness in ADI.
Austenite is slightly undercooled when quenched below Eutectoid temperature. When given more time, stable microconstituents can form: ferrite and cementite. Coarse pearlite is produced when atoms diffuse rapidly after phases that form pearlite nucleate. This transformation is complete at the pearlite finish time (P f).
Control-rolled steels: Hot rolled steels which have a highly deformed austenite structure that will transform to a very fine equiaxed ferrite structure upon cooling. Pearlite-reduced steels: Low carbon content steels which lead to little or no pearlite, but rather a very fine grain ferrite matrix. It is strengthened by precipitation hardening.