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Austenite (or gamma phase iron) is a metallic non-magnetic solid solution of iron
Iron

Iron is a chemical element with the symbol Fe and atomic number 26. Iron is a Group 8 element and period 4 element. Iron is lustrous and silvery in color....
 and an alloy
Alloy

An alloy is a partial or complete solid solution of one or more chemical element in a metallic matrix. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may be homogeneous in distribution depending on thermal history....
ing element. In plain-carbon steel
Plain-carbon steel

Carbon steel, also called plain carbon steel, is steel where the main alloying constituent is carbon. The AISI defines carbon steel as: "Steel is considered to be carbon steel when no minimum content is specified or required for chromium, cobalt, columbium [niobium], molybdenum, nickel, titanium, tungsten, vanadium or zirconium, or any...
, austenite exists above the critical eutectoid temperature of 1000 K (about 727 °C); other alloys of steel
Steel

Steel is an alloy consisting mostly of iron, with a carbon content between 0.2% and 2.14% by weight , depending on grade. Carbon is the most cost-effective alloying material for iron, but various other alloying elements are used such as manganese, chromium, vanadium, and tungsten....
 have different eutectoid temperatures. It is named after Sir William Chandler Roberts-Austen
William Chandler Roberts-Austen

William Chandler Roberts-Austen, , was a metallurgist noted for his research on the physical properties of metals and their alloys. Austenite is named in his honor....
 (1843-1902).

ustenite cools, it often transforms into a mixture of ferrite
Ferrite (iron)

Ferrite or alpha iron is a materials science term for iron, or a solid solution with iron as the main constituent, with a body centred cubic crystal structure....
 and cementite
Cementite

Cementite or iron carbide is a chemical compound with the formula Fe3C , and an orthorhombic crystal structure. It is a hard, brittle material, normally classified as a ceramic in its pure form, though it is more important in metallurgy....
 as the carbon diffuses.






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Austenite (or gamma phase iron) is a metallic non-magnetic solid solution of iron
Iron

Iron is a chemical element with the symbol Fe and atomic number 26. Iron is a Group 8 element and period 4 element. Iron is lustrous and silvery in color....
 and an alloy
Alloy

An alloy is a partial or complete solid solution of one or more chemical element in a metallic matrix. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may be homogeneous in distribution depending on thermal history....
ing element. In plain-carbon steel
Plain-carbon steel

Carbon steel, also called plain carbon steel, is steel where the main alloying constituent is carbon. The AISI defines carbon steel as: "Steel is considered to be carbon steel when no minimum content is specified or required for chromium, cobalt, columbium [niobium], molybdenum, nickel, titanium, tungsten, vanadium or zirconium, or any...
, austenite exists above the critical eutectoid temperature of 1000 K (about 727 °C); other alloys of steel
Steel

Steel is an alloy consisting mostly of iron, with a carbon content between 0.2% and 2.14% by weight , depending on grade. Carbon is the most cost-effective alloying material for iron, but various other alloying elements are used such as manganese, chromium, vanadium, and tungsten....
 have different eutectoid temperatures. It is named after Sir William Chandler Roberts-Austen
William Chandler Roberts-Austen

William Chandler Roberts-Austen, , was a metallurgist noted for his research on the physical properties of metals and their alloys. Austenite is named in his honor....
 (1843-1902).

Behavior in Plain-Carbon Steel

As austenite cools, it often transforms into a mixture of ferrite
Ferrite (iron)

Ferrite or alpha iron is a materials science term for iron, or a solid solution with iron as the main constituent, with a body centred cubic crystal structure....
 and cementite
Cementite

Cementite or iron carbide is a chemical compound with the formula Fe3C , and an orthorhombic crystal structure. It is a hard, brittle material, normally classified as a ceramic in its pure form, though it is more important in metallurgy....
 as the carbon diffuses. Depending on alloy composition and rate of cooling, pearlite
Pearlite

Pearlite is a Phase , Lamellar structure structure composed of alternating layers ofFerrite and cementite that occurs in some steels and cast irons....
 may form. If the rate of cooling is very fast, the alloy may experience a large lattice distortion known as martensitic transformation
Martensite

Martensite, named after the German :category:metallurgists Adolf Martens , most commonly refers to a very hard form of steel crystalline structure, but it is also any crystal structure that is formed by displacive transformation....
, instead of transforming into ferrite and cementite. In this industrially very important case, the carbon is not allowed to diffuse due to the cooling speed, resulting in a BCT-structure. The result is hard martensite
Martensite

Martensite, named after the German :category:metallurgists Adolf Martens , most commonly refers to a very hard form of steel crystalline structure, but it is also any crystal structure that is formed by displacive transformation....
. The rate of cooling determines the relative proportions of these materials and therefore the mechanical properties (e.g., hardness, tensile strength
Tensile strength

Tensile strength , or is the Stress at which a material breaks or permanently deforms. Tensile strength is an Intensive and extensive properties and, consequently, does not depend on the size of the test specimen....
) of the steel. Quench
Quench

A quench refers to a rapid cooling. In polymer chemistry and materials science, quenching is used to prevent low-temperature processes such as phase transformations from occurring by only providing a narrow window of time in which the reaction is both thermodynamically favorable and kinetically accessible....
ing (to induce martensitic transformation), followed by tempering
Tempering

Tempering is a heat treatment technique for metals, alloys and Toughened glass. In steels, tempering is done to "toughen" the metal by transforming brittle martensite into bainite or a combination of ferrite and cementite....
 will transform some of the brittle martensite into tempered martensite. If a low-hardenability steel is quenched, a significant amount of austenite will be retained in the microstructure.

Stabilization

The addition of certain alloying elements, such as manganese
Manganese

Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a Oxidation state in nature , and in many minerals....
 and nickel
Nickel

Nickel is a chemical element, with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge....
, can stabilize the austenitic structure, facilitating heat-treatment of low-alloy steels. In the extreme case of austenitic stainless steel
Stainless steel

In metallurgy, stainless steel is defined as a steel alloy with a minimum of 10% chromium content by mass. Stainless steel does not stain, corrode, or rust as easily as ordinary steel , but it is not stain-proof....
, much higher alloy content makes this structure stable even at room temperature. On the other hand, such elements as silicon
Silicon

Silicon is the most common metalloid. It is a chemical element, which has the symbol Si and atomic number 14. The atomic mass is 28.0855....
, molybdenum
Molybdenum

Molybdenum , is a Group 6 element chemical element with the symbol Mo and atomic number 42. It has the List of elements by melting point melting point of any element....
, and chromium
Chromium

Chromium is a chemical element which has the symbol Cr and atomic number 24. It is a steely-gray, Lustre , hard metal that takes a high polish and has a high melting point....
 tend to de-stabilize austenite, raising the eutectoid temperature.

Austenite transformation and Curie point

In many magnetic alloys, the Curie point
Curie point

The Curie point , or Curie temperature, is a term in physics and materials science, named after Pierre Curie , and refers to a characteristic property of a ferromagnetic or piezoelectric material....
, the temperature at which magnetic materials cease to behave magnetically, occurs at nearly the same temperature as the austenite transformation. This behavior is attributed to the paramagnetic nature of austenite, while both martensite and ferrite are strongly ferromagnetic.

Thermo-optical emission

A blacksmith
Blacksmith

A blacksmith is a person who processess iron or steel by forging the metal; i.e., by using tools to hammer, bend, cut, and otherwise shape it in its non-liquid form....
 causes phase changes in the iron-carbon system in order to control the material's mechanical properties, often using the annealing, quenching, and tempering processes. In this context, the color of light emitted by the workpiece is an approximate gauge of temperature
Black body

In physics, a black body is an Physical body that absorbs all electromagnetic radiation that falls on it. No electromagnetic radiation passes through it and none is Reflection ....
, with the transition from red to orange corresponding to the formation of austenite in medium- and high-carbon steel.

Maximum carbon solubility in austenite is 2.03% C at 1420 K (1147 °C).

See also

  • Cementite
    Cementite

    Cementite or iron carbide is a chemical compound with the formula Fe3C , and an orthorhombic crystal structure. It is a hard, brittle material, normally classified as a ceramic in its pure form, though it is more important in metallurgy....
  • Eutectoid
  • Ferrite (iron)
    Ferrite (iron)

    Ferrite or alpha iron is a materials science term for iron, or a solid solution with iron as the main constituent, with a body centred cubic crystal structure....
  • Martensite
    Martensite

    Martensite, named after the German :category:metallurgists Adolf Martens , most commonly refers to a very hard form of steel crystalline structure, but it is also any crystal structure that is formed by displacive transformation....


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