austenite

Iron-carbon phase diagram, showing the conditions under which austenite (γ) is stable in carbon steel.

Austenite (or gamma phase iron) is a metallic non-magnetic solid solution of iron and an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of 1000 K (about 727 °C); other alloys of steel have different eutectoid temperatures. It is named after Sir William Chandler Roberts-Austen (1843-1902).

Behavior in Plain-Carbon Steel

As austenite cools, it often transforms into a mixture of ferrite and cementite as the carbon diffuses. Depending on alloy composition and rate of cooling, pearlite may form. If the rate of cooling is very fast, the alloy may experience a large lattice distortion known as martensitic 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 BCC-structure. The result is hard martensite. The rate of cooling determines the relative proportions of these materials and therefore the mechanical properties (e.g., hardness, tensile strength) of the steel. Quenching (to induce martensitic transformation), followed by tempering 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 and nickel, can stabilize the austenitic structure, facilitating heat-treatment of low-alloy steels. In the extreme case of austenitic stainless steel, much higher alloy content makes this structure stable even at room temperature. On the other hand, such elements as silicon, molybdenum, and chromium tend to de-stabilize austenite, raising the eutectoid temperature.

Austenite transformation and Curie point

In many magnetic alloys, the Curie point, 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 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, 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
• Eutectoid
• Ferrite (iron)
• Martensite

References

• Robert Reed-Hill, Reza Abbaschian, Physical Metallurgy Principles, 3rd Edition, Boston: PWS-Kent Publishing, 1991. ISBN 0534921736.

External links

• Fe-Fe₃C phase diagram