Austenite

Austenite (or gamma phase iron) is a metallic, non-magnetic solid solution of carbon and iron that exists in steel above the critical temperature of 1333°F (about 723°C). It is named after Sir William Chandler Roberts-Austen (1843-1902). Its face-centred cubic (FCC) structure allows it to hold a high proportion of carbon in solution.

Behavior of Austenite during Cooling

As austenite cools, it transforms into a mixture of ferrite and cementite as the dissolved carbon falls out of solution. Depending on alloy composition and rate of cooling, pearlite may form. If the rate of cooling is very fast, the alloy may experience a slight lattice distortion known as martensitic transformation. 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 transfom some of the brittle martensite into bainite. If a low-hardenability steel is quenched, a significant amount of austentite will be retained in the microstructure.

Austenite 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.

Austentite Transformation and Curie Point

In many magnetic alloys, the Curie Point, the temperature at which magnentic materials cease to behave magneticly, occurs at nearly the same temperature as the austenite transfomation. This behavior is attributed to the paramagnetic nature of austenite, while martensite and ferrite are strongly magnetic.

Thermo-optical Emission of Austenite

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 accurate 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 1147°C.

References

"Physical Metallurgy Priciples". Reed-Hill, Robert. 3rd. Edition. PWS Publishing. Boston. 1991.

External link

Fe-Fe₃C phase diagram

Metallurgy