Everything about Austenite totally explained
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 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, instead of transforming into a mixture. In this industrially very important case, the carbon isn't allowed to blend out in the remaining melt due to the cooling speed, but is captured inside the FCC-structure of austenite, creating tension in the crystal when the alloy cools. The result is hard
martensite. The rate of cooling determines the relative proportions of these materials and therefore the mechanical properties (for example
hardness,
tensile strength) of the steel.
Quenching (to induce martensitic transformation), followed by
tempering will transform some of the brittle martensite into
bainite. 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).
Further Information
Get more info on 'Austenite'.
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