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 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 (about 727 °C); other alloys of steel have different eutectoid temperatures. It is named after Sir William Chandler Roberts-Austen (1843-1902).^[2]

Allotrope of iron

Above 912°C and up to 1394°C alpha iron undergoes a phase transition from body-centred cubic to the face-centred cubic configuration of gamma iron, also called austenite. This is similarly soft and ductile but can dissolve considerably more carbon (as much as 2.04% by mass at 1146°C). This gamma form of iron is exhibited by the most commonly used type of stainless steel for making hospital and food-service equipment.

Austenitization

Austenitization means to heat the iron, iron-based metal, or steel to a temperature at which it changes crystal structure from ferrite to austenite.^[3] An incomplete initial austenization can leave undissolved carbides in the matrix.^[4]

For some irons, iron-based metals, and steels, the presence of carbides may occur or be present during the austenitization step. The term commonly used for this is two-phase austenitization.^[5]

Austempering

Austempering is a hardening process that is used on iron-based metals to promote better mechanical properties. The metal is heated into the austenite region of the iron-cementite phase diagram and then quenched in a "salt bath" or heat extraction medium that is between temperatures of 300–375 °C (572–707 °F). The metal is annealed in this temperature range until the austenite turns to bainite or ausferrite (bainitic ferrite + high-carbon austenite).^[6]

By changing the temperature for austenitization, the austempering process can yield different and desired microstructures.^[7] A higher austenitization temperature can produce a higher carbon content in austenite, whereas a lower temperature produces a more uniform distribution of austempered structure.^[7] The carbon content in austenite as a function of austempering time has been established.^[8]

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 BCT-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.

Behavior in Cast Iron

Heating white hypereutectic cast iron above 730°C causes the formation of austenite in crystals of primary cementite.^[9] This austenization of white iron occurs in primary cementite at the interphase boundary with ferrite.^[9] When the grains of austenite form in cementite, they occur as lamellar clusters oriented along the cementite crystal layer surface.^[9] Austenite is formed by withdrawal of carbon atoms from cementite into ferrite.^[9]

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 is only stable above 910°C in bulk metal form. However, the use of a face-centered cubic (fcc) or diamond cubic substrate allows the epitaxial growth of fcc transition metals.^[10] The epitaxial growth of austenite on the diamond (100) face is feasible because of the close lattice match and the symmetry of the diamond (100) face is fcc. More than a monolayer of γ-iron can be grown because the critical thickness for the strained multilayer has been determined and is in close agreement with theory.^[10]

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
• Heat treatment
• Bainite
• Pearlite

References

External links

• Fe-Fe₃C phase diagram