Темірдің гамма фазасы немесе аустенит

Metallic, non magnetic allotrope of iron or a solid solution of iron, with an alloying element

the alloy and iron allotrope

Austenite, also known as gamma phase iron (γ Fe), is a metallic, non magnetic allotrope of iron or a solid solution of iron with an alloying element. In plain carbon steel, austenite exists above the critical eutectoid temperature of 1000 K (727 °C); other alloys of steel have different eutectoid temperatures. The austenite allotrope is named after Sir William Chandler Roberts Austen (1843–1902). It exists at room temperature in some stainless steels due to the presence of nickel stabilizing the austenite at lower temperatures.

From alpha iron undergoes a phase transition from body centered cubic (BCC) to the face centered cubic (FCC) configuration of gamma iron, also called austenite. This is similarly soft and ductile but can dissolve considerably more carbon (as much as 2.03% by mass at ). This gamma form of iron is present in the most commonly used type of stainless steel for making hospital and food service equipment.

Austenitization means to heat the iron, iron based metal, or steel to a temperature at which it changes crystal structure from ferrite to austenite. The more open structure of the austenite is then able to absorb carbon from the iron carbides in carbon steel. An incomplete initial austenitization can leave undissolved carbides in the matrix. For some iron metals, iron based metals, and steels, the presence of carbides may occur during the austenitization step. The term commonly used for this is two phase austenitization.

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 other heat extraction medium that is between temperatures of The metal is annealed in this temperature range until the austenite turns to bainite or ausferrite (bainitic ferrite + high carbon austenite). By changing the temperature for austenitization, the austempering process can yield different and desired microstructures. A higher austenitization temperature can produce a higher carbon content in austenite, whereas a lower temperature produces a more uniform distribution of austempered structure.

As austenite cools, the carbon diffuses out of the austenite and forms carbon rich iron carbide (cementite) and leaves behind carbon poor ferrite. Depending on alloy composition, a layering of ferrite and cementite, called pearlite, may form. If the rate of cooling is very swift, the carbon does not have sufficient time to diffuse, and the alloy may experience a large lattice distortion known as martensitic transformation in which it transforms into martensite, a body centered tetragonal structure (BCT). The rate of cooling determines the relative proportions of martensite, ferrite, and cementite, and therefore determines the mechanical properties of the resulting steel, such as hardness and tensile strength. A high cooling rate of thick sections will cause a steep thermal gradient in the material. The outer layers of the heat treated part will cool faster and shrink more, causing it to be under tension and thermal straining. At high cooling rates, the material will transform from austenite to martensite which is much harder and will generate cracks at much lower strains. The volume change (martensite is less dense than austenite) can generate stresses as well. The difference in strain rates of the inner and outer portion of the part may cause cracks to develop in the outer portion, compelling the use of slower quenching rates to avoid this. By alloying the steel with tungsten, the carbon diffusion is slowed and the transformation to BCT allotrope occurs at lower temperatures, thereby avoiding the cracking. Such a material is said to have its hardenability increased. Tempering following quenching 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, leaving the steel with internal stresses that leave the product prone to sudden fracture.

Heating white cast iron (containing iron carbide, i. e. cementite, but no uncombined carbon) above causes the formation of austenite in crystals of primary cementite. This austenisation of white iron occurs in primary cementite at the interphase boundary with ferrite.

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 in bulk metal form. However, fcc transition metals can be grown on a face centered cubic (fcc) or diamond cubic. 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 is greater than a monolayer. and ferrite are strongly ferromagnetic.

During heat treating, a blacksmith causes phase changes in the iron carbon system to control the material's mechanical properties, often using the annealing, quenching, and tempering processes. In this context, the color of light, or "blackbody radiation", emitted by the workpiece is an approximate gauge of temperature. Temperature is often gauged by watching the color temperature of the work, with the transition from a deep cherry red to orange red (815 °C (1499 °F) to 871 °C (1600 °F)) corresponding to the formation of austenite in medium and high carbon steel. In the visible spectrum, this glow increases in brightness as temperature increases. When cherry red, the glow is near its lowest intensity and may not be visible in ambient light. Hence blacksmiths usually austenitize steel in low light conditions to accurately judge the color of the glow.