Austenite is a compound of iron, carbon, and other alloying elements. It is an allotrope of iron and is also known as gamma iron. It is developed at high temperatures, and has a face-centered cubic crystal structure. Austenite is the most common form of steel and other iron alloys at temperatures above 870°C (1600°F).
At the temperature range of 814°C to 1393°C (1500°F to 2550°F), austenite is formed. This range is called the austenitizing range or the transformational range. This range is also known as the critical range, because in this temperature range, ferrite and cementite are continuously replaced by austenite. Above this range, the austenite phase is stable and cannot be changed back to ferrite and cementite. At 814°C, austenite has the maximum carbon solubility, which is about 0.76 mass%. Iron carbides begin to form at temperatures higher than 814°C.
Also, the austenite is not stable at low temperatures. Its atomic structure is disrupted as the temperature drops to room temperature. This disruption is called the transformation of austenite. During this transformation, the carbon atoms begin to precipitate from the solution and separate from the crystal lattice of the austenite. And then the carbon atoms form into ferrite and pearlite.
Austenitization is a process which causes the grain structure of a metal or alloy to transform into an austenite state. It is interpreted as the process of changing a metal or alloy’s grain structure from one state to another through heat treatment, and it is one of the processes in a heat-treatment cycle.
Austenitization is essential for obtaining desired mechanical properties in certain alloys. As the austenite grain structure increases, the hardness, ductility and strength of an alloy also increases.
The solubility of carbon in austenite is the maximum at 814°C and then decreases as the temperature increases beyond this. To obtain the maximum carbon solubility, austenitization is done between 814°C to 912°C (1500°F to 1675°F). Austenitization of more than 912°C (1675°F), results in the precipitation of carbon from the austenite lattice and makes it difficult to return the metal or alloy to an austenite state.
After forming an austenite grain structure, the material must be cooled to room temperature. This process is known as quenching and forms a hard, wear-resistant surface layer in many steels and iron alloys.
Generally, austenitization is done in two different ways: air cooling and water cooling. Air cooling is done by passing the hot material over a cooling frame, while the water cooling process requires the metal or alloy to be placed in a quench bath. Depending on the material, one of these processes can be used.
Austenitization is also used in combination with other processes such as tempering and annealing, to further improve the mechanical properties of the material.
In conclusion, austenite is the most common form of steel and other iron alloys at temperatures above 870°C (1600°F). Its atomic structure is disrupted as the temperature drops to room temperature, and this disruption is called the transformation of austenite. Austenitization is an essential process for obtaining desired mechanical properties in certain alloys, and it is done in two different ways: air cooling and water cooling. Furthermore, austenitization is also used in combination with other processes such as tempering and annealing, to further improve the mechanical properties of the material.
