What is austenite?

Austenite is a lamellar microstructure of steel, usually a non-magnetic solid solution of ɣ-Fe with a small amount of carbon solidly dissolved in it, also known as Worsted iron or ɣ-Fe. The name austenite comes from the British metallurgist Roberts Austin.

Austenite is generally composed of equiaxed polygonal grains with twins within the grains. At the end of the heating transformation, the austenite grains are relatively small, and the grain boundaries are irregularly curved. After a period of heating or holding, the grains will grow, grain boundaries can tend to straighten out. Iron and carbon phase diagram of austenite is a high-temperature phase, existing above the critical point A1 temperature, is a pearlite reverse eutectic transformation. When enough chemical elements are added to the steel to expand the austenite phase zone, Ni, Mn, etc., then the austenite can be stabilized at room temperature, such as austenitic steel.
Ferrite becomes austenitic in phase at 912°C to 1394°C, changing from a body-centered cubic structure to a face-centered cubic. Austenitic strength is lower, but its ability to dissolve carbon is greater (1146 ° C can be dissolved into 2.04% of the carbon). Austenitic series of stainless steel is commonly used in the food industry and surgical equipment.

Austenite is the most dense row of dotted structure, high density, so the volume mass of austenite than the steel ferrite, martensite and other phases of the volume mass is small. Therefore, when the steel is heated to the austenite phase area, the volume shrinks, and when it cools, the volume expands when the austenite is transformed into ferrite - pearlite and other organizations, which easily causes internal stress and deformation.
Austenite point slip system more, so austenite plasticity is good, low yield strength, easy to process plastic forming. Therefore, ingots, billets, steel is generally heated to 1100˚C or more austenite, and then forging and rolling, plastic processing into materials or processing into parts.
Austenite in general steel is paramagnetic, so austenitic steel can be used as non-magnetic steel. However, the special composition of Fe-Ni soft magnetic alloy, also has austenite organization, but has ferromagnetic properties.
Austenite has poor thermal conductivity and a large linear expansion coefficient, which is about twice as high as the average linear expansion coefficient of ferrite and carburized. Therefore, austenitic steel can be used to manufacture instrument components with sensitive thermal expansion. Among carbon steels, the thermal conductivity of ferrite, pearlite, martensite, austenite and carburized are 77.1,51.9,29.3,14.6 and 4.2, respectively. it can be seen that, except for carburized, austenite has the worst thermal conductivity, especially for austenitic steels with high alloying degree, so thick steel parts should be cooled and heated slowly during heat treatment to reduce the temperature difference thermal stress and avoid cracking.