Austeniidi

Austeniidi is a high-temperature phase of iron and steel. It is a solid solution of carbon in face-centered cubic (FCC) iron. At room temperature, iron typically exists as ferrite, which has a body-centered cubic (BCC) structure. However, as the temperature increases, the iron atoms rearrange themselves into the FCC structure, forming austenite. This transformation occurs at a specific temperature, known as the A3 transformation temperature, which varies depending on the carbon content of the steel.

Austenitic stainless steels are a prominent class of materials that contain austenite as their primary phase at room temperature. This is achieved by alloying iron with significant amounts of nickel, chromium, and sometimes other elements like manganese and molybdenum. These alloying additions stabilize the FCC structure of austenite even at lower temperatures. Austenitic stainless steels are known for their excellent corrosion resistance, good formability, and non-magnetic properties. Common examples include the 300 series stainless steels, such as 304 and 316.

The properties of austenite are distinct from those of ferrite. Austenite can dissolve a much larger amount of carbon than ferrite, with solubility reaching up to 2.14% by weight at about 1147 °C. This ability to hold carbon in solid solution is crucial for many heat treatment processes, such as hardening of steel. When steel containing austenite is rapidly cooled (quenched), the carbon atoms are trapped within the FCC structure, which can then transform into martensite, a very hard and brittle phase. Alternatively, slower cooling allows the carbon to precipitate out as carbides or form other phases, influencing the final microstructure and properties of the steel.