High-temperature phase of steel with a face-centered cubic lattice that can hold up to about 2.1 percent carbon in solid solution, far more than ferrite. Plain carbon steel starts forming it at 727 °C (1,341 °F). Every hardening cycle begins by turning the structure to austenite, because only austenite can transform to martensite on quenching. Austenite is nonmagnetic, and the loss of magnetism as steel heats through the critical range is the basis of the magnet check at the forge.
Give those 4-inch rounds another twenty minutes, they aren't austenite all the way to the core yet.
Also heard as
- gamma iron
Related terms
Austenitizing
Heating steel above its upper critical temperature and holding it until the structure becomes austenite.
Martensite
The hard, needle-like phase that forms when austenite is quenched fast enough to trap its carbon.
Critical temperature
A temperature at which steel's structure changes phase on heating or cooling, such as Ac1 or Ac3.
Retained austenite
Austenite that fails to transform during the quench and survives at room temperature inside hardened steel.
Magnet test
Checking steel with a magnet while heating, since it stops being magnetic near its hardening temperature.