Critical Temperature of Steel
Critical temperature of steel defines phase transition between two phases of steel. As the steel is heated above the critical temperature, about 1335°F (724°C), it undergoes a phase change, recrystallizing as austenite. There are two types of critical temperature:
- Lower critical temperature (Ac1). The temperature at which austenite starts to transform from ferrite.
- Upper critical temperature (Ac3). The temperature at which austenite is completely transformed from ferrite.
In the Fe-C system, there is a eutectoid point at approximately 0.8wt% C, 723°C. The phase just above the eutectoid temperature for plain carbon steels is known as austenite or gamma.