A1, A2, A3, A4

Designation for transformation temperatures of iron and iron alloys according to the
iron-carbon phase diagram.

The eutectic halts on the cooling curve are referred to as Ar points
(r = refroidissement), the halts on the heat-up curve as Ac points
(c = chauffage).

The temperature difference between the respective halt temperatures on the head-up and cooling curve, i.e. the hysteresis, is increased with faster temperature change.

With very slow temperature change, the position of the halts approximates the values for  equilibrium temperature as specified in the list below. This results in the following designation concept:

1536 °CSolidification temperature (melting point) delta iron
1392 °C:A4 point gamma iron
911 °C:A3 point non-magnetic alpha iron
769 °C:A2 point ferromagnetic alpha iron
723 °C:A1-point eutectoid transformation - pearlite point