Critical temperatures

-     Upper critical temperature (point) A₃ is the temperature, below which ferrite starts to form as a result of ejection from austenite in the hypo-eutectoid alloys.

-     Upper critical temperature (point) A_CM is the temperature, below which cementite starts to form as a result of ejection from austenite in the hyper-eutectoid alloys.

- Lower critical temperature (point) A₁ is the temperature of the austenite-to-Pearlite eutectoid transformation. Below this temperature austenite does not exist.

- Magnetic transformation temperature A₂ is the temperature below which α-ferrite is ferromagnetic.