The lower critical point for all steels is

Explanation

Explanation: Critical temperature: The temperature at which phase change of any substance happens. Lower critical temperature : The lower critical temperature of all steels is the same i.e. 723 °C. It is the temperature of Pearlite to Austenite transformation. Below this temperature austenite does not exist. Hypo-eutectoid steels : Carbon content from 0 – 0.83 % consists of primary ferrite (according to the curve A 3) and Pearlite. Eutectoid steel : Carbon content 0.83 % entirely consists of pearlite. Hyper-eutectoid steels : Carbon content from 0.83 – 2.06 % consists of primary cementite (according to the curve A cm ) and pearlite. Upper critical temperature (Point A 3): It 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 ): It is the temperature below which cementite starts to form as a result of ejection from austenite in the hyper-eutectoid alloys.