When steel is heated or cooled to a certain temperature, a phase transition occurs. The temperature at which the phase transition begins or ends is called a critical temperature or critical point. Taking ferrocarbon alloy as an example, A1, A3, Acm and other critical temperatures are commonly used in the state diagram of ferrocarbon alloy. The tipping point introduction is as follows: A1: A1 is iron carbon alloy state diagram of PSK line (723 ℃ eutectoid temperature), known as the lower critical temperature. At this temperature, iron - carbon alloys undergo eutectoid transformation. Ac1 is used for heating and Ar1 is used for cooling. Since steel is affected by various conditions during heating and cooling, the eutectoid transition cannot occur at A1 temperature, that is, Ac1 and Ar1 occur, and Ac1 is usually higher than A1. And Ar1 is less than A1. Such as 40 cr steel Ac1 = 735 ℃, while the Ar1 = 700 ℃. A3: A3 iron carbon alloy GS line in the state diagram (723 ~ 900 ℃), known as the critical temperature. When the steel is heated to the temperature of Ac3, the ferrite all melts into the austenite. At the time of cooling, the ferrite precipitates out from austenite. Acm: Acm is iron carbon alloy ES line in the state diagram (723 ~ 1140 ℃), called on critical temperature. Accm is used for heating and Arcm is used for cooling. It is the temperature at which the austenite begins to precipitate out the cementite. Such as GCr15 steel Accm = 900 ℃, and Arcm = 850 ℃.