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When steel is heated or cooled to a certain temperature, a phase transition occurs. The temperature at which this phase transition begins or ends is called the critical temperature or critical point. Taking the iron-carbon alloy as an example, the commonly used critical temperatures seen on the iron-carbon alloy state diagram are A1, A3, Acm, etc. These critical points are summarized as follows: A1: A1 is the PSK line (723°C eutectoid temperature) in the state diagram of the iron-carbon alloy, which is called the lower critical temperature. At this temperature, the iron-carbon alloy undergoes eutectoid transformation. It is represented by Ac1 during heating and Ar1 during cooling. Because steel is affected by various conditions during heating and cooling, the eutectoid transformation cannot occur at the temperature of A1, that is, Ac1 and Ar1 appear, usually Ac1 is higher than A1; and Ar1 is lower than A1. For example, Ac1=735℃ for 40Cr steel, and Ar1=700℃. A3: A3 is the GS line (723～900℃) on the state diagram of the iron-carbon alloy, which is called the upper critical temperature. It is represented by Ac3 during heating, and when the steel is heated to Ac3 temperature, the ferrite is completely incorporated into austenite; when it is cooled, it is represented by Ar3, and then the ferrite begins to precipitate from austenite. Acm: Acm is the ES line (723～1140℃) on the state diagram of the iron-carbon alloy, which is called the upper critical temperature. It is represented by Accm during heating and Arcm during cooling, and is the temperature at which cementite precipitates from austenite. For example, Gcm15 steel has Accm=900°C and Arcm=850°C.
