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In the process of heating or cooling the carbon steel, the orgnization of PSK line,GS line and ES line changes. For example, when the point S cooled to the temperature of S then A>P. When heated to the temperature of S, then P>S. Since in the process of heating, PSK line,GS line and ES line are very important, we will call them A1 line, A3 line and Acm line. During the process of heat treatment, whether these three lines are cooled or heated, the intersection between these three points is the eqilibrium critical point.

 

According to the equilibrium diagram of iron carbon, the eutectic steel is converted to austenite when it is heated over the temperature of A1.

 

Eutectoid and Eutectic steel are heated to A3 and Acm above to obtain single phase austenite.

 

The actual critical temperature at the time of heating is usually marked with the letter "c", such as Ac1, Ac3, Accm.

 

The actual critical temperature when cooling is marked with the letter "r", such as Ar1, Ar3, Arcm and so on.

 

Physical Significance:

 

Ac1: the temperature at which the pearlite changes to austenite during heating;

 

Ar1: the temperature at which austenite changes to pearlite during cooling;

 

Ac3: all the eutectoid ferrite is converted into the final temperature of the austenite when heated;

 

Ar3: the starting temperature of austenite transition to ferrite during cooling;

 

Accm: the final temperature of the secondary cementite into the austenite when heated;

 

Arcm: The temperature of the secondary cementite is precipitated from the austenite when cooled.

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