TRANSFORMATION TEMPERATURE. The temperature at which a change in phase occurs. This term is sometimes used to denote the limiting temperature of a transformation range. The following symbols are used for irons and steels:
Accm. In hypereutectoid steel, the temperature at which solution of cementite in austenite is completed during heating.
Ac1. The temperature at which austenite begins to form during heating.
Ac3. The temperature at which transformation of ferrite to austenite is completed during heating.
Ac4. The temperature at which austenite transforms to delta ferrite during heating.
Aecm, Ae1, Ae3, Ae4. The temperatures of phase changes at equilibrium.
Arcm. In hypereutectoid steel, the temperature at which precipitation of cementite starts during cooling.
Ar1. The temperature at which transformation of austenite to ferrite or to ferrite plus cementite is completed during cooling.
Ar3. The temperature at which austenite begins to transform to ferrite during cooling.
Ar4. The temperature at which delta ferrite transforms to austenite during cooling.
Ar(. The temperature at which transformation of austenite to pearlite starts during cooling.
M1. The temperature at which transformation of austenite to martensite is completed during cooling.
M1 (or Ar((). The temperature at which transformation of austenite to martensite starts during cooling.
NOTE: All these changes, except formation of martensite, occur at lower temperatures during cooling than during heating, and depend on the rate of change temperature.
