The temperature at which a change in phase occurs. The term is sometimes used to denote the limiting temperature of a transformation range. The following symbols are used for iron and steels: . Ac(cm) In hypereutectoid steel, the temperature at which the solution of cementite in austentite 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. . Ae(cm) Ae1 Ae3 Ae4 The temperatures of phase changes at equilibrium. . Ar(cm) 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 austentie during cooling. . M(s) (or Ar) The temperature at which transformation of austenite to martensite starts during cooling. . M(f) The temperature at which martensite formation finishes during cooling. . NOTE: All these changes except the formation of martensite occur at lower temperatures during cooling than during heating, and depend on the rate of change of temperature.The temperature at which a change in phase occurs. The term is sometimes used to denote the limiting temperature of a transformation range. The following symbols are used: . -A1-The temperature of the eutectoid transformation. . -A3- The temperature at which pro-eutectiod ferrite begins to separate from austenite under conditions of slow cooling. . -Acm- The temperature at which pro-eutectoid cementite begins to separate from austenite under conditions of slow cooling. . M(f)- The temperature at which transformation of austenite to martensite finishes during cooling. . M(s)-The temperature at which transformation of austenite to martensite starts during cooling.The temperature at which transformation occurs. The term is sometimes used to denote the limiting temperature of a transformation range.

This definition is in context to Metal. See more contextual defintions for Transformation Temperature.