The temperature at which a given reaction or transformation completes during heating, or begins during cooling. In the iron-carbon phase diagram, the upper transformation temperature is indicated by the A3 and Acm lines. During heating, it marks the point at which austenite and ferrite, or austenite and cementite complete their transformation and become pure austenite. During cooling, this temperature indicates the point at which pure austenite begins to transform into a mixture of austenite and ferrite or austenite and cementite.
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