The temperature at which a given reaction or transformation begins during heating, or completes during cooling. The lower transformation temperature, 1333 F, is the same for all iron-carbon alloys. In the iron-carbon phase diagram, the lower transformation temperature is indicated by the A1 line, and marks the point at which either ferrite, cementite, pearlite, or some combination thereof, begin their transformation into austenite when being heated. During cooling, the lower transformation temperature indicates the point at which austenite, or a mixture of austenite and ferrite or cementite, fully transform into ferrite, cementite, or pearlite.
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