A "Carbon Phase Diagram" has an upper and a lower transformation temperatures. It was my understanding that once a steel had cooled below the lower transformation temp (1333 deg F) that the austenite had changed completely to another structure (ie martensite, pearlite, cementite etc.) However when looking at the "TTT" diagram, it shows the transformation occurring between the two "C" curves. The "C" curves are below the 1333 deg F. The "TTT" diagram also shows that the transformation from austenite does not even start until the temp is below the lower transformation temp of 1333 deg F.
Can someone help me understand.