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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.

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  • $\begingroup$ Phase diagrams map the system at equilibrium. TTT diagrams map the kinetics to get to equilibrium. $\endgroup$ Nov 26 '18 at 20:52
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Somewhere on your IRON -carbon diagram is the word "equilibrium" ; It describes equilibrium conditions. TTT curves are sort of the antithesis of equilibrium ; The right hand boundary is static conditions. But the route or path taken to get to the right boundary makes a substantial difference in what you have when you get there . Although they have some common features these two diagrams do not work together. About one year of metallurgical education is needed to understand them.

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