Ferrite eutectoid/proeutectoid

The effect of this extrapolation can be seen in isothermal diagrams. Figure 7.8 is the isothermal transformation diagram for a 1050 steel. Note that proeutectoid ferrite must form before cementite for transformation temperatures above about 600 °C in accordance with Figure 7.8. The agreement is not perfect because in addition to 0.50% C, the 1050 steel contains 0.91 % Mn, which lowers the eutectoid temperature and composition. 

The microstructural product of an iron-carbon alloy of eutectoid composition is pearlite, a microconstituent consisting of alternating layers of ferrite and cementite. The microstructures of alloys having carbon contents less than the eutectoid (i.e., hy-poeutectoid alloys) are composed of a proeutectoid ferrite phase in addition to pearlite. Pearlite and proeutectoid cementite constitute the microconstituents for hypereutec-toid alloys—those with carbon contents in excess of the eutectoid composition. 

Mass fractions of a proeutectoid phase (ferrite or cementite) and pearlite may be computed using the lever rule and a tie line that extends to the eutectoid composition (0.76 wt% C) [e.g., Equations 9.20 and 9.21 (for hypoeutectoid alloys) and Equations 9.22 and 9.23 (for hypereutectoid alloys)]. 

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