Proeutectoid cementite

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. 

Ferrite that is formed directly from the decomposition of hypoeutectoid austenite during cooling, without the simultaneous formation of cementite. Also called proeutectoid ferrite. 

Steels with less than 0.76 wt% C are called h) oeutectoid steels. Before they reach the euctectoid temperature, some proeutectoid a-Fe is formed. Upon further cooling to the eutectoid temperature, the remaining 7 is transformed to pearlite, composed of euctectoid a and cementite. The final sfruefure fhen consists of proeutectoid a and pearlite. The relative amounts of each phase are defermined by the starting composition and can be found by applying fhe lever rule. 

Steels with greater than 0.76 wt% C are called h5q3ereutectoid steels. The development of their microstructure is similar to the h5 oeutectoid steels except the proeutectoid cementite forms before reaching fhe eutectoid temperature and the final solid consists of proeutectoid cementite and pearlite. 

Fractions of both total a (eutectoid and proeutectoid) and cementite are determined using the lever rule and a tie line that extends across the entirety of the a + FesC phase region, from 0.022 to 6.70 wt% C. 

Concept Check 9.9 Briefly explain why a proeutectoid phase (ferrite or cementite) forms along austenite grain boundaries. Hint Consult Section 4.6. 

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)]. 

The microstructure of an iron-carbon alloy con-O sists of proeutectoid cementite and pearlite the... 

For iron-carbon alloys of other compositions, a proeutectoid phase (either ferrite or cementite) coexists with pearhte, as discussed in Section 9.19. Thus, additional... 

Figure 10.16 Isothermal transformation diagram for a 1.13 wt% C iron-carbon alloy A, austenite C, proeutectoid cementite ...

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