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Callister - An introduction - 8th edition

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328 • Chapter 9 / Phase Diagrams<br />

Figure 9.33<br />

Photomicrograph of<br />

a 1.4 wt% C steel<br />

having a<br />

microstructure<br />

consisting of a white<br />

proeutectoid<br />

cementite network<br />

surrounding the<br />

pearlite colonies.<br />

1000. (Copyright<br />

1971 by United<br />

States Steel<br />

Corporation.)<br />

Pearlite<br />

Proeutectoid<br />

cementite<br />

20 m<br />

wt% C.Thus, for an alloy having composition C¿ 1 in Figure 9.31, fractions of pearlite<br />

W p and proeutectoid cementite W Fe3 C are determined from the following lever rule<br />

expressions:<br />

and<br />

W p <br />

W Fe3 C¿ <br />

X<br />

V X 6.70 C¿ 1<br />

6.70 0.76 6.70 C¿ 1<br />

5.94<br />

V<br />

V X C¿ 1 0.76<br />

6.70 0.76 C¿ 1 0.76<br />

5.94<br />

(9.22)<br />

(9.23)<br />

Concept Check 9.8<br />

Briefly explain why a proeutectoid phase (ferrite or cementite) forms along austenite<br />

grain boundaries. Hint: Consult Section 4.6.<br />

[The answer may be found at www.wiley.com/college/callister (Student Companion Site).]<br />

EXAMPLE PROBLEM 9.4<br />

Determination of Relative Amounts of Ferrite, Cementite,<br />

and Pearlite Microconstituents<br />

For a 99.65 wt% Fe–0.35 wt% C alloy at a temperature just below the eutectoid,<br />

determine the following:<br />

(a) The fractions of total ferrite and cementite phases<br />

(b) The fractions of the proeutectoid ferrite and pearlite<br />

(c) The fraction of eutectoid ferrite

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