Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

GTBL042-10 GTBL042-Callister-v2 August 16, 2007 8:49

388 • Chapter 10 / Phase Diagrams

Temperature

Composition (wt % C)

0.022 0.76

6.70

+ Fe 3 C

+ Fe 3 C





TUV X

C 0 C 1

Figure 10.35 A

portion of the

Fe–Fe 3 C phase

diagram used in

computations for

relative amounts of

proeutectoid and

pearlite

microconstituents

for hypoeutectoid

(C′ 0 ) and

hypereutectoid

(C′ 1 ) compositions.

composition. For example, let us consider an alloy of compositionC′ 0 in Figure 10.35.

Thus, the fraction of pearlite,Wp, may be determined according to

Lever rule expression

for computation of

pearlite mass fraction

(compositionC′ 0 ,

Figure 10.35)

Wp=

T

T+U

=

C 0 ′− 0. 022

0. 76 − 0. 022

=

C 0 ′− 0. 022

0. 74

(10.20)

Furthermore, the fraction of proeutectoidα,Wα′, is computed as follows:

Wα′=

U

T+U

=

0. 76 −C 0 ′

0. 76 − 0. 022

=

0. 76 −C 0 ′

0. 74

(10.21)

Lever rule expression

for computation of

proeutectoid ferrite

mass fraction

Of course, fractions of both totalα(eutectoid and proeutectoid) and cementite are

determined using the lever rule and a tie line that extends across the entirety of the

α+Fe 3 C phase region, from 0.022 to 6.70 wt% C.

Hypereutectoid Alloys

hypereutectoid alloy Analogous transformations and microstructures result forhypereutectoid alloys,

those containing between 0.76 and 2.14 wt% C, which are cooled from tempera-

tures within theγphase field. Consider an alloy of compositionC 1 in Figure 10.36

that, upon cooling, moves down the linezz′. At pointgonly theγphase will be

present with a composition ofC 1 ; the microstructure will appear as shown, having

onlyγgrains. Upon cooling into theγ+Fe 3 C phase field—say, to pointh—the

cementite phase will begin to form along the initialγgrain boundaries, similar to

theαphase in Figure 10.33, pointd. This cementite is calledproeutectoid cemen-

tite—that which forms before the eutectoid reaction. Of course, the cementite com-

proeutectoid

cementite

position remains constant (6.70 wt% C) as the temperature changes. However, the

composition of the austenite phase will move along linePOtoward the eutectoid. As