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Callister - An introduction - 8th edition

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10.5 Isothermal Transformation Diagrams • 357<br />

Percent pearlite<br />

100<br />

50<br />

600°C 650°C 675°C<br />

0<br />

50<br />

Percent austenite<br />

Figure 10.12 For an<br />

iron–carbon alloy of<br />

eutectoid composition<br />

(0.76 wt% C),<br />

isothermal fraction<br />

reacted versus the<br />

logarithm of time for<br />

the austenite-topearlite<br />

transformation.<br />

0 100<br />

1 10 10 2 10 3<br />

Time (s)<br />

composition is indicated in Figure 10.12, which plots S-shaped curves of the percentage<br />

transformation versus the logarithm of time at three different temperatures.<br />

For each curve, data were collected after rapidly cooling a specimen composed of<br />

100% austenite to the temperature indicated; that temperature was maintained constant<br />

throughout the course of the reaction.<br />

A more convenient way of representing both the time and temperature dependence<br />

of this transformation is in the bottom portion of Figure 10.13. Here, the vertical<br />

and horizontal axes are, respectively, temperature and the logarithm of time.<br />

Percent of austenite<br />

transformed to pearlite<br />

Temperature (°C)<br />

100<br />

50<br />

0<br />

700<br />

600<br />

500<br />

Transformation<br />

temperature 675°C<br />

Transformation<br />

begins<br />

1 10<br />

Austenite (stable)<br />

Austenite<br />

(unstable)<br />

Figure 10.13<br />

Transformation<br />

ends<br />

10 2 10 3 10 4 10 5<br />

Time (s)<br />

Pearlite<br />

50% Completion curve<br />

Completion curve<br />

(~100% pearlite)<br />

Eutectoid temperature<br />

1400<br />

1200<br />

1000<br />

Temperature (°F)<br />

Demonstration of how<br />

an isothermal<br />

transformation<br />

diagram (bottom) is<br />

generated from<br />

percentage<br />

transformation–versus–<br />

logarithm of time<br />

measurements (top).<br />

[Adapted from H.<br />

Boyer, (Editor), Atlas<br />

of Isothermal<br />

Transformation and<br />

Cooling<br />

Transformation<br />

Diagrams, American<br />

Society for Metals,<br />

1977, p. 369.]<br />

400<br />

Begin curve<br />

(~ 0% pearlite)<br />

1 10 10 2 10 3 10 4 10 5<br />

800<br />

Time (s)

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