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

GTBL042-11 GTBL042-Callister-v3 October 4, 2007 11:59

2nd Revised Pages

420 • Chapter 11 / Phase Transformations

Time (s)

Temperature (

°C)

Temperature (

°F)

10 –1 110102 103 104 105

400

600

800

1000

1200

A Eutectoid temperature^1400

A

A

50%

N

B

P

100

200

300

400

500

600

700

800

A

+

P

A + B

Figure 11.18

Isothermal

transformation

diagram for an

iron–carbon alloy of

eutectoid composition,

including austenite-

to-pearlite (A–P)

and austenite-to-

bainite (A–B)

transformations.

[Adapted from H.

Boyer (Editor),Atlas

of Isothermal

Transformation and

Cooling

Transformation

Diagrams, American

Society for Metals,

1977, p. 28.]

phase appears as sphere-like particles embedded in a continuousαphase matrix.

This transformation has occurred by additional carbon diffusion with no change in

the compositions or relative amounts of ferrite and cementite phases. The photomi-

crograph in Figure 11.20 shows a pearlitic steel that has partially transformed to

spheroidite. The driving force for this transformation is the reduction inα–Fe 3 C

phase boundary area. The kinetics of spheroidite formation are not included on

isothermal transformation diagrams.

Figure 11.19 Photomicrograph of a steel

having a spheroidite microstructure. The

small particles are cementite; the continuous

phase isαferrite. 1000×. (Copyright 1971 by

United States Steel Corporation.)