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

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

2nd Revised Pages

11.5 Isothermal Transformation Diagrams • 425

Time (s)

Temperature (

°C)

Temperature (

°F)

10 –1 110102 103 104 105

200

400

800

600

1000

1200

1400

Eutectoid temperature

A

A

P

B

(c)

(c)

(a)

(b)

A

M(start)

M(50%)

M(90%)

100

0

200

300

400

500

600

700

800

A + B

P + A

(b)

100%

Martensite

(c)

50% Pearlite

50% Bainite

(a)

100%

Bainite

Figure 11.25 Isothermal transformation diagram for an iron–carbon alloy

of eutectoid composition and the isothermal heat treatments (a), (b), and

(c) in Example Problem 11.2.

(c)Rapidly cool to 650◦C (1200◦F), hold for 20 s, rapidly cool to 400◦C (750◦F),

hold for 10^3 s, and quench to room temperature.

Solution

The time–temperature paths for all three treatments are shown in Figure 11.25.

In each case the initial cooling is rapid enough to prevent any transformation

from occurring.

(a)At 350◦C austenite isothermally transforms to bainite; this reaction begins

after about 10 s and reaches completion at about 500 s elapsed time. There-

fore, by 10^4 s, as stipulated in this problem, 100% of the specimen is bainite,

and no further transformation is possible, even though the final quenching

line passes through the martensite region of the diagram.

(b)In this case it takes about 150 s at 250◦C for the bainite transformation to

begin, so that at 100 s the specimen is still 100% austenite. As the specimen

is cooled through the martensite region, beginning at about 215◦C, pro-

gressively more of the austenite instantaneously transforms to martensite.