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

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

2nd Revised Pages

416 • Chapter 11 / Phase Transformations

Time (s)

Temperature (

°C)

Percent of austenitetransformed to pearlite

Temperature (

°F)

110102 103 104

800

1000

1200

1400

105

110

Transformation

temperature 675°C

Transformation

begins

Austenite

(unstable)

Pearlite

50% Completion curve

Completion curve

(~100% pearlite)

Begin curve

(~0% pearlite)

Eutectoid temperature

Transformation

ends

103 104 105

400

500

600

700

0

50

100

Austenite (stable)

Time (s)

102

Figure 11.13

Demonstration of

how an isothermal

transformation

diagram (bottom) is

generated from

percentage

transformation-

versus-logarithm of

time measurements

(top). [Adapted from

H. Boyer, (Editor),

Atlas of Isothermal

Transformation and

Cooling

Transformation

Diagrams, American

Society for Metals,

1977, p. 369.]

50% completion (to the dashed line in Figure 11.13). That is, the shorter this time, the

higher is the rate. Thus, from Figure 11.13, at temperatures just below the eutectoid

(corresponding to just a slight degree of undercooling) very long times (on the order

of 10^5 s) are required for the 50% transformation, and therefore the reaction rate is

very slow. The transformation rate increases with decreasing temperature in such a

way that at 540◦C (1000◦F) only about 3 s is required for the reaction to go to 50%

completion.

Several constraints are imposed on using diagrams like Figure 11.13. First,

this particular plot is valid only for an iron–carbon alloy of eutectoid composi-

tion; for other compositions, the curves will have different configurations. In ad-

dition, these plots are accurate only for transformations in which the temperature

of the alloy is held constant throughout the duration of the reaction. Conditions of

constant temperature are termedisothermal; thus, plots such as Figure 11.13 are re-

isothermal ferred to asisothermal transformation diagrams,or sometimes astime–temperature–

transformation

diagram

transformation(orT–T–T) plots.

An actual isothermal heat treatment curve (ABCD) is superimposed on the

isothermal transformation diagram for a eutectoid iron–carbon alloy in Figure 11.14.

Very rapid cooling of austenite to a temperature is indicated by the near-vertical

lineAB, and the isothermal treatment at this temperature is represented by the

horizontal segmentBCD. Of course, time increases from left to right along this line.

The transformation of austenite to pearlite begins at the intersection, pointC(after

approximately 3.5 s), and has reached completion by about 15 s, corresponding to

pointD. Figure 11.14 also shows schematic microstructures at various times during

the progression of the reaction.