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

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

2nd Revised Pages

428 • Chapter 11 / Phase Transformations

Time (s)

Temperature (

°C)

Temperature (

°F)

10 –1 110102 103 104 105

200

400

800

600

1000

1200

1400

Eutectoid temperature

M(start)

A

B

M(50%)

M(90%)

Denotes a

transformation

during cooling

100

0

200

300

400

Coarse

pearlite

Fine

pearlite

Moderately

rapid cooling

curve

(normalizing)

Slow cooling

curve

(full anneal)

500

600

700

800

Pearlite

Austenite

Figure 11.27

Moderately rapid

and slow cooling

curves superimposed

on a continuous

cooling

transformation

diagram for a

eutectoid

iron–carbon alloy.

sections. Figure 11.29 shows the continuous cooling transformation diagram for the

same alloy steel for which the isothermal transformation diagram is presented in Fig-

ure 11.24. The presence of the bainite nose accounts for the possibility of formation of

bainite for a continuous cooling heat treatment. Several cooling curves superimposed

on Figure 11.29 indicate the critical cooling rate, and also how the transformation

behavior and final microstructure are influenced by the rate of cooling.

Interestingly enough, the critical cooling rate is diminished even by the presence

of carbon. In fact, iron–carbon alloys containing less than about 0.25 wt% carbon

are not normally heat treated to form martensite because quenching rates too rapid

to be practical are required. Other alloying elements that are particularly effective

in rendering steels heat treatable are chromium, nickel, molybdenum, manganese,

silicon, and tungsten; however, these elements must be in solid solution with the

austenite at the time of quenching.

In summary, isothermal and continuous cooling transformation diagrams are,

in a sense, phase diagrams in which the parameter of time is introduced. Each is

experimentally determined for an alloy of specified composition, the variables being

temperature and time. These diagrams allow prediction of the microstructure after

some time period for constant temperature and continuous cooling heat treatments,

respectively.