Callister - An introduction - 8th edition

10.6 Continuous Cooling Transformation Diagrams • 367
Concept Check 10.3
Make a copy of the isothermal transformation diagram for an iron–carbon alloy of
eutectoid composition (Figure 10.22) and then sketch and label on this diagram a
time–temperature path that will produce 100% fine pearlite.
[The answer may be found at www.wiley.com/college/callister (Student Companion Site).]
10.6 CONTINUOUS COOLING TRANSFORMATION
DIAGRAMS
Isothermal heat treatments are not the most practical to conduct because an alloy
must be rapidly cooled to and maintained at an elevated temperature from a higher
temperature above the eutectoid. Most heat treatments for steels involve the continuous
cooling of a specimen to room temperature. An isothermal transformation
diagram is valid only for conditions of constant temperature; this diagram must be
modified for transformations that occur as the temperature is constantly changing.
For continuous cooling, the time required for a reaction to begin and end is delayed.
Thus the isothermal curves are shifted to longer times and lower temperatures,
as indicated in Figure 10.25 for an iron–carbon alloy of eutectoid composition.
Figure 10.25
Superimposition of
isothermal and
continuous cooling
transformation
diagrams for a
eutectoid
iron–carbon alloy.
[Adapted from H.
Boyer (Editor), Atlas
of Isothermal
Transformation
and Cooling
Transformation
Diagrams, American
Society for Metals,
1977, p. 376.]
Temperature (°C)
800
700
600
500
400
300
Austenite
Pearlite
Eutectoid temperature
Continuous cooling transformation
1400
1200
1000
800
600
Temperature (°F)
200
M(start)
M(50%)
M(90%)
400
100
200
0
10 –1 1 10 10 2 10 3 10 4 10 5
Time (s)