Callister - An introduction - 8th edition

10.6 Continuous Cooling Transformation Diagrams • 369
800
700
600
Austenite
Eutectoid temperature
Pearlite
1400
1200
Figure 10.27 Continuous cooling
transformation diagram for a eutectoid
iron–carbon alloy and superimposed cooling
curves, demonstrating the dependence of the
final microstructure on the transformations
that occur during cooling.
1000
500
Temperature (°C)
400
300
Critical
cooling
rate
140°C/s
35°C/s
800
600
Temperature (°F)
200
M (start)
400
100
Martensite
200
Martensite
+
Pearlite
Pearlite
0
10 –1 1 10 10 2 10 3 10 4 10 5
Time (s)
For the continuous cooling of a steel alloy, there exists a critical quenching rate,
which represents the minimum rate of quenching that will produce a totally martensitic
structure. This critical cooling rate, when included on the continuous transformation
diagram, will just miss the nose at which the pearlite transformation begins,
as illustrated in Figure 10.27. As the figure also shows, only martensite will exist for
quenching rates greater than the critical; in addition, there will be a range of rates
over which both pearlite and martensite are produced. Finally, a totally pearlitic
structure develops for low cooling rates.
Carbon and other alloying elements also shift the pearlite (as well as the proeutectoid
phase) and bainite noses to longer times, thus decreasing the critical cooling
rate. In fact, one of the reasons for alloying steels is to facilitate the formation of
martensite so that totally martensitic structures can develop in relatively thick cross
sections. Figure 10.28 shows the continuous cooling transformation diagram for the
same alloy steel for which the isothermal transformation diagram is presented in Figure
10.23. 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 10.28 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.