Callister - An introduction - 8th edition

370 • Chapter 10 / Phase Transformations
Figure 10.28 Continuous cooling
transformation diagram for an alloy steel (type
4340) and several superimposed cooling curves
demonstrating dependence of the final
microstructure of this alloy on the
transformations that occur during cooling.
[Adapted from H. E. McGannon (Editor), The
Making, Shaping and Treating of Steel, 9th
edition, United States Steel Corporation,
Pittsburgh, 1971, p. 1096.]
Temperature (°C)
800
700
600
500
300
Eutectoid temperature
8.3°C/s
400 Critical
cooling
rate
Bainite "nose"
M (start)
Austenite
0.3°C/s
Austenite
Austenite
Martensite
0.02°C/s
Bainite
Ferrite
Austenite
Pearlite
0.006°C/s
1400
1200
1000
800
600
Temperature (°F)
200
400
M + F +
P + B
100
200
M + F
F + P
M
M + B
+ B
0
1 10 10 2 10 3 10 4 10 5 10 6
Time (s)
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.
Concept Check 10.4
Briefly describe the simplest continuous cooling heat treatment procedure that would
be used to convert a 4340 steel from (martensite bainite) to (ferrite pearlite).
[The answer may be found at www.wiley.com/college/callister (Student Companion Site).]
10.7 MECHANICAL BEHAVIOR OF
IRON–CARBON ALLOYS
We shall now discuss the mechanical behavior of iron–carbon alloys having the microstructures
discussed heretofore—namely, fine and coarse pearlite, spheroidite,
bainite, and martensite. For all but martensite, two phases are present (ferrite and cementite),
and so an opportunity is provided to explore several mechanical property–
microstructure relationships that exist for these alloys.
Pearlite
Cementite is much harder but more brittle than ferrite. Thus, increasing the fraction
of Fe 3 C in a steel alloy while holding other microstructural elements constant will result
in a harder and stronger material. This is demonstrated in Figure 10.29a, in which