Callister - An introduction - 8th edition

360 • Chapter 10 / Phase Transformations
Temperature (°C)
900
800
700
600
500
A
A
A
+
C
A
+
1600
Eutectoid temperature
1400
1200
P
P
1000
1 10 10 2 10 3 10 4
Temperature (°F)
Figure 10.16
Isothermal
transformation
diagram for a 1.13
wt% C iron–carbon
alloy: A, austenite; C,
proeutectoid cementite;
P, pearlite. [Adapted
from H. Boyer (Editor),
Atlas of Isothermal
Transformation and
Cooling Transformation
Diagrams, American
Society for Metals,
1977, p. 33.]
Time (s)
For iron–carbon alloys of other compositions, a proeutectoid phase (either ferrite
or cementite) will coexist with pearlite, as discussed in Section 9.19. Thus
additional curves corresponding to a proeutectoid transformation also must be
included on the isothermal transformation diagram. A portion of one such diagram
for a 1.13 wt% C alloy is shown in Figure 10.16.
bainite
Bainite
In addition to pearlite, other microconstituents that are products of the austenitic
transformation exist; one of these is called bainite. The microstructure of bainite
consists of ferrite and cementite phases, and thus diffusional processes are involved
in its formation. Bainite forms as needles or plates, depending on the temperature
of the transformation; the microstructural details of bainite are so fine that their
resolution is possible only using electron microscopy. Figure 10.17 is an electron
micrograph that shows a grain of bainite (positioned diagonally from lower left to
Figure 10.17 Transmission electron
micrograph showing the structure of bainite.
A grain of bainite passes from lower left to
upper right corners; it consists of elongated
and needle-shaped particles of Fe 3 C within
a ferrite matrix. The phase surrounding the
bainite is martensite. (Reproduced with
permission from Metals Handbook, 8th
edition, Vol. 8, Metallography, Structures and
Phase Diagrams, American Society for
Metals, Materials Park, OH, 1973.)
Martensite
Cementite
Ferrite