Callister - An introduction - 8th edition

324 • Chapter 9 / Phase Diagrams
Austenite grain
boundary
Figure 9.28 Schematic
representation of the
formation of pearlite from
austenite; direction of carbon
diffusion indicated by arrows.
Cementite
(Fe 3 C)
Austenite
( )
Ferrite ( )
Ferrite ( )
Ferrite ( )
Ferrite ( )
Austenite
( )
Growth direction
of pearlite
Carbon diffusion
hypoeutectoid alloy
Hypoeutectoid Alloys
Microstructures for iron–iron carbide alloys having other than the eutectoid composition
are now explored; these are analogous to the fourth case described in
Section 9.12 and illustrated in Figure 9.16 for the eutectic system. Consider a composition
C 0 to the left of the eutectoid, between 0.022 and 0.76 wt% C; this is termed
a hypoeutectoid (less than eutectoid) alloy. Cooling an alloy of this composition is
represented by moving down the vertical line yy in Figure 9.29. At about 875C,
point c, the microstructure will consist entirely of grains of the phase, as shown
1100
1000
900
M
y
c
+ Fe 3 C
Figure 9.29 Schematic
representations of the
microstructures for an
iron–carbon alloy of
hypoeutectoid composition C 0
(containing less than 0.76 wt%
C) as it is cooled from within
the austenite phase region to
below the eutectoid
temperature.
Temperature (°C)
800
Te
700
d
e
N
f
O
Pearlite
600
Fe 3 C
500
Eutectoid
+ Fe 3 C
Proeutectoid
400
y
0 1.0
2.0
C 0
Composition (wt% C)