Callister - An introduction - 8th edition

The Iron–Carbon System
9.18 The Iron–Iron Carbide (Fe–Fe 3 C) Phase Diagram • 319
Of all binary alloy systems, the one that is possibly the most important is that
for iron and carbon. Both steels and cast irons, primary structural materials in
every technologically advanced culture, are essentially iron–carbon alloys. This
section is devoted to a study of the phase diagram for this system and the development
of several of the possible microstructures. The relationships among
heat treatment, microstructure, and mechanical properties are explored in Chapters
10 and 11.
9.18 THE IRON–IRON CARBIDE
(Fe–Fe 3 C) PHASE DIAGRAM
A portion of the iron–carbon phase diagram is presented in Figure 9.24. Pure iron,
ferrite
austenite
upon heating, experiences two changes in crystal structure before it melts. At room
temperature the stable form, called ferrite, or -iron, has a BCC crystal structure.
Ferrite experiences a polymorphic transformation to FCC austenite, or -iron, at
912C (1674F). This austenite persists to 1394C (2541F), at which temperature
the FCC austenite reverts back to a BCC phase known as -ferrite, which finally
1600 0 5
1538°C
1493°C
Composition (at% C)
10 15 20 25
1400
1394°C
+ L
L
2500
Temperature (°C)
1200
1000
912°C
, Austenite
1147°C
2.14 4.30
+ Fe 3 C
2000
Temperature (°F)
800
+
727°C
1500
0.76
600
0.022
, Ferrite + Fe 3 C
Cementite (Fe 3 C)
1000
400
0 1 2 3 4
(Fe)
Composition (wt% C)
5 6 6.70
Figure 9.24 The iron–iron carbide phase diagram. [Adapted from Binary Alloy Phase
Diagrams, 2nd edition, Vol. 1, T. B. Massalski (Editor-in-Chief), 1990. Reprinted by
permission of ASM International, Materials Park, OH.]