Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

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10.19 The Iron–Iron Carbide (Fe–Fe 3 C) Phase Diagram • 381

Temperature (

°C)

Temperature (

°F)

01234

Composition (wt% C)

Composition (at% C)

(Fe)

5 6 6.70

1000

1500

2000

2500

400

600

800

1000

912 °C

0.76

Cementite (Fe 3 C)

727 °C

0.022

, Ferrite  + Fe 3 C

1200

1400

1600

05

1493 °C

1147 °C

2.14 4.30

L

 + L

 + Fe 3 C

1394 °C



, Austenite

1538 °C

10 15 20 25



+



Figure 10.28 The iron–iron carbide phase diagram. [Adapted fromBinary Alloy Phase

Diagrams,2nd edition, Vol. 1, T. B. Massalski (Editor-in-Chief), 1990. Reprinted by

permission of ASM International, Materials Park, OH.]

austenite Ferrite experiences a polymorphic transformation to FCCaustenite,orγiron, at

912 ◦C (1674◦F). This austenite persists to 1394◦C (2541◦F), at which temperature

the FCC austenite reverts back to a BCC phase known asδferrite, which finally

melts at 1538◦C (2800◦F). All these changes are apparent along the left vertical axis

of the phase diagram.^1

The composition axis in Figure 10.28 extends only to 6.70 wt% C; at this con-

cementite centration the intermediate compound iron carbide, orcementite(Fe 3 C), is formed,

which is represented by a vertical line on the phase diagram. Thus, the iron–carbon

system may be divided into two parts: an iron-rich portion, as in Figure 10.28, and the

other (not shown) for compositions between 6.70 and 100 wt% C (pure graphite). In

practice, all steels and cast irons have carbon contents less than 6.70 wt% C; there-

fore, we consider only the iron–iron carbide system. Figure 10.28 would be more

appropriately labeled the Fe–Fe 3 C phase diagram, since Fe 3 C is now considered

to be a component. Convention and convenience dictate that composition still be

(^1) The reader may wonder why noβphase is found on the Fe–Fe 3 C phase diagram, Figure
10.28 (consistent with theα,β,γ, etc. labeling scheme described previously). Early
investigators observed that the ferromagnetic behavior of iron disappears at 768◦C and
attributed this phenomenon to a phase transformation; the “β” label was assigned to the
high-temperature phase. Later it was discovered that this loss of magnetism did not result
from a phase transformation (see Section 18.6), and that therefore the presumedβphase did
not exist.