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

GTBL042-13 GTBL042-Callister-v2 August 29, 2007 8:52

524 • Chapter 13 / Types and Applications of Materials

2.1 wt% C

Temperature (

°F)

Temperature (

°C)

Composition (wt% C)

Composition (at% C)

1600

1200

1000

2500

2000

1500

1000

800

600

(^40001234)
0 5 10 15 98
90 100
Graphite
Liquid Liquid

• Graphite
  1153 °C
  4.2 wt% C
  740 °C
  0.65 wt% C
  (Austenite)
  + Graphite
  (Ferrite) + Graphite
  + L
  1400
  Figure 13.2 The
  true equilibrium
  iron–carbon phase
  diagram with
  graphite instead of
  cementite as a stable
  phase. [Adapted
  fromBinary Alloy
  Phase Diagrams,
  T. B. Massalski,
  (Editor-in-Chief),

1990. Reprinted by
      permission of ASM
      International,
      Materials Park, OH.]

and 2350◦F), which is considerably lower than for steels. Thus, they are easily melted

and amenable to casting. Furthermore, some cast irons are very brittle, and casting

is the most convenient fabrication technique.

Cementite (Fe 3 C) is a metastable compound, and under some circumstances it

can be made to dissociate or decompose to formαferrite and graphite, according to

the reaction

Decomposition of

iron carbide to form

αferrite and graphite

Fe 3 C→3Fe (α)+C (graphite) (13.1)

Thus, the true equilibrium diagram for iron and carbon is not that presented

in Figure 10.28, but rather as shown in Figure 13.2. The two diagrams are virtually

identical on the iron-rich side (e.g., eutectic and eutectoid temperatures for the Fe–

Fe 3 C system are 1147 and 727◦C, respectively, as compared to 1153 and 740◦C for

Fe–C); however, Figure 13.2 extends to 100 wt% carbon such that graphite is the

carbon-rich phase, instead of cementite at 6.70 wt% C (Figure 10.28).

This tendency to form graphite is regulated by the composition and rate of cool-

ing. Graphite formation is promoted by the presence of silicon in concentrations

greater than about 1 wt%. Also, slower cooling rates during solidification favor

graphitization (the formation of graphite). For most cast irons, the carbon exists

as graphite, and both microstructure and mechanical behavior depend on composi-

tion and heat treatment. The most common cast iron types are gray, nodular, white,

malleable, and compacted graphite.

Gray Iron

gray cast iron The carbon and silicon contents ofgray cast ironsvary between 2.5 and 4.0 wt% and

1.0 and 3.0 wt%, respectively. For most of these cast irons, the graphite exists in the

form of flakes (similar to corn flakes), which are normally surrounded by anα-ferrite