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

GTBL042-11 GTBL042-Callister-v3 October 4, 2007 11:59

2nd Revised Pages

432 • Chapter 11 / Phase Transformations

Composition (wt% C)

Percent Fe 3 C

Brinell hardness number

Rockwell hardness

0 0.2 0.4 0.6

Spheroidite

Coarse pearlite

Fine pearlite

0.8 1.0

80

120

160

200

240

280

320

0 3 6 9 12 15

35

HRC

30

25

20

(a)

100

HRB

90

60

70

80

Composition (wt% C)

Percent Fe 3 C

Ductility (% RA)

0 0.2 0.4 0.6

Fine pearlite

Spheroidite

Coarse pearlite

0.8 1.0

10

0

20

30

40

50

60

90

80

70

0 3 6 9 12 15

(b)

Figure 11.31 (a) Brinell and Rockwell hardness as a function of carbon concentration for

plain carbon steels having fine and coarse pearlite as well as spheroidite microstructures.

(b) Ductility (%RA) as a function of carbon concentration for plain carbon steels having

fine and coarse pearlite as well as spheroidite microstructures. [Data taken fromMetals

Handbook: Heat Treating, Vol. 4, 9th edition, V. Masseria (Managing Editor), American

Society for Metals, 1981, pp. 9 and 17.]

Alloys containing pearlitic microstructures have greater strength and hardness than

do those with spheroidite. This is demonstrated in Figure 11.31a, which compares

the hardness as a function of the weight percent carbon for spheroidite with both

of the pearlite types. This behavior is again explained in terms of reinforcement at,

and impedance to, dislocation motion across the ferrite–cementite boundaries as

discussed above. There is less boundary area per unit volume in spheroidite, and

consequently plastic deformation is not nearly as constrained, which gives rise to a

relatively soft and weak material. In fact, of all steel alloys, those that are softest and

weakest have a spheroidite microstructure.

As would be expected, spheroidized steels are extremely ductile, much more

than either fine or coarse pearlite (Figure 11.31b). In addition, they are notably tough

because any crack can encounter only a very small fraction of the brittle cementite

particles as it propagates through the ductile ferrite matrix.

Bainite

Because bainitic steels have a finer structure (i.e., smallerα-ferrite and Fe 3 C parti-

cles), they are generally stronger and harder than pearlitic ones; yet they exhibit a

desirable combination of strength and ductility. Figure 11.32 shows the influence of

transformation temperature on the tensile strength and hardness for an iron–carbon

alloy of eutectoid composition; temperature ranges over which pearlite and bainite