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

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11.7 Mechanical Behavior of Iron–Carbon Alloys • 433

Transformation temperature (°C)

Brinell hardness number Tensile strength (MPa)

200 300 400 500 600 700 800

500

0

1000

1500

2000

0

100

200

300

400

500

600 Bainite Pearlite

Figure 11.32 Brinell hardness and tensile strength (at room temperature) as a function of

isothermal transformation temperature for an iron–carbon alloy of eutectoid composition,

taken over the temperature range at which bainitic and pearlitic microstructures form.

(Adapted from E. S. Davenport, “Isothermal Transformation in Steels,”Trans.ASM, 27 ,

1939, p. 847. Reprinted by permission of ASM International.)

form (consistent with the isothermal transformation diagram for this alloy, Figure

11.18) are noted at the top of Figure 11.32.

Martensite

Of the various microstructures that may be produced for a given steel alloy, marten-

site is the hardest and strongest and, in addition, the most brittle; it has, in fact,

negligible ductility. Its hardness is dependent on the carbon content up to about

0.6 wt%, as demonstrated in Figure 11.33, which plots the hardness of martensite

and fine pearlite as a function of weight percent carbon (top and bottom curves). In

Composition (wt% C)

Brinell hardness number

Rockwell hardness, HRC

0 0.2 0.4 0.6 0.8 1.0

20

30

40

50

60

65

0

100

200

300

400

500

600

700

0369

Percent Fe 3 C

12 15

Martensite

Tempered martensite

(tempered at 371°C)

Fine pearlite

Figure 11.33 Hardness (at

room temperature) as a

function of carbon

concentration for plain

carbon martensitic, tempered

martensitic [tempered at

371 ◦C (700◦F)], and pearlitic

steels. (Adapted from Edgar

C. Bain,Functions of the

Alloying Elements in Steel,

American Society for Metals,

1939, p. 36; and R. A. Grange,

C. R. Hribal, and L. F. Porter,

Metall. Trans.A, Vol. 8A,

p. 1776.)