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

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2nd Revised Pages

11.6 Continuous Cooling Transformation Diagrams • 427

Time (s)

Temperature (

°C)

Temperature (

°F)

10 –1 110102 103 104 105

200

400

800

600

1000

1200

1400

Eutectoid temperature

M(start)

M(50%)

M(90%)

100

0

200

300

400

500

600

700

800

Pearlite

Austenite Continuous cooling transformation

Figure 11.26

Superimposition of

isothermal and

continuous cooling

transformation

diagrams for a

eutectoid iron–

carbon alloy.

[Adapted from H.

Boyer (Editor),Atlas

of Isothermal

Transformation and

Cooling

Transformation

Diagrams, American

Society for Metals,

1977, p. 376.]

austenite–pearlite transformation terminates just below the nose (Figure 11.27) as

indicated by the curveAB. For any cooling curve passing throughABin Figure 11.27,

the transformation ceases at the point of intersection; with continued cooling, the

unreacted austenite begins transforming to martensite upon crossing theM(start)

line.

With regard to the representation of the martensitic transformation, theM(start),

M(50%), andM(90%) lines occur at identical temperatures for both isothermal and

continuous cooling transformation diagrams. This may be verified for an iron–carbon

alloy of eutectoid composition by comparison of Figures 11.23 and 11.26.

For the continuous cooling of a steel alloy, there exists a critical quenching rate

that represents the minimum rate of quenching that will produce a totally martensitic

structure. The critical cooling rate curve, when included on the continuous transfor-

mation diagram, will just miss the nose at which the pearlite transformation begins,

as illustrated in Figure 11.28. As the figure also shows, only martensite will exist for

quenching rates greater than the critical; in addition, there will be a range of rates

over which both pearlite and martensite are produced. Finally, a totally pearlitic

structure develops for low cooling rates.

Carbon and other alloying elements also shift the pearlite (as well as the proeu-

tectoid phase) and bainite noses to longer times, thus decreasing the critical cooling

rate. In fact, one of the reasons for alloying steels is to facilitate the formation of

martensite so that totally martensitic structures can develop in relatively thick cross