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

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2nd Revised Pages

11.5 Isothermal Transformation Diagrams • 417

110102 103 104 105

Time (s)

Denotes that a transformation

is occurring

Austenite → pearlite

transformation

Austenite (stable)

Eutectoid

temperature

Ferrite Coarse pearlite

Fe 3 C

Fine pearlite

800

500

600

700

1000

1200

1400

1 s 1 min 1 h 1 day

Temperature (

°C)

727 °C

Temperature (

°F)

 

 



 



A

B

C

D

Figure 11.14 Isothermal transformation diagram for a eutectoid iron–carbon alloy, with

superimposed isothermal heat treatment curve (ABCD). Microstructures before, during,

and after the austenite-to-pearlite transformation are shown. [Adapted from H. Boyer

(Editor),Atlas of Isothermal Transformation and Cooling Transformation Diagrams,

American Society for Metals, 1977, p. 28.]

The thickness ratio of the ferrite and cementite layers in pearlite is approximately

8 to 1. However, the absolute layer thickness depends on the temperature at which

the isothermal transformation is allowed to occur. At temperatures just below the

eutectoid, relatively thick layers of both theα-ferrite and Fe 3 C phases are produced;

coarse pearlite this microstructure is calledcoarse pearlite,and the region at which it forms is indi-

cated to the right of the completion curve on Figure 11.14. At these temperatures,

diffusion rates are relatively high, so that during the transformation illustrated in

Figure 10.32 carbon atoms can diffuse relatively long distances, which results in the

formation of thick lamellae. With decreasing temperature, the carbon diffusion rate

decreases, and the layers become progressively thinner. The thin-layered structure

fine pearlite produced in the vicinity of 540◦C is termedfine pearlite;this is also indicated in

Figure 11.14. To be discussed in Section 11.7 is the dependence of mechanical prop-

erties on lamellar thickness. Photomicrographs of coarse and fine pearlite for a eu-

tectoid composition are shown in Figure 11.15.

For iron–carbon alloys of other compositions, a proeutectoid phase (either ferrite

or cementite) will coexist with pearlite, as discussed in Section 10.20. Thus additional

curves corresponding to a proeutectoid transformation also must be included on the

isothermal transformation diagram. A portion of one such diagram for a 1.13 wt%

C alloy is shown in Figure 11.16.

Bainite

In addition to pearlite, other microconstituents that are products of the austenitic

bainite transformation exist; one of these is calledbainite.The microstructure of bainite