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

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

2nd Revised Pages

11.5 Isothermal Transformation Diagrams • 419

Martensite

Cementite

Ferrite

Figure 11.17 Transmission electron micrograph showing the structure of bainite. A grain of

bainite passes from lower left to upper right-hand corners, which consists of elongated and

needle-shaped particles of Fe 3 C within a ferrite matrix. The phase surrounding the bainite is

martensite. (Reproduced with permission fromMetals Handbook, 8th edition, Vol. 8,

Metallography, Structures and Phase Diagrams, American Society for Metals, Materials

Park, OH, 1973.)

this micrograph have been labeled. In addition, the phase that surrounds the needle

is martensite, the topic to which a subsequent section is addressed. Furthermore, no

proeutectoid phase forms with bainite.

The time–temperature dependence of the bainite transformation may also be

represented on the isothermal transformation diagram. It occurs at temperatures

below those at which pearlite forms; begin-, end-, and half-reaction curves are just

extensions of those for the pearlitic transformation, as shown in Figure 11.18, the

isothermal transformation diagram for an iron–carbon alloy of eutectoid composition

that has been extended to lower temperatures. All three curves are C-shaped and

have a “nose” at pointN, where the rate of transformation is a maximum. As may

be noted, whereas pearlite forms above the nose [i.e., over the temperature range of

about 540 to 727◦C (1000 to 1341◦F)], at temperatures between about 215 and 540◦C

(420 and 1000◦F), bainite is the transformation product.

It should also be noted that pearlitic and bainitic transformations are really

competitive with each other, and once some portion of an alloy has transformed to

either pearlite or bainite, transformation to the other microconstituent is not possible

without reheating to form austenite.

Spheroidite

If a steel alloy having either pearlitic or bainitic microstructures is heated to, and left

at, a temperature below the eutectoid for a sufficiently long period of time—for ex-

ample, at about 700◦C (1300◦F) for between 18 and 24 h—yet another microstructure

spheroidite will form. It is calledspheroidite(Figure 11.19). Instead of the alternating ferrite and

cementite lamellae (pearlite), or the microstructure observed for bainite, the Fe 3 C