Page 133 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Chapter 4 Metal Alloys: Their Structure and Strengthening by Heat Treatment

grain growth (Section 1.7) are examples of thermal treatment, involving changes in
the grain structure of the alloy.
This section focuses on the microstructural changes in the iron-carbon system.
Because of their technological significance, the structures considered are pearlite,
spheroidite, bainite, martensite, and tempered martensite. The heat-treatment
processes described are annealing, quenching, and tempering.

Pearlite. If the ferrite and cementite lamellae in the pearlite structure of the eutec-
toid steel shown in Fig. 4.1 1 are thin and closely packed, the microstructure is called
fine pearlite; if they are thick and widely spaced, it is called coarse pearlite. The
difference between the two depends on the rate of cooling through the eutectoid
temperature, which is the site of a reaction in which austenite is transformed
into pearlite. If the rate of cooling is relatively high (as it is in air), fine pearlite is
produced; if cooling is slow (as it is in a furnace), coarse pearlite is produced.

Spheroidite. When pearlite is heated to just below the eutectoid temperature and
then held at that temperature for a period of time (subcritical annealing, Section 4.1 1),
such as for a day at 700°C, the cementite lamellae transform to roughly spherical
shapes (Fig. 4.14). Unlike the lamellar shapes of cementite, which act as stress rais-
ers, spheroidites (spherical particles) are less conducive to stress concentration be-
cause of their rounded shapes. Consequently, this structure has higher toughness
and lower hardness than the pearlite structure. In this form, it can be cold worked,
because the ductile ferrite has high toughness and the spheroidal carbide particles
prevent the initiation of cracks within the material.

Bainite. Visible only through electron microscopy, bainite is a very fine microstruc-
ture consisting of ferrite and cementite, similar to pearlite, but having a different
morphology. Bainite can be produced in steels with alloying elements and at cooling
rates that are higher than those required for transformation to pearlite. This struc-
ture, called bainitic steel (after E.C. Bain, 1891-1971 ), is generally stronger and more
ductile than pearlitic steels at the same hardness level.

Martensite. When austenite is cooled at a high rate, such as by quenching it in
water, its fcc structure is transformed into a body-centered tetragonal (bct) struc-
ture. This structure can be described as a body-centered rectangular prism that is
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slightly elongated along one of its principal axes (see Fig. 4.9d). This
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Martensite transformation takes place almost instantaneously because
3.4
it involves, not the diffusion process, but a slip mechanism (and thus
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mechanism in other transformations as well.
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The material undergoes volume changes because of the differences
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austenite transforms to martensite, its volume increases (and hence its
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density decreases) by as much as 4%. A similar, but smaller, volume
expansion also occurs when austenite transforms to pearlite. These
expansions, and the thermal gradients present in a quenched part, cause
FIGURE 4.l4 Microstructure of eutectoid
steel. Spheroidite is formed by tempering internal stresses within the body. They may cause parts to undergo
distortion or even to crack during heat treatment; quench cracking of
the steel at 700°C. Magnification: 1000><.
steels is caused by rapid cooling during quenching.