Page 99 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Chapter 2 Mechanical Behavior, Testing, and Manufacturing Properties of Materials
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voids
Strong direction
Void
~ l
Matrix ._ _,i , _
Inclusion Q or 5 5 H g. Weak direction
Softinclusion Hard inclusion Hard inclusion
of deformed metal
(a) Before deformation (b) After deformation

FIGURE 2.24 Schematic illustration of the deformation of soft and hard inclusions and of
their effect on void formation in plastic deformation. Note that, because they do not
conform to the overall deformation of the ductile matrix, hard inclusions can cause internal
voids.

metalworking processes such as drawing and extrusion (Chapter 15). Two factors
affect void formation:
a. The strength of the bond at the interface between an inclusion and the matrix.
If the bond is strong, there is less tendency for void formation during plastic
deformation.
b. The hardness of the inclusion. If the inclusion is soft, such as one of manganese
sulfide, it will conform to the overall shape change of the workpiece during
plastic deformation. If the inclusion is hard (as, for example, in carbides and
oxides-see also Section 8.2), it could lead to void formation (Fig. 2.24). Hard
inclusions, because of their brittle nature, may also break up into smaller
particles during deformation.
The alignment of inclusions during plastic deformation leads to mechanical
fibering (Section 1.5). Subsequent processing of such a material must, therefore,
involve considerations of the proper direction of working for maximum ductility
and strength.

Transition Temperature. Many metals undergo a sharp change in ductility and
toughness across a narrow temperature range called the transition temperature
(Fig. 2.25). This phenomenon occurs mostly in body-centered cubic, and in some
hexagonal close-packed, metals; it is rarely exhibited by face-centered cubic
metals. The transition temperature depends on such factors as the composition,
microstructure, grain size, surface finish, and shape of the specimen, and the de-
formation rate. High rates, abrupt changes in workpiece shape, and the pres-
ence of surface notches raise the transition temperature.

Strain Aging. Strain aging is a phenomenon in which carbon atoms in steels
Transition
temperature segregate to dislocations, thereby pinning the dislocations and, in this way,
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increasing the resistance to their movement; the result is increased strength and
Temperature --> reduced ductility. Instead of taking place over several days at room temperature,
this phenomenon can occur in just a few hours at a higher temperature; it is
then called accelerated strain aging. An example of accelerated strain aging in
steels is blue brittleness, so named because it occurs in the blue-heat range,
where the steel develops a bluish oxide film. Blue brittleness causes a marked

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