Page 70 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 1.5 Grains and Gram Boundanes

A grain size of 7 is generally acceptable for sheet metals for making car bodies,
appliances, and kitchen utensils (Chapter 16).

V IT =*f
EXAMPLE l.l Number of Grains in the Ball of a Ballpoint Pen

Assume that the ball of a ballpoint pen is 1 mm in 4.M3 47,-(0_5 mmf' 3
diameter and has an ASTM grain size of 10. Calculate 0 5236 mm
the number of grains in the ball. _ _
The total number of grams is calculated by multiplying
Solution A metal with an ASTM grain size of 10 has the Volume by the grams per mm or
520,000 grains per mm3. (See Table 1.1.) The volume No. grains = (0.5236 mm3) (520 000 grains/mm
of the 1-mm-diameter ball is = 272,300

l.5.2 Influence of Grain Boundaries
Grain boundaries have an important influence on the strength and ductility of
metals, and because they interfere with the movement of dislocations, grain
boundaries also influence strain hardening. These effects depend on temperature,
deformation rate, and the type and amount of impurities present along the grain
boundaries.
Because the atoms along the grain boundaries are packed less efficiently and
are more disordered, grain boundaries are more reactive than the grains them-
selves. As a result, the boundaries have lower energy than the atoms in the orderly
lattice within the grains, and thus they can be more easily removed or chemically
bonded to another atom. For example, a metal surface becomes rougher when
etched or subjected to corrosive environments. (See also end grains in forging, in
Section 14.5).
At elevated temperatures, and in metals whose properties depend on the rate at
which they are deformed, plastic deformation also takes place by means of grain-
boundary sliding. The creep mechanism (elongation under stress over time, usually at
elevated temperatures) results from grain-boundary sliding (see Section 2.8).
Grain-boundary embrittlement. When exposed to certain low-melting-point
metals, a normally ductile and strong metal can crack when subjected to very low exter-
nal stresses. Examples of such behavior are (a) aluminum wetted with a mercury-zinc
amalgam or liquid gallium, and (b) copper at elevated temperature wetted with lead
or bismuth. These added elements weaken the grain boundaries of the metal by
embrittlement. The term liquid-metal embrittlement is used to describe such phe-
nomena, because the embrittling element is in a liquid state. However, embrittlement
can also occur at temperatures well below the melting point of the embrittling element,
a phenomenon known as solid-metal embrittlement.
Hot shortness is caused by local melting of a constituent or of an impurity in
the grain boundary at a temperature below the melting point of the metal itself.
When subjected to plastic deformation at elevated temperatures (hot working), a
piece of metal crumbles along its grain boundaries; examples are antimony in cop-
per, leaded steels (Section 21.7.1), and leaded brass. To avoid hot shortness, the
metal is usually worked at a lower temperature in order to prevent softening and
melting along the grain boundaries. Another form of embrittlement is temper em-
brittlement in alloy steels, which is caused by segregation (movement) of impurities
to the grain boundaries (Section 4.11).