Page 72 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 1.7 Recovery, Recrystallization, and Grain Growth 5|

the direction of the tensile force. By contrast, slip planes under compression tend
to align themselves in a direction perpendicular to the direction of the compressive
force.

Mechanical Fibering. This is a type of anistropy that results from the alignment of
inclusions (stringers), impurities, and voids in the metal during deformation. Note
that if the spherical grains in Fig. 1.11a were coated with impurities, these impurities
would align themselves in a generally horizontal direction after deformation.
Because impurities weaken the grain boundaries, this piece of metal will now be
weaker and less ductile when tested in the vertical direction. As an analogy, consider
lowers the strength, and raises the ductility of the
plywood, which is strong in tension along its planar direction, but peels off (splits)
easily when pulled in tension in its thickness direction.

|.7 Recovery, Recrystallization, and Grain Growth

We have seen that plastic deformation at room temperature causes distortion of the
grains and grain boundaries (leading to anisotropic behavior), a general increase in
strength, and a decrease in ductility. These effects can be reversed, and the properties
of the metal can be brought back to their original levels, by heating the metal to a
specific temperature range for a given period of time-a process called annealing
(described in detail in Section 4.11). Three events take place consecutively during
the heating process:
l. Recovery. During recovery, which occurs at a certain temperature range below
the recrystallization temperature of the metal (described next), the stresses in the
highly deformed regions of the metal piece are relieved. Subgrain boundaries
begin to form (a process called polygonization), with
no significant change in mechanical properties such as
hardness and strength (Fig. 1.13).
2. Recrystallization. This is the process in which, within a Residual
certain temperature range, new equiaxed and strain-free stresses
grains are formed, replacing the older grains. The tem-
perature required for recrystallization ranges approxi-
mately between 0.3T,,, and 0.5 Tm, where Tm is the
melting point of the metal on the absolute scale.
Strength, Strength Ductmty
Generally, the recrystallization temperature is
hardness,
defined as the temperature at which complete recrys- ductility Hardness
tallization occurs within approximately one hour.
Cold-worked I
Recrystallization decreases the density of dislocations, and recovered New grains
size I
metal (Fig. 1.13). Lead, tin, cadmium, and zinc recrys- Grain fe ear ` 'T' "
it
tallize at about room temperature; consequently, they T "i”` "`
do not work harden when cold worked. Recovery iFIecrysta|-i Grain
The recrystallization temperature depends on Iizatiorl growth
4
the degree of prior cold work (work hardening): The
Temperature
more the cold work, the lower the temperature re-
quired for recrystallization. The reason is that, as the
FIGURE l.l3 Schematic illustration of the effects of
amount of cold work increases, the number of dislo-
recovery, recrystallization, and grain growth on
cations and the amount of energy stored in dislocations mechanical properties and on the shape and size of grains.
(stored energy) also increase. This energy supplies Note the formation of small new grains during recrystal-
some of the work required for recrystallization. lization. Source: After G. Sachs.