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Recrystallization
and Grain Growth
When materials are altered in shape by plastic
deformation their grain shape tends to change. This is illustrated below
for a wire being "drawn" into a dimension suitable for a bicycle
spoke. While passing through the drawing die the material decreases in cross
section and is elongated (material volume is conserved). This process also
work hardens the material and makes it less ductile. Heat treatment is used
to counter these effects.
By heating the steel wire to 650 C, the process
of recrystallization occurs under the influence of the strain fiends produced
during deformation. Small approximately equiaxed grains are produced
from the elongated grains of the deformed material by this stage of heat treatment.
The grain size of the material is then controlled by heating it at 1000
C where diffusion processes are more rapid. These processes cause the growth
of large grains and the dissolution of small grains. Time and temperature
are used to control the final grain size and achieve the desired mechanical
properties: yield stress, hardness, ductility and toughness. The graph below
shows the variation in hardness of the spoke. |
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