Introduction to Basic Manufacturing Processes and ... - always yours

122 Introduction to Basic Manufacturing Processes and Workshop Technology
7.2.2 Recrystallisation
Formation of new equiaxed grains in the heating process of metal, instead of the oriented
fibrous structure of the deformed metal, is called recrystallisation. The process of
recrystallisation is illustrated through Fig. 7.1. The first effect of heating of metal is to form
new minute grains and these rapidly enlarge until further growth is restricted by grain
meeting another. The original system of grains go out of the picture and the new crystallized
structure is formed in the metal. Recrystallisation does not produce new structures however
it produces new grains or crystals of the same structure in the metal. It consists in having
the atoms of the deformed metal overcome the bonds of the distorted lattice, the formation
of nuclei of equiaxed grains and subsequent growth of these grains due to transfer of atoms
from deformed to un-deformed crystallites. Finer grains get refined and acquire a shape
resembling fibres. The temperature at which crystallization starts, that is new grains are
formed, is called recrystallisation temperature. Recrystallisation temperature is also
defined as that temperature at which half of the cold worked material will recrystallise in 60
minutes.
Internal
residual
stress
Strength
hardness
du ctility
Strength
Ductility
Hardness
Grain growth
Cold worked and reworked New grains
Recovery Recrystallization Grain growth
Fig. 7.1 Recovery, recrystalisation and grain growth
7.2.3 Grain Growth
On recrystallisation of metal, the grains are smaller and somewhat regular in shape. The
grains in metal will grow if the temperature is high enough or if the temperature is allowed
to exceed the minimum required for recrystallisation and this growth of grain is the result
of a tendency to return to more stable and larger state. It appears to depend primarily on
the shape of the grain. For any temperature above the recrystallization temperature, normally
there is practical maximum size at which the grains will reach equilibrium and cease to grow
significantly. However, there are certain kinds of abnormal grains growth in metal that occur
as a result of applied or residual gradients of strain due to non-uniform impurity distribution,
and which permits growing very large single grain in metal.
7.3 TESTING OF METALS
Metal testing is accomplished for the purpose of for estimating the behavior of metal under
loading (tensile, compressive, shear, tortion and impact, cyclic loading etc.) of metal and for