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

12. Creep
Porperties and Testing of Metals 121
When a metal part when is subjected to a high constant stress at high temperature for
a longer period of time, it will undergo a slow and permanent deformation (in form of a crack
which may further propagate further towards creep failure) called creep.
13. Formability
It is the property of metals which denotes the ease in its forming in to various shapes
and sizes. The different factors that affect the formability are crystal structure of metal, grain
size of metal hot and cold working, alloying element present in the parent metal. Metals with
smal1 grain size are suitable for shallow forming while metal with size are suitable for heavy
forming. Hot working increases formability. Low carbon steel possesses good formability.
14. Castability
Castability is defined as the property of metal, which indicates the ease with it can be
casted into different shapes and sizes. Cast iron, aluminium and brass are possessing good
castability.
15. Weldability
Weldability is defined as the property of a metal which indicates the two similar or
dissimilar metals are joined by fusion with or without the application of pressure and with
or without the use of filler metal (welding) efficiently. Metals having weldability in the
descending order are iron, steel, cast steels and stainless steels.
7.2 RECOVERY, RECRYSTALLISATION AND GRAIN GROWTH
When metal is subjected to hot working and cold working processes, plastic deformation
occurs which is an important phenomenon. Plastic deformation of metal distorts the crystal
lattice. It breaks up the blocks of initial equiaxed grains to produce fibrous structure and
increases the energy level of metal. Deformed metal, during comparison with its un-deformed
state, is in non-equilibrium, thermodynamically unstable state. Therefore, spontaneous
processes occur in strain-hardened metal, even at room temperature that brings it into a
more stable condition. When the temperature of metal is increased, the metal attempts to
approach equilibrium through three processes: (i) recovery, (ii) recrystallisation, and (iii) grain
growth. Fig.7.1 reflects the recovery, recrystallisation and grain growth and the main property
changes in each region.
7.2.1 Recovery
When a strain-hardened metal is heated to a low temperature, the elastic distortions of the
crystal lattice are reduced due to the increase in amplitude of thermal oscillation of the
atoms. This heating will decrease the strength of the strain-hardened metal but there is an
increase in the elastic limit and ductility of metal, though they will not react the values
possessed by the initial material before strain-hardening. No changes in microstructure of
metal are observed in this period. The partial restoration of the original characteristics,
produced by reducing the distortion of the crystal lattice without remarkable changes in
microstructure, is called recovery. At the initial state, the rate of the recovery is fastest and
it drops off at longer times at given temperature. Hence the amount of recovery that occurs
in a practical time increases with increasing temperature. The individual characteristic recover
at different rates and gain various degrees of completion in a given cold worked metal.