Mechanical working is a process of shaping of metals by plastic deformation. When a metal is subjected to external force beyond yield strength but less than fracture strength of the metal, metal is deformed by slip or twin formation. There are two types of mechanical working process: cold working and hot working.
When metal is deformed between room temperature and recrystallization temperature, it is called cold working. This process is suitable for highly ductile metal. Recrystallization temperature is the temperature at which the crystal structure of the metal starts to change. For low ductility, hot working is used. When metal is deformed between recrystallization temperature and melting point of the metal, it is called hot working. In this process, crystal structure of the metal is not deformed or distorted but it is rearranged. The comparison between cold working and hot working is shown in Table 21.1.
Table 21.1 Comparison Between Cold Working and Hot Working
| Cold working | Hot working |
|---|---|
| Advantages | |
| No oxidation or scaling occurs on work surface.Surface defects are removed.Good surface finish.High dimensional accuracy.Heavy work hardening occurs and so the inherent strength of the material is permanently increased. | Due to low strength of materials at high temperature power requirement is low.Very large workpiece can be deformed with equipment of reasonable size.Due to high ductility at high temperature and absence of work hardening, large deformations can be undertaken in a single stage and complex parts can be fabricated.Inter stage annealing and stress relieving are not required.Blow holes and porosities are eliminated by welding action at high temperature and pressure. |
| Disadvantages | |
| Large force is required for deformation, therefore high capacity machine is used.Several stage of deformation with inter stage annealing is required.Complexity of shapes that can be readily produced is limited.Secondary stress relieving heat treatment is required.Only ductile material is cold worked. | Poor dimensional accuracy and surface finish, material loss due to oxidation and scaling.Thin parts cannot be produced due to loss of ductility because of high rate of loss of heat.Higher cost to heat the metal.Automation is difficult due to high working temperature.Reduces hardness and strength due to decarbonization. |
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