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Metallurgical advantages of Hot-working over cold working Processes

 Hot working :

1. Hot - Working is done at a temperature above recrystallization temperature. It can therefore be regarded as a simultaneous process of deformation and recovery.

2. Hardening due to plastic deformation is completely eliminated, by recovery and recrystallization. This is true, however, only if the rate of crystallization is higher than that of deformation.

3. Mechanical properties such as elongation, reduction of area, and impact values are improved. Ultimate tensile strength, hardness, and resistance to corrosion, are not affected by hot-working if this is properly done. 

4. Refinement of crystals occurs.

5. Breaks up brittle film of hard constituents, and promotes uniformity of  materials by facilitating diffusion of constituent alloys.

6. Cracks and blow holes are welded up.

7. Internal of residual stresses are not developed in the metal.


Hot working vs cold working

Cold Working :

1. Cold working is done at a temperature below recrystallization temperature. So no appreciable recovery can take place during deformation.

2. Hardening is not eliminated since working is done at a temperature below critical temperature and this is always accompanied by work-hardening.

3. Cold-working decreases elongation, reduction of area, and impact values, while it increases ultimate tensile strength, yield point, fatigue strength, and hardness. But resistance to corrosion is decreased by cold-working. If severely worked, yield point may coincide with ultimate tensile strength.

4. Crystallization does not occur. Grains are only elongated and /or distorted. The extent of distortion depends on the degree of cold - working.

5. Uniformity of materials is lost and the properties are affected.

6. Possibility of crack formation and its propagation is great.

7. Internal and residual stresses are developed in the metal.

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