Hot working and cold working

Hot working process

It is a process in which the material is heated above its re-crystallization temperature but below the melting temperature of the material. less amount of power is required for deforming the metal in hot working as compared to cold working.
Re-crystallization temperature of Fe= 450°c
But the melting temperature is approx 1500°c

Cold working

The process of forming in which the metal is heated below its re-crystallization temperature is known as cold working. It gives less finished surface compared to hot working.
E.g.- In cold working, Fe is heated below the 450°c.
Example- rolling at normal temperature,
Extrusion at normal temperature etc.

Advantage of hot working

1. No strain hardening because the working temperature is above its re-crystallization temperature
2. No chance of porosity in the material.
3. As shear stress gets reduced at a higher temperature, therefore, less force is required for defamation purposes.
4. Ductility and impact resistance are improved
5. A very large workpiece can be deformed with reasonable size equipment.
6. When course-sized grains are refined they result in a fine grain structure.
7. Further stress removal process is not required.
8. High reduction is possible without any fracture.

Disadvantage

1. Due to high-temperature cooling handling of hot metal is costly and difficult
2. Due to high temperature, there is a scaling of surface which leads to the poor surface finish of the finished product
3. Close tolerance of dimensions is not possible
4. Brittle material can't be hard work
5. Metallurgical structure of workpiece may be non-uniform.
6. example -hot rolling,hot extrusion,hot forging etc.

Advantages of cold working

1. Increased strength
2. Improved surface finish
3. Controlled dimensional tolerance and concentricity
4. Improved straightness
5. Improved machinability

Disadvantages of cold working-

1. Higher forces are required to initiate and complete the deformation
2. Less ductility is available
3. Intermediate anneals may be required to compensate for the loss of ductility that accompanies strain hardening
4.  Heavier and more powerful equipment is required
5.  Metal surfaces must be clean and scale-free
6.  Imparted directional properties may be detrimental
7.  Undesirable residual stresses may be produced