Hot working and cold working of metals

HOT WORKING AND COLD WORKING OF METALS

The metal forming or working processes are mainly classified into hot working and cold working processes. The above division is on the basis of working temperature.

1. Principle of Hot Working

Deforming the material at a temperature higher or equal to the recrystallization temperature of the work metal but below the melting point is called hot working. It may also be defined as the plastic deformation of metals and alloys under the conditions of temperature and strain rate. The recrystallization temperature is defined as the "minimum temperature at which the complete recrystallization of a metal takes place within a specified time”.

Being above the recrystallization temperature, the material is recrystallized during deformation. It is important because recrystallization keeps the materials from strain hardening which ultimately keeps the yield strength, hardness low and ductility high.

Some metals such as lead and tin have low recrystallization temperature (normally below room temperature) so can be hot worked at room temperature only. Hence lead would almost always be hot-worked. But most of the metals have high recrystallization temperature. 

Figure 3.1 shows the hot working of metal above the recrystallization temperature for steel.

In hot working, the refinement of grain size occurs. Thus, mechanical properties are improved. Even a brittle material can be hot worked. It requires much less force for deformation but the finally formed surface finish and dimensional accuracy are not good. There is no work hardening.

1. Hot working Operations

The important hot working operations are as follows:

1. Hot forging

a) Hammer forging

b) Drop forging

c) Upset forging

d) Press forging

e) Roll forging.

2. Hot Rolling

3. Hot extrusion

4. Drawing

5. Swaging

6. Hot spinning.

2. Advantages and Disadvantages of Hot working Process

Advantages of hot working:

1. Force requirement is less when compared to cold working process for making the required shape.

2. As grain structure is refined, toughness, ductility and resistance can be improved.

3. Since large deformation is obtained, it is a quick and economical process.

4. Porosity is considerably minimised and density of the metal is increased.

5. This process is more suitable for all metals.

Disadvantages of hot working:

1. Surface finish may be poor due to oxidation and scaling.

2. Close tolerances and automation cannot be achieved due to high working temperature.

3. Tooling and handling cost are high.

4. Sheets and wires cannot be produced.

5. On account of the loss of carbon from the surface of the steel workpiece, the surface layer loses its strength. It may give rise to a fatigue crack.

2. Principle of Cold Working

Deforming the material at a temperature lower than the recrystallization temperature of the work metal is called cold working. The recrystallization temperature is about one half of the absolute melting temperature but generally cold working is carried out only at room temperature. In cold working process, greater pressure is required than the required pressure in hot working. As the metal is in a more rigid state, it is not permanently deform until the stress exceeds the elastic limit.

In cold working process, both strength and hardness increase due to strain hardening but the ductility decreases: Good surface finish and high dimensional accuracy are achieved. If cold working is beyond certain limits, the metal will fracture before reaching the desired shape and size. Usually, cold working operations are performed in many steps with intermediate annealing operation.

1. Cold Working Operations

2. Advantages and Disadvantages of Cold working Process

Advantages of cold working:

1. There is no heating required.

2. Better surface finish can be being obtained.

3. This process provides higher dimensional accuracy.

4. Thin material can be obtained.

5. Physical properties can be improved.

6. It is more suitable for mass production.

Limitations of cold working:

1. Higher forces are required for deformation.

2. Heavier and more powerful equipment are required.

3. Metal surfaces must be clean and scale-free.

4. Stress formation in the metal during cold working is higher.

5. This process may produce undesirable residual stresses.

3. Comparison between Hot Working and Cold Working