Mechanical Working of Metals (Hot Working and Cold Working)
Mechanical working of metals (Hot working and cold working) is a simple plastic deformation performed to change the dimensions, properties, and surface conditions with the help of mechanical pressure.
Depending upon the temperature and strain rate, mechanical working may be either a hot working or cold working, such that the recovery process takes place simultaneously with the deformation.
During deformation, the metal is said to flow, which is called as the plastic flow of the metal, and grain shapes are changed.
If the deformation is carried out at higher temperatures, the new grains start growing at the locations of internal stresses.
When the temperature is sufficiently high, the grain growth is accelerated and continues till the metal comprises entirely of new grains only.
This process of formation of new grains is called as recrystallisation and the corresponding temperature is the recrystallisation temperature of the metal.
Recrystallisation temperature is the point which differentiates hot working and cold working.
Mechanical working of metals above the recrystallisation temperature, but below the melting or burning point is known as hot working whereas; below the recrystallisation temperature, is known as cold working.
Hot Working and Cold Working
Hot working is accomplished at a temperature above the recrystallisation temperature but below the melting or the burning point of the metal, because above the melting or the burning point, the metal will burn and become unsuitable for use.
Every metal has a characteristic hot working temperature range over which hot working may be performed.
The upper limit of working temperature depends on composition of metal, prior deformation and impurities within the metal.
The changes in structure from hot working improves mechanical properties such as ductility, toughness, resistance to shock and vibration, % elongation, % reduction in area, etc.
Hot working of metals have following advantages and disadvantages:
Advantages of Hot Working
Due to hot working, no residual stresses are introduced in the metal.
Hot working refines grain structure and improves physical properties of the metal.
Any impurities in the metal are disintegrated and distributed throughout the metal.
Porosity of the metal is minimised by the hot working.
During hot working, as the metal is in plastic state, larger deformation can be accomplished and more rapidly.
Hot working produces raw material which is to be used for subsequent cold working operations.
Disadvantages of Hot Working
As hot working is carried out at high temperature, a rapid oxidation or scale formation takes place on the metal surface which leads to poor surface finish and loss of metal.
Due to the loss of carbon from the surface of the steel piece being worked, the surface layer loses its strength.
This weakening of the surface layer may give rise to fatigue crack which results in failure of the part.
Close tolerance cannot be obtained.
Hot working involves excessive expenditure on account of high tooling cost.
The working of metals at temperatures below their recrystallisation temperature is called as cold working.
Most of the cold working processes are performed at room temperature.
Unlike hot working, it distorts the grain structure and does not provide an appreciable reduction in size.
Cold working requires much higher pressure than hot working.
If the material is more ductile, it can be more cold worked.
Residual stresses are setup during the process, hence to neutralise these stresses a suitable heat treatment is required.
Cold working of metals have following advantages and disadvantages.
Advantages of Cold Working
Better dimensional control is possible because there is not much reduction in size.
Surface finish of the component is better because no oxidation takes place during the process.
Strength (tensile strength and yield strength) and hardness of metal are increased.
It is an ideal method for increasing hardness of those metals which do not respond to the heat treatment.
Disadvantages of Cold Working
Ductility of the metal is decreased during the process.
Only ductile metals can be shaped through the cold working.
Over-working of metal results in brittleness and it has to be annealed to remove this brittleness.
To remove the residual stresses setup during the process, subsequent heat treatment is mostly required.
Difference between Hot Working and Cold Working
Hot working is carried out above the recrystallisation temperature but below the melting point, hence deformation of metal and recovery takes place simultaneously.
During the process, residual stresses are not developed in the metal.
Because of higher deformation temperature used, the stress required for deformation is less.
Hot working refines metal grains, resulting in improved mechanical properties.
No hardening of metal takes place.
If the process is properly performed, it does not affect ultimate tensile strength, hardness, corrosion and fatigue resistance of the metal.
It also improves some mechanical properties like impact strength and elongation.
Due to oxidation and scaling, poor surface finish is obtained.
Close dimensional tolerances cannot be maintained.
Hot working is most preferred where heavy deformation is required.
Cold working is carried out below the recrystallisation temperature and such there is no appreciable recovery of metal.
During the process, residual stresses are developed in the metal.
The stress required to cause deformation is much higher.
Cold working leads to distortion of grains.
Metal gets work hardened.
It improves ultimate tensile strength, yield and fatigue strength but reduces corrosion resistance of the metal.
During the process, impact strength and elongation are reduced.
Cold worked parts carry better surface finish.
Superior dimensional accuracy can be obtained.
Cold working is preferred where work hardening is required.