Metal forming operations are mostly used to produce a new shape or to improve the properties of the metal.
The main objectives of metal working processes are to provide the desired shape and size, under the action of externally applied forces in metals.
Such processes are used to achieve optimum mechanical properties in the metal and reduce any internal voids or cavities present and thus make the metal dense.
By application of mechanical force or by heating the metal and then applying a small force, the required amount of deformation in a metal can be achieved.
Due to the elongation of grains the impurities get broken and dispersed through out the metal.
The harmful effects of the impurities are reduced which helps in improving the mechanical strength.
When the stress caused in the metal reaches the yield point, plastic deformation of the metal takes place.
Plasticity, ductility and malleability are the properties of a material, which retains the deformation produced under applied forces permanently and hence these metal properties are important for metal working processes.
Some amount of permanent deformation without rupture or failure is known as plasticity.
The ability of a material to be drawn into wire with the application of tensile force is known as ductility.
Ductile material shows the property of both strong and plastic.
Malleability is the ability of the material to be flattened into thin sheets without cracking by hot or cold working.
A malleable material should be plastic by nature but need not be so strong.
Plastic flow of the metal takes place and the shapes of the grains are changed during the process of plastic deformation in metal forming.
Formation of new grains start at the location of internal stresses developed in the metal when the plastic deformation occurs at higher temperature.
The growth of new grains is accelerated when the temperature is sufficiently high.
This process of formation of new grains is known as recrystallisation.
The temperature at which recrystallisation is completed is known as the recrystallisation temperature of the metal.
Mechanical working of a metal below its recrystallisation temperature is called as cold working and that accomplished above this temperature but below the melting or burning point is known as hot working.
Hot working processes are the mechanical working processes which are done above recrystallisation temperature of the metal.
In this process, the temperature of completion of metal working is important since any extra heat left after working aid in grain growth.
This increase in size of the grains occurs by a process of coalescence of adjoining grains and is a function of time and temperature.
Grain growth results in poor mechanical properties.
If this process is completed just above the recrystallisation temperature then the resultant grain size would be fine.
EFFECT OF HOT WORKING ON MECHANICAL PROPERTIES OF METALS:
This process is generally performed on a metal held at such a temperature that the metal does not work-harden.
Where large deformations are produced there the hot working process is done.
It has the capability to produce the same results on a metal as cold working and annealing.
It does not strain harden the metal.
Grain structure is refined due to hot working processes such as rolling process.
Surface finish of hot worked metal is not nearly as good as with cold working, because of oxidation and scaling.
The temperatures at which to start hot work and at which to stop the process is very important because this affects the properties to be introduced in the hot worked metal.
During hot working, defects in the metal such as blowholes, internal porosity and cracks get removed or welded up.
During this process, self-annealing occurs and recrystallization takes place immediately following plastic deformation.
MERITS OF HOT WORKING:
Any amount of working can be imparted if the material is above recrystallization temperature.
There is no limit on the amount of hot working that can be done on a material at a high temperature as the material has higher amount of ductility.
The grain structure of the metal is refined which improves mechanical properties due to hot working process.
Porosity of the metal is considerably minimised.
Hot working does not affect tensile strength, hardness, corrosion resistance, etc. if the process is properly carried out.
Due to hot working process, no residual stresses are introduced in the metal.
Mechanical properties, especially elongation and reduction of area are improved, but fibre and directional properties are produced.
Hot work promotes uniformity of material.
DEMERITS OF HOT WORKING:
Rapid oxidation or scale formation and surface de-carburization take place on the metal surface leading to poor surface
finish and loss of metal due to high temperature in hot working.
The loss of carbon from the surface of the steel piece causes the surface layer to lose its strength.
Fatigue crack is produced due to weakening of the surface layer.
Hot working does not occur for metals those have brittleness at high temperatures.
Dimensional accuracy in hot working is difficult to achieve due to thermal expansion of metals.
High cost of tooling.
Handling and maintenance charges are also high.