In this article we will discuss about:- 1. Definition of Hot Working 2. Methods of Hot Working 3. Factors Affecting 4. Advantages 5. Disadvantages.
Definition of Hot Working:
Mechanical working of metals can be carried out in the form of either hot working or cold working. Working above the recrystallization temperature (lower critical temperature) but below the burning point is termed as hot working.
The metals are given desired shape in hot working process by subjecting them to forces, which cause them to undergo plastic deformation when above the recrystallization temperature. The recrystallization temperature of steel is 800°C (50 – 100°C above the lower critical temperature) and for lead, tin and zinc it is at or near room temperature.
To get best results in each shaping operation, important points such as amount of energy consumed, the best temperature, rate of shaping, design of equipment, crystal structure and properties of the finished product must be given due consideration.
Methods of Hot Working:
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The various methods of hot working are as follows:
(i) Rolling,
(ii) Forging,
(iii) Extrusion,
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(iv) Piercing,
(v) Drawing or Cupping,
(vi) Spinning.
Factors Affecting Metal Deformation of Hot Working:
Temperature, friction and speed of deformation have great influence in shaping metals by deformation (both hot working and cold working).
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As the temperature of work piece is increased, it loses its strength and hardness but regains ductility.
Strain rate is proportional to the rate at which metal is deformed (deformation velocity) and inversely proportional to instantaneous size of deformation of metal. High strain rate tends to increase temperature of work material due to adiabatic heating, increases the flow stress, increases tensile strength (if seen at a particular temperature), and also tends to improve lubrication at the tool work piece interface.
Friction exists between tool and work piece during deformation process. Friction affects both deformation forces and the pattern of deformation. In cold working, coefficient of friction may be around 0.1 with efficient lubrication. But in hot working it is high (of the order of 0.6).
Advantages of Hot Working:
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Hot working has the following advantages:
(1) Above the recrystallization temperature, the metal becomes plastic and causes the growth of grains. By hot working, the grains are broken up and the parts are deformed into small and more numerous crystals or in other words the refinement of grains occurs. Metals possess little elasticity and low load is required to shape the metal as the strength and hardness decrease at elevated temperatures.
(2) The porosity of the steel ingot can be eliminated to a greater extent.
(3) Great latitude in shape and size of form is possible due to reduction of elastic limit.
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(4) A uniformity is established either by squeezing other impurities into fibre slags or distributing them throughout the mass.
(5) Directional property resulting from a fibre structure is obtained.
(6) Due to refinement of grains, mechanical properties such as toughness, ductility, elongation and reduction in area are improved.
(7) The power required to finish the part from ingot is less.
(8) It can be used on most of the metals, because it is a rapid and economical process.
Disadvantages of Hot Working:
(a) There is rapid oxidation or scaling of the surface due to high temperature of metal which results in poor surface finish.
(b) Close tolerances cannot be maintained; oxide films are a surface characteristic.
(c) Tooling and handling costs are high.
(d) The life of the tools used is less as they have to work at high temperature.