In this article we will discuss about:- 1. Meaning of Cold Working 2. Effects of Cold Working 3. Limitations.
Meaning of Cold Working:
Metal is said to be cold worked if it is mechanically processed below the recrystallization temperature of the metal. Most of cold working processes are performed at room temperature. Hot working usually results in the refinement of grain structure whereas cold working merely distorts the grains and does little towards reducing its size.
As the handling of heated material and use of soaking pits and furnaces are avoided in cold working, it results into faster production. In cold working, the deformation of metals is brought about by the process of slip of planes. Also the force required for cold working is greater than the force required for hot working of metals, because in cold working the metals are not deformed permanently till the elastic limit is exceeded.
As there is no recrystallization of grains, the recovery of distorted grains does not take place and on subsequent cold working, greater and greater resistance to the action of deformation is experienced. This results in the increased strength and hardness of metal, thus permitting use of plain carbon steel in place of costly alloy steels. This method of hardening is known as work hardening or strain hardening.
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For certain metals which do not respond to heat treatment, this is the only method available for strengthening the material, whereas in other metals further working is possible only after it is annealed to remove hardness and the residual stresses as otherwise huge force would be needed to work the metal further. Cold forging results in considerable savings in materials costs because all of the material is utilised in the finished component. There is no production of swarf as in metal cutting.
Cold working produces an improved surface finish, scale-free and bright surface and closer dimensional tolerances. Cold working processes are generally used in making end-use products because of their economy in producing the desired shapes. It is also used to produce residual stress into certain metals in order to have improved fatigue life, e.g., by shot peening in case of leaf springs.
A large proportion of hot rolled steel is subsequently cold rolled into strip or sheet from which a multitude of pressed parts are manufactured. The scale from hot rolled steel is removed by immersing it in dilute sulphuric acid (pickling).
Major part (90-95%) of the mechanical work of deformation in metal forming is converted into heat and it results into rise of temperature. Rise in temperature could be of the order of 75°C for aluminium, 280°C for steel and 570°C for titanium. It can be determined by dividing the work of plastic deformation per unit volume by the product of density and specific heat of work piece, and the mechanical equivalent of heat (J).
Effects of Cold Working:
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The effects of cold working on metals are summarised below:
(i) The grain structure is distorted and resistance to working keeps on increasing due to the lattice distortion.
(ii) Residual stresses are set up in the metal which remains unless they are removed by subsequent heat treatment. When reheating is done below the crystallisation temperature, the residual stresses are removed without appreciable change in physical properties of grain structure. Further heating into the recrystallize range eliminates the effect of cold working and restores the metal to its original condition.
(iii) Cold working results in loss of ductility and increase of strength and hardness of metal.
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(iv) Surface finish is improved and close tolerances can be maintained.
(v) Re-Crystallisation temperature for steel is raised.
Limitations of Cold Working:
(i) Only small sized components can be easily cold worked as greater forces are required for large sections. Sections of more than 25 mm diameter are rarely could rolled. Due to large deforming forces, heavy and expensive capital equipment is required.
(ii) The grain structure is not refined and residual stresses have harmful effects on certain properties of metals.
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(iii) Many of the metals having less ductility, e.g., carbon steel and certain alloy steels cannot be cold worked at room temperature. It is, therefore, limited to ductile metals and the range of shapes produced is not as wide as can be obtained by machining.
(iv) Tooling costs are high and as such it is used when large quantities of similar components are required.
Metals which can be suitably cold worked in the form of sheets or other forms are given below:
(i) Mild steel of low carbon content.
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(ii) Copper and its alloys including brass and bronze.
(iii) Aluminium bronze having aluminium less than 7%.
(iv) Nickel brasses.
(v) Monel metal.
(vi) Stainless steel.
(vii) Duralumin and several other aluminium alloys.
Effect of Friction:
(a) Friction increases the work load and thus more force and energy is required in shaping and forming of material.
(b) It causes fast wear out of dies, rolls and other working tools.
(c) It results in modification of the deformation pattern of the metal because the friction force superimposed on deforming force changes the direction of principal stress direction which is the deciding factor for determining the orientation of the atomic planes along which the slip occurs.
(d) Since friction force occurs only at the surface and not throughout the thickness of metal, it introduces microscopic inhomogeneity resulting in micro cracks on surface and weaker products having lower fatigue strength.
(e) It results in poor surface finish and build up edges.