NATURE OF PLASTIC DEFORMAIION

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1 NATURE OF PLASTIC DEFORMAIION Plastic deformation is the deformation which is permanent and beyond the elastic range of the material often, metals are worked by plastic deformation because of the beneficial effect that is imparted to the mechanical properties by it. The necessary deformation in a metal can be achieved by application of large amount of mechanical force only or by heating the metal and then applying a small force. The deformation of metals which is caused by the displacement of the atoms is achieved by one or both of the processes called slip and twinning. On the macroscopic scale when plastic deformation occurs, the metal appears to flow in the solid state along specific directions which are dependent on the type of processing and the direction of applied force. The crystals or grains of the metal get elongated in the direction of metal flow. This flow of metal can be seen under microscope after polishing and suitable etching of the metal surface. These visible lines are called as fibre flow lines". Since the grains are elongated in the direction of flow, they would be able to offer more resistance to stresses acting across them. As a result, the mechanically worked metals called wrought products would be able to achieve better mechanical strength in specific orientation, that of the flow direction. Since it is possible to control these flow lines in any specific direction by careful manipulation of the applied fibres. It is possible to achieve optimum mechanical properties. The metal of course, would be weak along the flow lines. The wastage of material in metal working processes is either negligible or very small and the production rate is in general very high. These two factors give rise to the economy in production. of the amount of heating applied to the metal before applying the mechanical force. Those processes, working above the recrystallisation temperature, are termed as hot working processes whereas those below are termed as cold working processes. Under the action of heat and the force, when the atoms reach a certain higher energy level, the new crystals start forming which is termed as recrystallisation. Recrystallisation destroys the old grain structure deformed by the mechanical working, and entirely new crystals which are strain free are formed. The grains in fact start nucleating at the points of severest deformation. Recrystallisation temperature as defined by American Society of Metals is "the approximate minimum temperature at which complete recrystallisation of a cold worked metal occurs within a specified time'" The recrystallisation temperature is generally between one-third to half the melting point of most of the metals. The recrystallisation temperature also depends on the amount of cold work a material has already received. Higher the cold work, lower would be the recrystallisation temperature as shown in Fig. HOT WORKING AND COLD WORKING The metal working processes are traditionally divided into hot working and cold working processes. The division is on the basis Page 1

2 Though cold work affects the recrystallisation temperature to a great extent, there are other variables which also affect as given in the Table below. In hot working, the process may be carried above the recrystallisation temperature with or without actual heating. For example, for lead and tin the recrystallisation temperature is below the room temperature and hence working of these metals at room temperature is always hot working. Similarly for steels, the recrystallisation temperature is of the order of 1000oC, and therefore working below that temperature is still cold working only. increase the size of the grains occurs by a process of coalescence of adjoining grains and is a function of time and temperature. This is not generally desirable. If the hot working is completed just above the recrystallisation temperature as in D, then the resultant grain size would be fine. The same is schematically shown for hot rolling operation. In hot working, the temperature at which the working is completed is important since any extra heal left after working will aid in the grain growth, thus giving poor mechanical properties' The effect of temperature of completion of hot working is shown schematically. In Fig. A is shown a, simple heating where the grain start growing after the metal crosses the recrytallizayion temperature. When it is cooled without any hot working as in B, the final grain size would be larger than when started in A. After heating, when the metal is worked, because of recrystallisation, the grain size is reduced. This is made possible because the working of metal gives rise to a large number of nucleation sites for the new crystals to form. But if the hot working is completed much above the recrystallisation temperature as in C the grain size start increasing and finally may end up with coarse grain size. This Hot working Advantages: 1. As the material is above the recrystallisation temperature, any amount of working can be imparted since no strain hardening taking place. 2. At a high temperature the material would have higher amount of ductility and therefore there is no limit on the amount of hot working that can be done on a material. Even brittle materials can be hot worked. Page 2

3 3. Since the shear stress gets reduced at higher temperatures, the hot working requires much less force to achieve the necessary deformation. 4. It is possible to continuously deform the grains in metal working and if the temperature and rate of working are properly controlled, a very favorable grain size could be achieved giving rise the better mechanical properties. Disadvantages: 1. Some metals cannot be hot worked because of their brittleness at high temperatures. 2. Higher temperature of metal give rise to scaling of the surface and as a result the surface finish obtained is poor. Although there is a possibility of the decarburization of skin in steels due to high temperatures. 3. Because of the thermal expansion of metals, the dimensional accuracy in hot working is difficult to achieve since it is difficult to control the temperature of work pieces. 4. Handling and maintaining of hot metal is difficult. Difference Between Hot & Cold Working: Page 3

4 Rolling Principle: Page 4

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6 Forging: Page 6

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