WORKING OF METALSDOE-HDBK-1017/1-93Properties of MetalsThe bainite that has been formed occupies more space than the original austenite lattice. Thiselongation of the material causes residual compressive and tensile stresses in the material.Welding stresses can be minimized by using heat sink welding, which results in lower metaltemperatures, and by annealing.Annealingis another common heat treating process for carbon steel components. Duringannealing, the component is heated slowly to an elevated temperature and held there for a longperiod of time, then cooled. The annealing process is done to obtain the following effects.a.to soften the steel and improve ductilityb.to relieve internal stresses caused by previous processes such as heat treatment,welding, or machiningc.to refine the grain structurePlastic deformation which is carried out in a temperature region and over a time interval such thatthe strain hardening is not relieved is called cold work. Considerable knowledge on the structureof the cold-worked state has been obtained. In the early stages of plastic deformation, slip isessentially on primary glide planes and the dislocations form coplanar arrays. As deformationproceeds, cross slip takes place. The cold-worked structure forms high dislocation density regionsthat soon develop into networks. The grain size decreases with strain at low deformation butsoon reaches a fixed size. Cold working will decrease ductility.Hot workingrefers to the process where metals are deformed above their recrystallizationtemperature and strain hardening does not occur. Hot working is usually performed at elevatedtemperatures. Lead, however, is hot-worked at room temperature because of its low meltingtemperature. At the other extreme, molybdenum is cold-worked when deformed even at red heatbecause of its high recrystallization temperature.The resistance of metals to plastic deformation generally falls with temperature. For this reason,larger massive sections are always worked hot by forging, rolling, or extrusion. Metals displaydistinctly viscous characteristics at sufficiently high temperatures, and their resistance to flowincreases at high forming rates. This occurs not only because it is a characteristic of viscoussubstances, but because the rate of recrystallization may not be fast enough.MS-02Page 30Rev. 0
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