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Tuesday, 12 November 2013

Cold-Working Processes

Cold-Working Processes
Points : Cold-Working Processes, Definition Definition Cold-working from the ingot to the finished product, with, of course, the necessary intermediate annealing stages, is applied only in the case of a few alloys. These include both alloys which are very malleable in the cold and, on the other hand, those which become weak and brittle when heated. Cold-working is more often applied in the finishing stages of production.

Then its functions are:
(a) to enable accurate dimensions to be attained in the finished product;
(b) to obtain a clean, smooth finish;
(c) in some cases to straighten the work piece;
(d) by adjusting the amount of cold-work in the final operation after annealing, to obtain the required degree of hardness, or ‘temper’, in alloys which cannot be hardened by heat-treatment.

Raising the temperature of an alloy generally increases its malleability but reduces its ductility because tensile strength falls as the temperature increases. This causes the metal to tear apart in tension before appreciable extension has occurred. Thus in hot-working processes we are always pushing the alloy into shape, whilst in cold-working operations we frequently make use of the high ductility of some alloys when cold, by pilling them into their required shapes. Therefore, processes involving the pulling or ‘drawing’ of metal through a die are always cold-working operations.
There are many cold-working processes, but the principal ones used in ‘‘metallurgical industries are discussed below.

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