Metal Finishing Guide Book


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Page 134 of 903

graphic changes produced within the interior of the material find expression internally in the form of so-called strain hardening. An example is an increase in tensile strength and hardness. Characteristic of the cold forming of steel compared with other metals is the high amount of mechanical energy required. This means that high temperatures occur in the forming zone where workpiece and tool come into contact with each other, which may impair or even completely nullify the effect of normal lubricants. Another factor is that the change in form of the workpiece may in some circumstances entail a considerable increase in the specific surface, so that additional stress is imposed on the lubricant. The character of friction prevailing in the forming zone under industrial conditions of cold forming largely corresponds to the concept of mixed friction. That is, in the range of the roughness peaks, there is an extensive lubricant coating up to several molecule layers deep, which because of its texture can prevent galling and seizing in the case of pressures that are not too high (boundary friction). In the depression between the roughness peaks occur hydrostatic pressure areas, which may be regarded as a lubricant reservoir. The lubricant forced out of these pressure areas or cavities during forming produces locally limited hydrodynamic friction states. The tendency of one surface to become bonded by galling and seizing on direct contact with the other surface is a major problem in cold-forming operations. The chemical and physical-chemical properties of the surfaces of the tool and workpiece have a considerable influence on the tendency to galling and seizing. Under favorable conditions the workpiece surface can be altered to such an extent that galling and seizing during the cold-forming process practically never occur. To achieve this is one of the main aims of chemical surface treatment. Fig. 4. These SEM pictures show the internal surface of a steel pipe before (a) and after (b) drawing and also after cleaning (c). 131

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