Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 42 of 331

customer demands. (Fig. 3 shows a summary of the possible sequences.). COLD-FORMING PROCESSES FOR TUBE WIRE, COLD EXTRUSION The application of chemical processes for the purpose of assisting the cold form- ing of metals was already in general use at the beginning of this century. The in- dustrial importance of cold-forming technology for the entire steel working in- dustry sector today would be inconceivable without the metal surface technol- ogy including zinc phosphate and lube. The expression cold forming implies that the change of form is effected with- out preheating the workpiece. The noncutting of a workpiece is performed un- der the action of outside forces (tensile, compressive, and shear stresses), which strain the material beyond the yield point and then force the material, now in the plastic state, to assume the form imposed on it by the tool. During this process the mass and composition of the material remain unchanged. The internal processes taking place in the material consist of permanent changes in the position of the atoms within the crystal lattice. The crystallo- 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 com- pletely 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 Fig. 4. These SEM pictures show the internal surface of a steel pipe before (a) and after (b) drawing and also after cleaning (c). 41

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