Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 45 of 331

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 sev- eral molecule layers deep, which because of its texture can prevent galling and seiz- ing in the case of pressures that are not too high (boundary friction). In the de- pression 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 pro- duces 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-chem- ical properties of the surfaces of the tool and workpiece have a considerable in- fluence on the tendency to galling and seizing. Under favorable conditions the workpiece surface can be altered to such an extent that galling and seizing dur- ing the cold-forming process practically never occur. To achieve this is one of the main aims of chemical surface treatment. ZINC PHOSPHATE COATINGS Coatings composed of zinc phosphate have been successfully used in all areas of the cold forming of nonalloy and low-alloy steels. The main reasons for the spe- cial suitability of zinc phosphate as a separating medium and as a wear-reducing material are as follows: zinc phosphate coatings are associated with strong bind- ing forces with the iron surface; the specific structure of the zinc and zinc-iron crystal structure enables the plastic deformation of crystals under the action of the compressive and shear stresses arising during forming operations; zinc phos- phate coatings are able to react with alkali metal soaps to give high-efficiency lu- bricant systems; the actual phosphating process is performed by immersion or by an in-line method. The workpieces are treated for a period of 5 to 10 minutes (dip process) or 20 to 30 seconds (in-line process) with zinc phosphating process- es working on the "iron side" or accelerated with nitrite. Coating weights can vary as much as 300 to 2,500 mg/ft2 . The properties of the zinc phosphate coatings produced are mainly determined by the following factors: type of phosphate forming the coating (zinc phosphate, zinc- iron phosphate, and zinc-calcium phosphate); type of accelerators used (nitrite/ni- trate, chlorate, and nitrate); concentrations of components of the process (total acid, free acid, metal components in the bath, and accelerators); process parameters such as process temperature and process time; type of application (dip operation ver- sus in-line process); application of activation prerinses; type and mode of previous pickling processes; and type and mode of previous annealing. Lubrication and its different processes will not be discussed in detail here. Plas- tic forming of a zinc phosphate coating after being treated with metallic soap is shown in Fig. 4. The SEM pictures show the internal surface of a steel pipe before and after drawing and also after cleaning. Some application differences are listed in Table VI. MANGANESE PHOSPHATE COATINGS TO IMPROVE SLIDING PROPERTIES Manganese phosphate coatings are extensively employed to improve the slid- ing properties of engine, gear, and power transmission systems. Manganese 44

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