NEW SURFACE PROCESSES FOR COLD-FORGING OF STEELS

The authors present a new generation of surface processings for cold f orging of steels. These coatings, designed to replace phosphatation, a re easy to implement and less polluting. A drawing-seizure test proced ure is also proposed to evaluate the tribological performances of thes e new coatings more objectively and to compare them to standard zinc p hosphatation. Test results, including the occurrence of boundary lubri cation, mixed lubrication and seizure at the slider-coated specimen in terface, are related to effective bonding and to lubricant effects of the coatings. The new surface processes concern two types of chemicals : a boron salts-based processing (sodium tetraborate: B4O7Na2.5H(2)O a nd derivatives) and a new generation of silicate-based processings rec ently developed to comply to environmental constraints. The seizure re sistance of the coatings is mainly analyzed by considering different p rocessing conditions such as bath temperature and drying conditions. T his parameter, which is very sensitive to treatment parameters, has gi ven rise to an easy and objective optimization of the new surface proc essings. It has been shown that the silicate-based processing presents characteristics very close to those of phosphatation. This new genera tion of coatings could therefore replace phosphatation for severe cold forging operations in the short term. (C) 1998 Elsevier Science S.A.