EVALUATION OF QUALITY OF ROLLED STEEL FOR COLD VOLUME FORGING

Abstract

Metal products obtained by cold heading from rolled metal are used in all branches of mechanical engineering. Its quality is estimated by the required chemical composition and plasticity, the absence of a dispersion of mechanical characteristics along the entire length, the absence of internal and surface defects. The competitive advantage of metalware is the result of optimization during all technological pro­cesses: from rolled metal smelting to heading of finished metalware. At the same time, to reduce costs and to achieve the required quality of metalware, an important condition is to ensure safety and to reduce the energy intensity and labor input of its manufacturing process. An important controlling role in this technological chain is the preparation of the material for its cold volume forging. High-strength fasteners, obtained in conditions of cold heading, are most often obtained from chromium steels. Recently, alternative boron-containing steels have been actively introduced. However, because of the possible formation of boron oxides and nitrides, which lead to a decrease in hardenability, they exhibit instability of thermal hardening during thermohardening of metal products. In addition, the rolled metal of chromium steels, as a rule, is cheaper by 12 – 16 %. And as the fact that foreign deliveries of such steels are associated with additional costs, the fasteners produced from boron-containing steel are obtained with even higher increase in value, which again shows the favor of chromium steels. The standard mechanical characteristics were obtained as well as the destruction cri­teria of 40Kh rolled steel, subjected to patenting in a nylon bath with different temperatures and subsequent drawing with different degrees of deformation during cobbing. The optimum mode of preparation of structure parameters and mechanical characteristics of rolled products before the operation of metalware cold volume forging was identified: patenting (temperature of salt bath of 400 °С) and drawing (deforma­tion degree in the range of 5 – 10 %). It was established that treatment under this mode ensures the obtaining of the required quality of rolled products and is more preferable than the one that operates in produc­tion.