Kondratenko P. Improvement of production technology and testing methods of high strength fasteners made of carbon and low-alloy steels

The object of research - the processes of formation of the structure and properties of low-carbon non-alloy steel under the combined influence of active and cyclic deformation, and the influence of the structural state on the properties of metal products with stress concentrators. The purpose of the dissertation is to scientifically substantiate and test a new technology for the manufacture of high-strength bolts from non-alloy low-carbon steel based on the method of deformation reinforcement of the workpiece, followed by additional strengthening of bolts during cold volume stamping. At the same time, the problem of improving the methods of passing tests of finished bolts in comparison with existing ones was considered, since the properties of finished products along with the structural state are also determined by the geometry of the product. Solving the problem of improving the geometry of the bolts in order to increase the resistance to brittle fracture. Research methods: mathematical statistics; light microscopy; X-ray diffraction analysis; static mechanical tests; scanning electron microscopy. Theoretical and practical results: the proposed technological parameters in comparison with the standard technological parameters of cold-formed metal products at one level of the obtained performance characteristics will save 40 - 50% on the workpiece by replacing alloy steel with ordinary carbon. In this case, the proposed technological parameters of the process of obtaining cold-deformed products in comparison with current technologies increase the value of performance by 20-40%, which further increases the efficiency of the proposed technological solutions while reducing their cost above. For 20kp steel, new modes of active and cyclic deformation are proposed, which allow to obtain a substructure due to the consolidation of dislocations by vacancies, which increases the fatigue strength of fasteners and to obtain immunity to deformation aging. Scientific novelty: the principles of forming the structure of low-carbon non-alloy steel were further developed, which allowed to substantiate for the first time the technology of obtaining high-strength fasteners by combining cold active and cyclic deformation with thermal effects based on the formation of specific structural state at different structural levels; for the first time it is shown that the nature of the influence of the notching system on the properties of metal products depends on the level of their strengthening and a semi-empirical equation is proposed, which describes the regularity of such influence of the notching system; for the first time the regularities of the influence of their mass on the resistance to brittle fracture and the opposite influence of mass on the resistance to viscous fracture are proposed for bolts; for the first time the characteristics of resistance to viscous fracture based on the use of conditions of macrolocalization of plastic deformation in a loaded object are proposed and substantiated. Degree of implementation: the results of the dissertation are introduced into the educational process of the Department of Heat Treatment of Metals of the National Metallurgical Academy of Ukraine in teaching the discipline "Structure and Properties" and in the activities of PJSC "Dniprometyz" in the production of fasteners. Scope: production of high-strength fasteners from low-carbon non-alloy steel for machine-building purposes, carrying out mechanical tests of metal products.