DETERMINATION OF THE CAUSES OF DESTRUCTION OF HIGH-STRENGTH FASTENING ELEMENTS

Abstract

Intensively operated modern technology requires the use of high-strength fasteners. The article presents the results of studies of the structure and physical and mechanical properties of the material of fastening elements (threaded hardware) in order to determine the causes of their destruction during operation. The fractographic analysis of fractures of fractured bolts revealed pronounced centers of crack initiation with traces of delayed brittle fracture, which is typical for high-strength hardware that are under high load for a long time, close to the yield point. Mechanical tests of the studied fastening elements showed increased values of ultimate strength. The performed relaxation tests of the material of broken bolts also revealed an increased value of the microplasticity limit, which is responsible for the onset of plastic deformation processes in microvolumes of the material, and for the material of whole hardware, the value of the limit is included in the permissible interval. At the same time, the average hardness of the material of all products meets the regulatory requirements. The maximum spread of hardness 70 HB is fixed on the destroyed bolt. Using the experimental values of microhardness, theoretical estimates of the coefficient of plasticity, which characterize the ability of a material to perceive elastic and plastic deformations, were carried out. To ensure a sufficient level of ductility of a long-term working metal material, the ductility coefficient must be at least 0.8. The average value of the hardness of the material of the fastening elements satisfies this criterion for assessing ductility. According to the results of a step-by-step analysis of the microstructure of the samples, the inhomogeneity of the distribution of the carbide phase over the cross-section of the bolts was established. The two-phase composition (a-Fe + Fe3C) of steel was established by X-ray structural analysis, which confirms the results of microstructural studies. It should be noted that the phase composition of all studied samples is identical. Based on the results of the studies carried out, it was found that the destruction occurred due to the reduced strength and increased fragility of the material due to the presence of microdefects. The material of the whole bolts in terms of structure and mechanical properties comply with regulatory requirements.