Savkyn S. Improving the method of calculation and technology of production of welded pipes using vibration

The dissertation on competition of a scientific degree of the doctor of philosophy on a specialty 132 – "Materials science". – National Metallurgical Academy of Ukraine, Dnipro, 2021. The dissertation is devoted to the research of the influence of vibration in the process of welding the billet on the level of residual stresses, metal microstructure and mechanical properties of the weld and seam zone. In the dissertation work new scientifically substantiated theoretical and experimental results are obtained, which together make a significant contribution to solving the urgent problem of reducing residual stresses formed in the pipe billet during welding, which ultimately allowed to propose a new method of reducing residual stresses based on vibration. in the welding process. Every year the requirements for the produced welded pipes increase and special attention is paid to the quality of the weld, namely the residual stress in the weld and the seam zone, which occur during the shrinkage of the metal during 8 crystallization. At the moment, on an industrial scale, the problem of negative impact on the pipe product of the welding process is solved by heat treatment of the entire cross section of the pipe or local treatment of the weld and seam area by annealing. However, this process is one of the most expensive, time consuming and not always environmentally friendly. This gives real prerequisites for finding an alternative way to relieve residual stresses in welds and seamless area of straight pipes. The aim of the dissertation is to improve the technological process of production of welded straight-seam pipes by removing residual stresses in the weld and seamless area by vibration. This goal is achieved by developing mathematical models that describe the pattern of the effect of vibration frequency during welding on the residual stress level and is confirmed by experimental studies of the residual stress level and microstructure of samples welded using different vibration frequencies. Theoretical researches are executed with use of fundamental laws and analytical methods of mathematical modeling of welded connections and vibration processing of material. Laboratory and industrial studies are performed using modern equipment and measuring instruments subjected to metrological verification. Processing of experimental results and construction of models was performed using experimental planning methods and the theory of statistical data processing. Object of research: the process of processing the weld of the pipe in order to remove residual stresses. Subject of research: the level of residual stresses in the weld and seam zone, the grain size of the structure and the variety of thermal impact zone, the mechanical properties of the weld. The practical significance of the results is that according to research, the method of removing residual stresses using vibration during welding allows 90% to replace heat treatment by the level of residual stress and at the same time avoid high-cost equipment and negative consequences in the form of scale, high duration and energy consumption of the process. The hypothesis of reduction of residual stresses in a weld at vibration influence on it in the course of welding received the further development. The effect is achieved due to changes in the crystallization process of the liquid metal under the vibration-resonant influence with a frequency equal to the natural frequency of oscillations at the micro and microstructural level of the melt structure. This allows you to replace the process of removing residual stresses in the manufacture of pipes using heat treatment with the process of vibration treatment of the weld. For the first time, the influence of the frequency of vibration in the process of welding a billet of carbon steels on the level of residual stresses and the structure of the weld and seam zone was experimentally established. The development differs in that under the influence of vibration with a frequency of 25-200 Hertz in the welding process there is a reduction of residual stresses in the weld and seam zone by 41-71% and a decrease in the grain size of the microstructure to 16%. This proves the positive effect of vibration when reducing the level of residual stresses in welded pipes. For the first time the mathematical dependence of the required duration of vibration influence on the cooling rate of the weld metal during welding of the billet was developed. For the first time the method of calculation of residual stresses of a weld at welding of pipe preparation with an application of vibration is developed.