A study of the influence of high frequency cyclic loading on the structure and properties of weld connections in process pipelines

Introduction. Pipeline systems are exposed to several conditions that lead to a drastic reduction in their durability, primarily due to variable low-frequency and high-frequency loads arising in a process pipeline due to the operation of compressor units. Hence, fatigue failure occurs, leading to the pipeline failure. As early as at the pipeline installation stage, sections of process pipelines have weld connections, and thermal welding cycles have an adverse effect on the properties of materials exposed to fatigue loading. The study of weld connections in steel pipelines exposed to high-frequency vibrations and effects of weld seam defects on durability characteristics are the focus of this research. Materials and methods. Low-carbon pipeline steel St20 was selected for the study. The radiographic inspection method, optical metallography, microhardness of structural phases, and the method of high-frequency fatigue tests were used. Results. The results and principal conclusions about the effect of welding defects on durability characteristics of welded samples, made of pipeline steel and exposed to high-frequency fatigue tests, are presented; structural changes in weld connections are analyzed using optical metallography and microhardness methods. Defects of weld seams and their dimensions were identified by means of radiographic inspection. A comparative analysis of durability limits, demonstrated by the parent metal of the model material that has defective weld connections, and the same limits of defect-free samples is provided. The main causes of failure of weld joints, exposed to high-frequency vibrations, are identified. Conclusions. Having summarized the research findings, we can argue that high-frequency vibrations have a negative impact on the parent metal of a process pipeline and its weld joints. The weld seam is the point of failure; defects trigger destruction, and their presence has a higher impact on fatigue characteristics than their dimensions or types. Characteristics of durability in case of exposure to high frequency loading applied to a weld joint in the gigacycle range are 67 percent below those of the parent metal.