OBRABOTKAMETALLOV Vol. 25 No. 4 2023 technology Fig. 8. Distribution of pipes strength from different sources in the X80 pipeline project [41] The current principle of matching the strength and toughness of welded joints due to increasing the strength and a wide range of the actual strength of the base metal of the steel pipe is not entirely correct because it reduces the safety of pipeline operation [40, 41]. Due to the nature of the welding process, it is more difficult to balance the strength and toughness in the weld metal than in the pipe steel that has passed the TMCT, since the metal is a casting structure formed during heating, melting and solidification. The higher the strength class of pipe steel, the more difficult it is to achieve equilibrium in the weld metal. On the other hand, the wide range of the actual strength of the steel pipe makes it difficult to implement a standard compliance of above or equal strength fig. 8 [41]. Conclusion In the course of the analysis of literature sources related to the pipe production technologies and subsequent welding, two ways to increase the tension of a linear pipe are revealed. One of it is the careful design of the metallurgical chemical composition and precise control of the alloy composition during melting. Another is the precise control of the cooling rate during rolling. It perfectly protects the high-strength pipeline steel from cold cracks and HAZ brittleness. However, when welding new technical difficulties arise. Adopting the method of thermal and mechanical sheet rolling for pipes, domestic steel requires a differentiated alloying elements system and better control over the parameters of sheet rolling. Domestic steel type Cr80 requires special care when welding, especially when assembling pipelines in the field. Regardless of the API classification class, welding is the main process in the manufacture and assembly of pipelines. Welding processes require a lot of time in the production and assembly of these structures. This fact should be taken into account in the approach to studying the issues of increasing the efficiency of the welding processes used or the introduction of new processes in order to improve the cost-benefit ratio when implementing of these structures. Even compared to high-performance welding processes such as hybrid laser and electron-beam welding, the most commonly used process in pipe production is still submerged arc, applied using tandem technology. Among the welding processes used in the assembly of pipelines in the field, the coated electrode continues to be widely used. However, transfer controlled MAG (TC) welding process combined with the flux-cored welding process provide an excellent alternative to a conventional covered electrode. Although the ultimate tensile strength of circumferential welded joints is not lower than the minimum specified ultimate tensile strength of the pipe, the circumferential welded joint corresponds to a strength less or equal to the actual strength of the steel pipe, which requires careful selection of welding materials and process.
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