OBRABOTKAMETALLOV Vol. 25 No. 4 2023 technology In [38], microstructural mechanisms for reducing the impact strength values of the coarse-grained heataffected zone of two microalloyed K60 steels were studied. It is shown that the greatest influence on the impact toughness of the heat-affected zone is exerted by titanium nitride inclusions, the spalling of which within large bainite packets can lead to macrobrittle fracture of the specimens. When evaluating the effect of welding on changes in the properties of pipe steel, it is necessary to understand that the ultimate strength and yield strength of pipes has a wide range. For example, fig. 7 shows an example of the permissible ranges of change in yield strength and ultimate strength of pipes of the X series according to API 5L. Fig. 7. Permissible variation of pipe strength according to API 5L [39] The upper ultimate tensile strength limit of grades X80 and higher increases as the grade of pipe increases. Even for the same class, the permissible strength range is in a large range of variations. Nevertheless, studies have shown that the critical strength matching factor satisfying the deformation requirements does not depend on the strength of the pipe [39–50]. Thus, the requirements for the ultimate tensile strength of the weld metal should be very high if the upper strength limit of the pipe is used to set the strength requirements for the weld metal, especially if the misalignment of the pipes from high to low and the apparent fracture toughness are conservatively set [39, 40, 45–50]. The strength requirement remains difficult to satisfy in practice when designing pipelines, taking into account the currently available welding methods and other limitations [39–50]. Since most of the parameters affecting the deformability are uncertain, the ultimate tensile strength requirements obtained by deterministic design methods may not be realistic enough. Therefore, when considering the probabilistic distribution of parameters, a reliability-based approach should be applied. In addition, the appropriate requirements for the strength matching coefficient of circumferential weld metals can be scientifically determined by adopting the theory of structural reliability [38, 39]. Modern requirements for the strength and other mechanical properties of circumferentially welded pipe joints are mainly reflected in the qualification requirements for welders. Anomalies in the form of skewed joints of pipes and pipelines in general, microcracks are inevitable for large-diameter pipelines made of high-strength steel [40]. In recent decades, many studies have been carried out on circumferential welds of onshore and offshore pipelines with cracks under operational load [38–41]. Table 6 presents generalized requirements of regulatory documents adopted in different countries for the ultimate tensile strength of circumferential welds. Almost all standards indicate that fractured specimens at the welding site can be accepted if the ultimate tensile strength of the weld is higher than the established minimum ultimate tensile strength σu of the main pipe.
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