Investigation of the state of the surface layer of oil and gas pipelines

The existing pipelines are constructed from pipes manufactured in accordance with different technological eras. One of the main features of the production of modern rolled products for pipes is the thermomechanical processing of metal, in which it is subjected to high degrees of plastic deformation without subsequent phase transformations and recrystallization. In addition to the positive effect of such treatment, it may also have a negative effect of preserving undesirable technological heredity, especially in the near-surface layers of contact deformation. It is also known that due to the operational characteristics of pipelines, the outer surface of the pipes is a zone of increased stress concentration and initiation of destruction. In connection with the above, the study of the state of the surface layer of new and used oil and gas pipelines and connecting parts of pipelines made using the most common rolling technologies was carried out. The study consisted in determining the brittleness index of the pipe metal at various depths, calculated using a specially developed technique: based on processing the values of microhardness and morphology of localized shifts next to the print during microindentation. It has been established that embrittlement of metal can be caused at all stages of the design life cycle. At the same time, the modes of controlled sheet rolling have a decisive influence on the formation of the initial embrittlement of the metal of pipes. It begins to appear in the production of steels of strength class K60, and increases in steels of strength class K65. In this case, there is also a significant heterogeneity of metal embrittlement in depth: in the near-surface layers with a depth of 0.5 – 1.0 mm, there is a sharp increase in metal embrittlement relative to the central volume. Such a feature can have a decisive influence on the origin and development of stress-corrosion cracks, since it correlates well with the existing statistics on the detection of these defects. The possibility of assessing the degree of embrittlement of metal during the operation of pipelines using specialized equipment has also been confirmed.

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