National University of Science and Technology “MISIS” (Moscow, Russia):
V. N. Shinkin, Dr. Phys.-Math., Prof., Dept. of Physics, e-mail: shinkin-korolev@yandex.ru

The durability of the large-diameter thick-walled steel pipes on the main gas and oil pipelines significantly depends on the quality and strength of the isolation of the outer and inner surfaces of the pipes. Usually, such pipes have an external anticorrosive three-layer polyethylene or polypropylene coating. To do this, first the outer surface of the pipe is chrome-plated, then the outer surface is heated, then an epoxy primer is applied to the chromium layer, then an adhesive is applied to the primer and, finally, the outer surface of the pipe is coated with polyethylene or polypropylene. To insulate the inner surface of large diameter pipes, the pipe’s inner surface is heated and the thin layer (about 2 mm thick) of a smooth (antifriction) coating or an internal anticorrosion epoxy coating is applied. The internal anticorrosive coating of large diameter pipes is much weaker and less durable than the external anticorrosive coating, and may eventually break down, when pumping some types of gas and oil from Russian Siberian fields containing acidic aggressive components. Therefore, in recent years, PJSC Gazprom has recommended the largest Russian metallurgical plants (such as the Zagorsk Pipe Plant, the Vyksa Steel Works, the Chelyabinsk Tube Rolling Plant, the Izhora Pipe Mill, and so on) to pay attention to the production of the largediameter thick-walled steel pipes with the internal coating from stainless steel with a thickness of about 2 mm. Such pipes can be produced (for example, on the step-by-step molding presses or the three-roll rollers) from the bimetallic sheet, the basis of which is a high-strength pipe steel (substrate) with a thickness of 19−48 mm of strength class K55−K65, covered by a layer of stainless steel (coating) with a thickness of about 2 mm. Thus, the relative thickness ratio of the substrate and the coating can reach 2/19 ≈ 10.5 %. The mechanical deformation of such bimetallic sheet is qualitatively different from the deformation of a monometallic sheet (sheet from homogeneous steel). For example, at elastoplastic stretching of a straight metal beam (sheet), the residual stresses inside the beam after deformation can significantly bend the beam’s longitudinal axis. We note that the elastoplastic transverse stretching of the pipe billets’ walls on the expander plays the main role at the expansion of the large-diameter pipes and can strongly influence on the pipes’ final diameter. Besides, thermal heating of the direct bimetallic beam (sheet) can also strongly bend the beam (sheet). Below, the mathematical model for calculating the curvature of the straight metal beam (sheet) after its elastoplastic stretching without taking into account and with taking into account thermal heating is obtained.

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