National University of Science and Technology “MISiS” (Moscow, Russia):
A. P. Kolikov, Dr. Eng., Prof., Metal Forming Dept., e-mail: apkolikov@mail.ru
S. O. Ti, Undedrgraduate, Metal Forming Dept.
T. Yu. Sidorova, Senior Lecturer, Metal Forming Dept.

The results of physical and mathematical modeling of the stress-strain state of metal and residual stresses during plastic deformation of a sheet blank and welding of large-diameter pipes are generalized. One of the analytical methods for calculation of residual stresses during elastoplastic deformation of a rectangular bar and a wide strip is described. From the standpoint of continuum mechanics, it is shown that under the action of a bending moment, tensile stresses appear in the outer layers, and compressive stresses appear in the inner layers, which change sign during unloading and residual stresses remain in the metal, compressive stresses in the outer layers, and tensile stresses on the inner surface. As a result of the studies carried out, the authors for the first time obtained a picture of residual stresses distribution and found that at each technological operation of the formed pipe billet after its completion, tangential and axial residual tensile stresses σ_ост^max/σ_в ≤ 0,31, act along the entire outer perimeter and length, which after expansion-calibration of pipes decrease to the magnitude σт σ_ост/= 0,23_{σв and σост}^∞_{/= 0,18σв.} Ii was established that the value and distribution of residual stresses are significantly influenced by the blank shape, the order of welds imposition, welding modes and mechanical properties of the metal. This conclusion was also confirmed by experimental studies of residual stresses, which were obtained by X-ray diffractometry and sequential etching of surface layers using the MerCulon «Tensor-3» system. The results of experimental studies have shown that during plastic deformation of the sheet blank and after welding, residual tensile stresses are observed on the outer surface in the pipe metal and in the weld, which can cause pipeline failure. The paper describes the results of numerical modeling of non-uniformity of the stress-strain state and residual stresses in the metal of pipe processing at all stages of production according to the JCO scheme: bending of the sheet edges → forming on a step-forming press → re-forming of the «spline pipe» - a pipe profile with divorced edges → calibration - pipe expansion. Based on the results of modeling by the finite element method, it is shown that the required geometric shape of the pipe and the dimensional accuracy of the LDP diameter are achieved when expanding the workpiece with an ovality of no more than 5 mm, while ensuring high-quality assembly and welding of edges of pipes to be joined in the line of the pipeline system.

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