PSI - Issue 68

Oleksii Milenin et al. / Procedia Structural Integrity 68 (2025) 1010–1016 Oleksii Milenin et al./ Structural Integrity Procedia 00 (2025) 000–000

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2.3. Case study A case study was conducted to analyse the impact of technological parameters of in-service repair welding of main gas pipelines on cold cracking of pipe metal, considering the effects of diffusible hydrogen concentration V H under gas-hydrogen blend transportation conditions. The study focused on a straight section of a main pipeline with a diameter of 1420 mm and a wall thickness of 20 mm, made from X65 steel, operational pressure 7.5 MPa, internal pressure during repair is 4.0 MPa. The operational defect considered was a local wall thinning with dimensions of 150 mm in length, 100 mm in width, and 10 mm in depth, which required repair through in-service welding. A defect of this size can be remedied by multi-pass weld deposition, following the scheme illustrated in Fig. 2. The welding parameters used were a welding speed of 5 mm/s and a welding current of 100 A. The repair area was preheated to a specific temperature p , and the time between consecutive weld passes was controlled to prevent the metal from cooling below the preheat temperature.

Fig. 2. Scheme of multi-pass weld deposition of corroded pipe.

3. Results and discussion The finite element analysis of the temperature field kinetics, phase transformations, hydrogen distribution, and stress-strain state during in-service repair welding of a main gas pipeline revealed a significantly nonuniform spatial distribution of these physical fields in the repair area (see Fig. 3). This heterogeneity arises not only from the localized heating during welding and the varying temperature cycles across the defective structure's cross-section but also from the gradual alteration of the pipe's geometry as metal is deposited. This observation further underscores the rationale behind selecting an integral probability criterion for cold cracking, as it effectively accounts for both the specific physical processes and the technological intricacies associated with repair welding.

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Fig. 3. Distribution of residual circumferential stresses (a), martensitic phase ratio (b), and diffusible hydrogen concentration (c) in the cross section of the corroded main pipeline in the region of in-service welding.