Specifics of diagnostics and investigation of material properties in pipes with planar defects

The paper presents findings of a study on planar defects formed on the inner surface of gas pipeline metal in the operation process. The diagnostic indicators of a defective section were determined and analyzed using the intelligent pigging data. A broad range of research was carried out, including visual and measuring control of the object, step-by-step ultrasonic thickness measurement, chemical analysis, optical and electron microscopy, microhardness measurement of defective and defect-free areas, static tensile and impact bending tests. The structure and morphology of defective and defect-free areas were considered. It was determined that the cause of the defects is contamination of the source material with gas pores and fluorspar residues. The defects of planar nature are formed during rolling and can be characterized as metal delamination. The maximum detected thickness of the inclusions was 1.0 mm, however, the main part was rolled to 10.0 μm. The key feature of defects is their location on various levels throughout the wall cross-sections, ranging from one to four detected layers or groups of rolled discontinuities. It is shown that such defects lead to a change in the strength properties of defective area, uneven distribution of loads and reduction in safety margin of the object. • The intelligent pigging by magnetic flaw detectors is ineffective for inner laminations at different depths of occurrence. • The precise detection of zones with laminations is provided only by the ultrasonic thickness measurement. • The material containing inner defects with a varying thickness has considerably different physical parameters.