Morphology and development dynamics of rolled steel products manufacturing defects during long-term operation in main gas pipelines

Abstract

The study describes the morphology and development dynamics of «backfin» type manufacturing defects on a steel pipe, and outlines their generalized diagnostic signs. The inspected pipe was made of 17G1S structural steel by controlled rolling and was in operation on the main gas pipeline in the central part of the Russian Federation for more than 20 years under conditions of cyclic loading with internal pressure up to 2.5 MPa, under the influence of atmospheric corrosion and a seasonal temperature difference of –35 to +30 °C. The paper analyzes in detail the objectivity of routine non-destructive control data, and includes data from a comprehensive metallographic, microscopic, and X-ray microprobe analysis of the morphology of the surface phase composition around the defect. The study reveals conditions, when readings of non-destructive control equipment are represented incorrectly. A defect is discovered to be an extended underlying crack with a variable opening, located parallel to the surface and filled up with corrosion products and oxide scale. Corrosion products inside the defect are composed of oxides formed when rolling and due to the atmosphere corrosion of a pipe section metal. The residual strength of the repaired pipe section is determined based on the finite element modeling. It has been pointed out a maximal stress to arise around the defect, being less than 152 MPa and complying, therefore, with 33% of the yielding point and 27% of the tensile strength of a pipe section.