Microstructure, texture, and property characteristics of high-strength K65–K80 grade pipes for next-generation gas pipelines

Abstract

This study investigates the microstructure, texture, and properties of high-strength K70 and K80 steel pipes that were subjected to full-scale pneumatic testing. The testing was done to evaluate the suitability of the pipes for next-generation gas trunk pipelines that operate at a pressure of 14.7 MPa (150 atm). The results are compared with previously accumulated data on K65 grade pipes, which were developed for and used in the construction of the Bovanenkovo–Ukhta gas pipeline, which was designed to operate at an elevated pressure of 11.8 MPa (120 atm). A characteristic feature of fracture in pipe base metal with insufficient crack resistance includes a limited plastic deformation zone, a relatively low degree of deformation at the fracture point, and numerous splittings along the rolling plane on the fracture surface. This fracture mode involves uncontrolled crack propagation along the pipe string under high test pressure and is linked to these microstructural characteristics. Despite its high impact toughness values, the K80 grade pipe steel, which has higher strength but significantly lower ductility than the K70 grade, does not effectively arrest running cracks. This study investigated how structural banding and texture characteristics influence the propensity for splitting formation upon fracture in a series of experimental pipes from both strength classes. The results suggest that increasing the intensity of the rotated-cube texture component decreases the toughness measured by instrumented drop-weight tests. Additionally, deformation zone characteristics from full-scale field tests with those from drop-weight tear test specimens for the examined strength grades were compared. Analysis revealed that K80 pipe metal exhibits the least plastic deformation and consequently the lowest fracture energy. In contrast, the K65 and K70 grades demonstrate significantly higher values for these characteristics, confirming their suitability for high-pressure gas pipeline construction.