Influence of strain and micro-zone mechanical properties at root beads around girth welds on crack propagation

Abstract

[Objective] Girth welds are known to be vulnerable locations along high-pressure transmission pipelines due to their susceptibility to cracking accidents under the combined action of internal and external loads, as well as defects and stress concentration. Considering the significant safety hazards associated with root beads around girth welds in high-grade steel pipelines, comprehending the failure mechanisms of girth welds becomes essential in ensuring operational safety. [Methods] This study investigated the mechanical properties of materials in different zones of root beads around girth welds along pipelines, using impact and tensile tests on small-size specimens, with a focus on microzones. Since cracking failures of girth welds predominantly occur at root beads in high-grade steel pipelines, this study explored the causes of crack initiation and propagation at these specific locations, examining mechanical properties within micro-zones, strain concentration across the weld's wall thickness, and characteristics of plastic strain aging within micro-zones. Optical full-field strain tests based on Digital Image Correlation(DIC) were performed to quantitatively analyze local strain concentration in welds under three different strength matching conditions during the tensile process. [Results] The micro-zone mechanical properties at root beads round girth welds exhibited significant variation depending on the welding procedures and strength matching conditions. When subjected to tension across the wall thickness, the root beads experienced substantial local strain concentration, especially in the presence of defects such as incomplete fusion, slag inclusion, porosity, and poor formation. These defects contributed to conditions conducive to cracking at the root beads. Moreover, the toughness at the root beads was significantly reduced during the strain aging process,ranging from 42.6%to 52.1%,thereby increasing the risk of brittle cracking. [Conclusion] In pipeline design and construction,managing the upper limit of base metal strength is crucial to ensure equal-matching or over-matching welds.By reducing the risk of weld cracking,the inherent safety of high-grade steel pipelines can be enhanced.

Keywords

• high-grade steel pipeline
• weld
• root welding position
• crack propagation
• fracture
• failure analysis