高強度ラインパイプ円周溶接部の延性破壊クライテリオンと歪ベース設計

Abstract

Fracture behavior of high strength linepipes with weld defects is of great interest for the integrity of pipeline system. Especially, in the seismic or permafrost area, where large grand displacement can be expected, linepipe materials need to have sufficient resistance against brittle and ductile fracture under large deformation. Since large weld flaw can be eliminated by recent advance in material, welding and inspection technology, ductile fracture must be a next concern for high strength linepipe. Therefore, ductile fracture behavior of girth weld joints of Grade X80 and X100 linepipes were investigated in this study. Wide plate tensile tests were conducted using girth weld joints with the surface notch in the weld metal. Close observations were conducted in order to determine crack initiation from the notch root of the wide plate specimens. Local stress-strain conditions were evaluated by 3-D FE analysis, and criterion for ductile cracking were compared with that was obtained by small-scale specimen. Results showed that ductile cracking behavior is strongly affected by hardening properties of the base materials. Resistance to cracking in girth weld metal can be improved by applying the materials with lower Y/T ratio even with the similar weld metal property, and over-matching of the weld metal should be important condition. Concepts for defect assessment in terms of preventing ductile cracking are discussed with regard to the effect of base metal properties and strength matching of girth weld joint.