Influence of microstructural gradient on Charpy V-notch impact toughness variability in X65 pipeline girth weld

Abstract

This study investigates the ductile-to-brittle transition behaviour of weld metal in a multipass girth weld extracted from a North American CO₂ steel pipeline (X65 grade), with a focus on understanding the causes of scatter in Charpy V-notch (CVN) impact energy values, particularly between −60 °C and −25 °C. CVN specimens were tested between −80 °C and 25 °C, and selected samples underwent post-test characterization to correlate fracture behaviour with microstructural variation through the weld thickness. Fracture surfaces and microstructures along the notch front and fracture edges were examined using optical microscopy, scanning electron microscopy, and electron backscattered diffraction. The results showed that the observed scatter in toughness data was primarily due to variations in the extent of the coarse columnar ferritic structure zone intersecting the CVN notch front. This microstructure, predominantly found in the weld cap and outer fill passes, was associated with lower impact toughness. A low density of high-angle grain boundaries (HAGBs) in these regions reduced resistance to crack propagation, contributing to the formation of large cleavage facets. The alignment of {001} cleavage planes with fracture edges and the presence of microcracks further highlighted the role of crystallography in fracture behaviour. Conversely, zones with finer equiaxed grains, present in the initial fill passes and characterized by a high density of HAGBs, exhibited enhanced plasticity and absorbed higher energy. The proposed integrated approach provides a microstructure-sensitive framework for interpreting scatter in weld metal toughness, supporting improved structural integrity assessments for pipeline welds.