Mechanical characterization via sub-size specimens: Application to a longitudinal seam weld in vintage X52 pipeline steel

Repurposing vintage natural-gas pipelines for hydrogen transport makes the integrity of longitudinal seam welds critical because the heat-affected zone (HAZ) is highly heterogeneous and poorly characterized. Standard specimens cannot resolve individual sub-zones in thin-walled pipes or probe crack propagation through the wall thickness. Here, sub-size specimens combined with non-contact optical Edge-Tracing extensometry characterize a double-pass submerged-arc weld in an 11.3 mm-thick API 5L X52 steel, including the through-thickness T–S orientation inaccessible to conventional compact-tension specimens at this thickness. For the base metal tested in the T–L orientation, mDCT and standard CT specimens yielded comparable mean 𝐽 𝐼 ⁢ 𝐶 values within experimental scatter, supporting the miniature geometry. The joint contains two critical zones depending on loading mode. Under tension, the sub-critical HAZ (SCHAZ) is the softest region, exhibits the lowest reduction of area ( 𝑍 = 1 5 . 6 %), and consistently triggers cross-weld failure. Under crack-growth loading, the coarse-grained HAZ (CGHAZ) shows the lowest resistance ( 𝐽 𝐼 ⁢ 𝐶 = 3 0 kJ/m less than half the base-metal value) and forms the preferential crack path. The weld-metal bottom exhibits the highest ductility and toughness ( 𝑍 = 7 2 %, 𝐽 𝐼 ⁢ 𝐶 = 1 1 0 kJ/m 2 ), while specimens targeting the fine-grained HAZ show relatively high apparent toughness followed by crack deviation toward the CGHAZ. SCHAZ softening is associated with pearlite spheroidization, whereas low CGHAZ resistance is associated with coarse acicular ferrite, high hardness ( ∼ 2 0 5 HV0.5), and elevated stored deformation. This methodology provides a transferable framework for local integrity assessment of vintage welds and is being extended to gaseous-hydrogen environments.