Tailored heat treatments to characterise the fracture resistance of critical weld regions in hydrogen transmission pipelines

TL;DR

This work addresses hydrogen embrittlement risk in hydrogen-transmission pipelines by focusing on the weld HAZ, a region rich in non-equilibrium microstructures. It introduces a protocol to tailor heat treatments that replicate ferrite–bainite, bainite, and martensite microstructures in bulk specimens, enabling direct SENT fracture testing in air and 100 bar $H_2$ across vintage X60, modern X60, and X80 steels. The main findings show that HAZ microstructures dramatically reduce fracture resistance in hydrogen, with $K_{J0}$ as low as $32~ ext{MPa} oot ext{m}$ for martensite in a vintage steel, and that microstructure effects predominate over steel age or grade. Practically, the results provide microstructure-specific toughness data to inform design, fitness-for-service assessments, and computational weld models for hydrogen pipelines, highlighting the need to account for brittle HAZ regions in hydrogen service.

Abstract

A new protocol is presented to directly characterise the toughness of microstructural regions present within the weld heat-affected zone (HAZ), the most vulnerable location governing the structural integrity of hydrogen transport pipelines. Heat treatments are tailored to obtain bulk specimens that replicate predominantly ferritic-bainitic, bainitic, and martensitic microstructures present in the HAZ. These are applied to a range of pipeline steels to investigate the role of manufacturing era (vintage versus modern), chemical composition, and grade. The heat treatments successfully reproduce the hardness levels and microstructures observed in the HAZ of existing natural gas pipelines. Subsequently, fracture experiments are conducted in air and pure H2 at 100 bar, revealing a reduced fracture resistance and higher hydrogen embrittlement susceptibility of the HAZ microstructures, with initiation toughness values as low as 32 MPa$\sqrt{\text{m}}$. The findings emphasise the need to adequately consider the influence of microstructure and hard, brittle zones within the HAZ.

Figures (15)

• Figure 1: Local hard, brittle regions within the weld compromise the integrity of hydrogen transmission pipelines. Hardness maps of welds associated with natural gas pipelines taken out of service: (a) submerged metal-arc welding (SMAW) girth weld of the X80 steel studied in this work, (b) electric resistance weld (ERW) seam weld of a vintage X46 steel, and (c) SMAW girth weld of the X60 vintage steel studied herein, with a corresponding optical micrograph confirming the presence of martensite in the HAZ. In all cases hardness values exceeding 350 HV (and, in some cases, 500 HV) are visible (vs base metal values in the range 150-200 HV).
• Figure 2: Base metal microstructure for (a) the vintage X60 steel (X60V), showing the presence of MnS inclusions (red dashed lines), (b) the modern X60 steel (X60M), and (c) the X80 steel.
• Figure 3: Representative illustration of local weld heterogeneities: (a) Girth and (b) seam welds of the X60V pipeline characterised in this work. The optical micrographs in (c) illustrate the HAZ sub-zones between the fusion zone (FZ) and the base metal (BM), going from coarse-grain bainitic microstructures near the FZ to fine-grain ferritic-bainitic microstructures near the BM.
• Figure 4: Combining experiments and modelling to establish suitable heat treatments: (a) Temperature-time profile for the heat treatments conducted to replicate the HAZ microstructure in base metal cylinders; (b) Maximum temperature contour along the HAZ, obtained through a two-pass weld simulation using the model developed in Ref. wijnen2025computational; and comparison of the experimental and predicted cooling curves for peak temperatures of (c) 1000 $^\circ\text{C}$ followed by oil quenching (OQ), (d) 1100 $^\circ\text{C}$ followed by oil quenching (OQ), and (e) 1300 $^\circ\text{C}$ followed by water quenching (WQ). The simulation points correspond to those highlighted in the $T_{max}$ contours. The second increase in temperature observed in the dashed curve for the first pass of the weld corresponds to the deposition of the second bead, shown by the solid line.
• Figure 5: Single-edge notched tension (SENT) specimen geometry and characteristics: (a) sample extraction location from the steel pipes, and (b) specimen layout (all dimensions in mm).