A computational framework to predict weld integrity and microstructural heterogeneity: application to hydrogen transmission

TL;DR

The paper addresses the challenge of predicting weld-induced microstructural heterogeneity and its effect on hydrogen embrittlement in pipelines. It develops a two-stage framework that first predicts local phase fractions via a thermo-metallurgical-weld model and then uses those maps in a coupled elastoplastic phase-field fracture model with hydrogen diffusion to compute failure pressures. Key contributions include a Li-based metallurgical kinetics model, integration with a diffusion-fracture framework, and validation against microhardness maps and residual-stress data, demonstrated on X60/X52 pipelines. The approach enables mechanistic assessment of weld configurations and hydrogen loading, offering insights into defect tolerance and critical pressures for repurposed hydrogen transport infrastructure.

Abstract

We present a novel computational framework to assess the structural integrity of welds. In the first stage of the simulation framework, local fractions of microstructural constituents within weld regions are predicted based on steel composition and welding parameters. The resulting phase fraction maps are used to define heterogeneous properties that are subsequently employed in structural integrity assessments using an elastoplastic phase field fracture model. The framework is particularised to predicting failure in hydrogen pipelines, demonstrating its potential to assess the feasibility of repurposing existing pipeline infrastructure to transport hydrogen. First, the process model is validated against experimental microhardness maps for vintage and modern pipeline welds. Additionally, the influence of welding conditions on hardness and residual stresses is investigated, demonstrating that variations in heat input, filler material composition, and weld bead order can significantly affect the properties within the weld region. Coupled hydrogen diffusion-fracture simulations are then conducted to determine the critical pressure at which hydrogen transport pipelines will fail. To this end, the model is enriched with a microstructure-sensitive description of hydrogen transport and hydrogen-dependent fracture resistance. The analysis of an X52 pipeline reveals that even 2 mm defects in a hard heat-affected zone can drastically reduce the critical failure pressure.

Figures (15)

• Figure 1: Schematic of the different zones in a weld and outline of the two-stage modeling framework. In stage 1, the welding process, including solid-state phase transformations in the underlying microstructure, is simulated. In stage 2, welding process results are used as initial conditions for an elastoplastic phase field fracture model.
• Figure 2: Phase transformation behavior of the implemented model. (a) Time-temperature-transformation diagram obtained by \ref{['eq:transf_f', 'eq:transf_p', 'eq:transf_b']}. (b) Continuous cooling transformation diagram obtained by making use of the additivity rule.
• Figure 3: Phase transformation behavior upon cooling down from the fully austenitic phase. (a) Volumetric strain due to thermal expansion and phase transformations for thermal cycles with different cooling rates. (b) Evolution of the phase fractions upon cooling with a cooling rate of -3$^\circ$C/s.
• Figure 4: Fracture toughness data and model fits. (a) J-R curves fitted on experimental data of X52 and X100 grades Ronevich2018Ronevich2021 (b) Hydrogen degradation law fitted on experimental data of X52 and X100 grades Ronevich2021SanMarchi2021.
• Figure 5: Outline of the implementation of the coupled metallurgical-thermal model at integration point level.