Source-linked AI summary
Coupled thermo-chemo-mechanical phase field-based modelling of hydrogen-assisted cracking in girth welds
L. Castro, Y. Navidtehrani. C. Betegón, E. Martínez-Pañeda
TL;DR
Hydrogen can reduce fracture toughness at concentrations as low as 6 ppm, motivating a computational framework that combines thermo-mechanical weld-process modelling with coupled deformation-diffusion-fracture simulations. The framework reports substantial pressure reductions from weld defects, including a 32% reduction for 1% porosity versus a defect-free case.
Problem
As little as 6 ppm hydrogen can reduce fracture toughness, highlighting the need to assess hydrogen-related weld integrity.
Method
The framework combines thermo-mechanical weld-process modelling with coupled deformation-diffusion-fracture simulations.
Results
32% porosity-related reduction in maximum H2 pipeline pressure was reported for 1% porosity versus the defect-free case.
Takeaways & Limitations
Porosity, root contraction defects, and weld imperfections reduce failure pressure by 44%, 36%, and 28%, respectively.
Takeaways & Limitations
Laboratory settings have difficulty mimicking real pipeline working conditions, including residual stresses and non-conventional defects.
Abstract
from arXiv · showhide
A new computational framework is presented to predict the structural integrity of welds in hydrogen transmission pipelines. The framework combines: (i) a thermo-mechanical weld process model, and (ii) a coupled deformation-diffusion-fracture phase field-based model that accounts for plasticity and hydrogen trapping, considering multiple trap types, with stationary and evolving trap densities. This enables capturing, for the first time, the interplay between residual stresses, trap creation, hydrogen transport, and fracture. The computational framework is particularised and applied to the study of weld integrity in X80 pipeline steel. The focus is on girth welds, as they are more complex due to their multi-pass nature. The weld process model enables identifying the dimensions and characteristics of the three weld regions: base metal, heat-affected zone, and weld metal, and these are treated distinctively. This is followed by virtual fracture experiments, which reveal a very good agreement with laboratory studies. Then, weld pipeline integrity is assessed, estimating critical failure pressures for a wide range of scenarios. Of particular interest is to assess the structural integrity implications of welding defects present in existing natural gas pipelines under consideration for hydrogen transport: pores, lack of penetration, imperfections, lack of fusion, root contraction, and undercutting. The results obtained in hydrogen-containing environments reveal an important role of the weld microstructure and the detrimental effect of weld defects that are likely to be present in existing natural gas pipelines, as they are considered safe in gas pipeline standards.
1. Introduction
The paper addresses the difficulty of assessing hydrogen-assisted cracking in heterogeneous, residually stressed welds, especially when existing pipelines contain varied defects. It introduces a coupled computational framework for evaluating girth-weld integrity across such conditions.
- Hydrogen can reduce pressure-vessel steel fracture toughness from above 200 MPa√m to around 20 MPa√m at 6 ppm hydrogen.
- Weld integrity is challenging because weld metal, heat-affected zone, and base metal have different properties, fracture behaviours, and hydrogen-embrittlement susceptibility.
- Residual stresses, slanted HAZ orientation, and the need for sufficiently large fracture-test samples complicate experimental characterization of weld regions.
- Retrofitting natural-gas pipelines for hydrogen requires evaluating many pre-existing defects, weld and base materials, hydrogen purities, and pressures.
- The proposed framework combines thermo-mechanical welding simulations with elastic-plastic deformation, multi-trap hydrogen diffusion, and hydrogen-sensitive phase-field fracture.
- It examines girth welds and a broad defect set, including porosity, lack of penetration, lack of fusion, root contraction, and undercutting.
2. Welding process modeling
The welding model sequentially solves temperature and mechanical fields for a multi-pass X80 girth weld, using temperature-dependent material behaviour to predict thermal cycles and residual stresses. Results identify a roughly 3 mm HAZ and strong tensile stresses near the weld root.
- The model simulates X80 steel girth welds fabricated by shielded metal arc welding through sequential thermal and mechanical analyses.
- Temperature fields are computed first and transferred to the mechanical model to capture thermal strains and residual stresses from multi-pass welding.
- Material properties include temperature-dependent thermal expansion, conductivity, heat capacity, Young’s modulus, yield strength, density, and elasticity.
- The weld bead is activated at 1500 °C and cooled to a 125 °C interpass temperature, during which constrained thermal contraction generates the largest residual stresses.
- The modeled HAZ spans approximately 3 mm, consistent with existing experimental studies.
3. Coupled deformation-diffusion-fracture predictions of girth weld integrity
The coupled structural-integrity model uses the thermo-mechanical weld results as its starting point and is assessed against fracture experiments before predicting pipeline failure with varied defects. Its scope includes elastic-plastic phase-field fracture and multi-trap hydrogen diffusion.
- The thermo-mechanical weld simulation supplies residual-stress distributions and distinct weld regions for subsequent structural-integrity predictions.
- The coupled model combines elastic-plastic phase-field fracture with multi-trap hydrogen diffusion.
- The model is benchmarked against fracture experiments on X80 base-metal, weld-metal, and heat-affected-zone samples.
- After validation, the framework predicts structural integrity for hydrogen-transmission pipelines containing a wide range of defects.
3.1. Modelling framework
The framework couples elastic-plastic deformation, hydrogen diffusion with multiple traps, and phase-field fracture. It further represents trap evolution, hydrogen-dependent toughness, and heterogeneous trapping across base metal, weld metal, and heat-affected zones.
- Coupled formulation: The coupled formulation solves deformation, lattice hydrogen diffusion, and fracture using displacement, lattice concentration, and phase-field variables.The deformation, diffusion, and fracture sub-problems are expressed through coupled balance equations.
- Hydrogen transport: Hydrogen transport uses an effective diffusivity that combines lattice concentration with occupancy of multiple trapping-site types.The model decomposes hydrogen into lattice and trapped concentrations and accommodates multiple trap types.
- Trap evolution: Dislocation trap density evolves with equivalent plastic strain, whereas grain-boundary, α-Fe3C, and martensite-austenite trap densities remain constant.This distinction enables plastic deformation to create additional dislocation traps during the analysis.
- Hydrogen-dependent fracture: The fracture phase field degrades stiffness and evolves when stored energy reaches a toughness that depends on lattice hydrogen concentration.The fracture driving force includes tensile elastic energy and 10% of plastic energy, reflecting the assumed dissipation of the remaining plastic work as heat.
- Weld heterogeneity: The weld model assigns distinct material properties and trap densities to base metal, weld metal, and heat-affected zone regions.These heterogeneous trap choices incorporate literature data, microstructural considerations, and one-dimensional permeation experiments.
- Model calibration: The hydrogen degradation law approximates normalized toughness data with R2 = 0.992.The same degradation law is applied to each weld region as a first-order approximation because region-specific data are unavailable.
3.2. Model validation
The coupled framework is validated against fracture experiments for the distinct BM, HAZ, and WM regions of X80 pipeline steel. Boundary-layer virtual fracture experiments reproduce region-specific crack-growth resistance and fracture behavior with very good agreement to laboratory data.
- Validation results: The BM is the tougher region, whereas the HAZ is the least fracture-resistant among the weld regions.The framework also captures increasing crack-growth resistance from plastic dissipation without prescribing toughness variation a priori.
- Material calibration: Material strength is set to σc = 4σy0 for BM, 3.55σy0 for WM, and 3.75σy0 for HAZ to capture plastic dissipation during crack growth.The WM and HAZ choices reflect their lower strain hardening and available experimental data.
- Virtual fracture experiments: Boundary-layer virtual fracture experiments apply a remote KI or equivalent JI field under small-scale yielding and compare crack extension with laboratory tests.The model uses Williams’ elastic solution and exploits reflective symmetry by simulating half the domain.
- Model setup: Three independently parameterized models represent the BM, HAZ, and WM regions using their reported mechanical and fracture properties.The simulations use the same geometry and loading configuration while varying material properties to assess distinct regional fracture behavior.
- Validation results: Numerical predictions show very good agreement with experimental data for each weld region, capturing crack initiation and growth in welded X80 steel.The predicted J-R curves reproduce the distinct fracture behavior of BM, HAZ, and WM.
3.3. Predictions of girth weld integrity
The validated framework transfers welding residual stresses and region dimensions into a hydrogen-exposed girth-weld model, then evaluates defect-free and defective pipelines. Hydrogen-assisted fracture occurs below yield pressure, while several standards-acceptable defects substantially reduce failure pressure.
- Assessment scope: The weld-integrity study evaluates defect-free pipelines and defects including porosity, lack of penetration, imperfections, lack of fusion, root contraction, and undercutting.The analysis quantifies how these defects interact with hydrogen to reduce admissible pressures.
- Boundary-value model: Residual stresses from the welding model and its weld-region dimensions are transferred into the hydrogen-integrity model for a 2D axisymmetric girth-weld representation.The model applies hydrogen at the pipeline interior and allows it to exit through the exterior surface.
- Defect-free integrity: 25 MPa hydrogen-assisted failure occurs below the 30 MPa yield pressure, with cracking from the weld root along the HAZ to the outer surface.Without hydrogen, the pipeline instead fails by plastic collapse; hydrogen content and material toughness govern the earlier fracture.
- Hydrogen trapping: Dislocation-trapped hydrogen is highest near the weld root and constitutes a much larger fraction of trapped hydrogen than lattice hydrogen.The results indicate that welding residual stresses dominate hydrogen trapping and that total hydrogen varies across weld regions.
- Defect effects: 1% porosity lowers critical pressure to 17 MPa, an approximately 32% reduction, and redirects cracking from the HAZ toward a higher-porosity path through the WM.This porosity level is allowed by pipeline standards for the cited scenario.
- Defect effects: Lack of penetration and root contraction are the most harmful defects, whereas undercutting is least harmful with pf = 24 MPa and an approximately 4% reduction.Internal lack of fusion is more detrimental than outer lack of fusion, with failure pressures of 24 MPa and 18 MPa, respectively.
4. Conclusions
The framework couples thermo-mechanical weld-process modelling with deformation-diffusion-fracture simulations to assess hydrogen-assisted cracking in X80 girth welds. Results show that residual stresses, heterogeneous microstructure, and weld defects strongly influence trapping, crack paths, failure modes, and pipeline pressure capacity.
- Framework and scope: The framework combines multi-pass weld-process modelling with coupled elastic-plastic deformation, hydrogen diffusion, trapping, and phase-field fracture simulations for X80 girth welds.The process model identifies BM, WM, and HAZ dimensions and residual stresses before structural-integrity simulations quantify pipeline behaviour.
- Hydrogen trapping: Residual stresses dominate hydrogen trapping, with dislocation-trapped hydrogen reaching maximum values near the weld root.The model resolves the interaction between welding-induced residual stresses and trap populations.
- Fracture and weld heterogeneity: Elastic-plastic phase-field fracture modelling captures crack-growth resistance across BM, WM, and HAZ, including toughness increases from plastic dissipation.The simulations also reproduce distinct hydrogen trapping and embrittlement behaviour across heterogeneous weld regions, with cracking localising along the HAZ.
- Failure mechanisms: Hydrogen, residual stresses, and heterogeneous weld properties can change failure from plastic collapse to rapid fracture, even without pre-existing defects.The weld microstructure produces distinct trapping and embrittlement susceptibility across regions.
- Defect effects: Allowed 1% porosity reduces maximum H2 pipeline pressure by 32% relative to the defect-free case and shifts peak susceptibility away from the HAZ.Porosity significantly affects both failure pressure and crack trajectory.
- Defect effects: Relative to defect-free pipelines, lack of penetration reduces failure pressure by 44%, root contraction by 36%, imperfections and inner lack of fusion by 28%, and outer lack of fusion and undercutting by 4%.Multiple defects further compromise integrity, but behaviour is mainly governed by the most harmful defect; the results motivate stricter weld quality control and revised standards.
CRediT authorship contribution statement
The authors’ contributions span conceptualization, investigation, methodology, software, validation, analysis, supervision, project administration, funding, and writing.
- L. Castro contributed conceptualization, investigation, methodology, software, validation, formal analysis, and writing.
- Y. Navidtehrani contributed investigation, methodology, software, and writing–review and editing.
- C. Betegón contributed investigation, funding acquisition, supervision, project administration, and writing–review and editing.
- E. Martínez-Pañeda contributed conceptualization, investigation, methodology, resources, supervision, project administration, funding acquisition, and writing–review and editing.
Declaration of Competing Interest
The authors declare no known competing financial interests or personal relationships that could have influenced the reported work.
- The authors declare no known competing financial interests or personal relationships that could have influenced the reported work.