Hydrogen-assisted fatigue cracking presents a critical challenge to the structural integrity of legacy carbon steel natural gas pipelines being repurposed for hydrogen transport, posing a major barrier to the deployment of hydrogen infrastructure. This study systematically evaluates the fatigue crack growth (FCG) behavior of API 5L X60 pipeline steel under varying hydrogen–natural gas (H₂–NG) blending conditions to assess its suitability for long-term hydrogen service. Experiments are conducted using a custom-designed autoclave to replicate field-relevant environmental conditions. Gas mixtures range from 0% to 100% hydrogen by volume, with tests performed at a constant pressure of 6.9 MPa and a temperature of 25 °C. A fixed loading frequency of 8.8 Hz and load ratio (R) of 0.60 ± 0.1 are applied to simulate operational fatigue loading. The test matrix is designed to capture FCG behavior across a broad range of stress intensity factor values (ΔK), spanning from near-threshold to moderate levels consistent with real-world pipeline pressure fluctuations. The results demonstrate a clear correlation between increasing hydrogen concentration and elevated FCG rates. Notably, at 100% hydrogen, API X60 specimens exhibit crack propagation rates up to two orders of magnitude higher than those in 0% hydrogen (natural gas) conditions, particularly within the Paris regime. In the lower threshold region (ΔK ≈ 10 MPa·√m), the FCG rate (da/dN) increased nonlinearly with hydrogen concentration, indicating early crack activation and reduced crack initiation resistance. In the upper Paris regime (ΔK ≈ 20 MPa·√m), da/dNs remained significantly elevated but exhibited signs of saturation, suggesting a potential limiting effect of hydrogen concentration on crack propagation kinetics. Fatigue life declined substantially with hydrogen addition, decreasing by ~33% at 50% H₂ and more than 55% in pure hydrogen. To complement the experimental investigation and enable predictive capability, a modular machine learning (ML) framework was developed and validated. The framework integrates sequential models for predicting hydrogen-induced reduction of area (RA), fracture toughness (FT), and FCG rate (da/dN), using CatBoost regression algorithms. This approach allows upstream degradation effects to be propagated through nested model layers, enhancing predictive accuracy. The ML models accurately captured nonlinear trends in fatigue behavior across varying hydrogen concentrations and environmental conditions, offering a transferable tool for integrity assessment of hydrogen-compatible pipeline steels. These findings confirm that even low-to-moderate hydrogen blends significantly reduce fatigue resistance, underscoring the importance of data-driven approaches in guiding material selection and infrastructure retrofitting for future hydrogen energy systems.

Hydrogen embrittlement (HE) poses a major challenge for the safe transportation of hydrogen (H2) through existing pipeline systems, especially in carbon steel pipelines originally designed for natural gas. This study systematically examines the susceptibility of three common pipeline steels (API 5L X52, X60, and X70) to HE under controlled hydrogen exposure. It examines the effects of key factors such as temperature (10–50 °C), hydrogen concentration (100% H2 versus 50% H2 in natural gas), and oxygen (O2) levels (0–1000 ppm) on their mechanical performance using slow strain rate tensile (SSRT) tests at 1.12 × 10−4 s−1 in high-pressure hydrogen environments. Results indicate that X52 is most vulnerable to embrittlement, with the reduction of area (RA) falling to 35.3% in 100% hydrogen at 0 ppm oxygen, while X70 displays better resistance, maintaining 58.5% RA under the same conditions. Introducing oxygen into the hydrogen environment significantly reduces embrittlement, with mechanical properties remaining stable at 100 ppm and nearly fully preserved at 1000 ppm. Fractographic analysis shows brittle intergranular fracture at low oxygen levels, whereas ductile microvoid coalescence occurs at higher oxygen concentrations. The effect of temperature on embrittlement is nonlinear: it is most severe at 10 °C due to hydrogen trapping at microstructural defects, whereas at higher temperatures (35–50 °C) increased hydrogen mobility reduces localized accumulation, enhancing ductility. These findings provide quantitative insights into the potential of gas-phase inhibition as a mitigation strategy and underscore the importance of material selection and environmental control in ensuring the long-term safety and reliability of hydrogen transportation pipelines.

As hydrogen–natural gas blending emerges as a near-term pathway to decarbonize gas transmission systems, understanding its implications for the fatigue integrity of legacy pipelines remains a critical challenge. This study investigates hydrogen-assisted fatigue crack growth (HA-FCG) in vintage, service-aged API 5L X52 and X70 pipeline steels to quantify grade-dependent degradation across the full hydrogen-blending range and to develop a predictive framework that explicitly accounts for the hydrogen-degraded material state. The fatigue behavior of compact-tension specimens cut from API pipes was tested in situ in a high-pressure autoclave at a constant total pressure of 6.9 MPa and ambient temperature, under hydrogen–natural gas blends ranging from 0% to 100% hydrogen. Fatigue crack propagation was characterized by crack-length and crack-growth-rate analyses as functions of the stress-intensity-factor range, complemented by scanning electron microscopy to elucidate hydrogen-induced fracture mechanisms. In parallel with the experimental investigation, a machine-learning framework was developed in which hydrogen-degraded reduction of area (RA) and fracture toughness (FT) were first inferred from independent mechanical-property datasets and then integrated as material-state descriptors for fatigue-crack-growth prediction. The results show a systematic acceleration of crack growth with increasing hydrogen fraction for both steels, as evidenced by leftward shifts in crack-length–cycle curves and upward shifts in da/dN–ΔK responses. Hydrogen sensitivity is strongly dependent on steel grade. X70 exhibits substantially greater degradation than X52, with fatigue life reductions approaching 60% at 100% hydrogen, compared with approximately 30% for X52. Pronounced early-life sensitivity is observed even at low hydrogen contents. Hydrogen effects are nonlinear and tend to stabilize at high blend fractions, consistent with fractographic observations indicating a transition from ductile tearing in natural gas to terrace- and facet-controlled crack advance with secondary cracking and river-pattern features in hydrogen-rich environments. The proposed modeling framework accurately reproduces hydrogen- and grade-dependent crack growth trends across regimes and provides predictions useful for pipeline integrity screening. Collectively, this work presents new experimental evidence of hydrogen-induced fatigue degradation in service-aged pipeline steels under realistic blending conditions and advances beyond grade-based descriptions by incorporating hydrogen-degraded material properties into predictive fatigue crack growth models.

This study investigates hydrogen-assisted fatigue crack growth (FCG) in vintage pipeline steels to quantify grade-dependent degradation under hydrogen–natural gas blending conditions. Fatigue behavior was evaluated using compact-tension specimens extracted from API X52 and X70 pipeline steels and tested in natural gas–hydrogen mixtures at a total pressure of 6.9 MPa and ambient temperature. Hydrogen concentration was systematically varied from 0% to 100% H₂ to assess its influence on crack-length evolution, fatigue crack growth rate, and fracture morphology. Crack propagation was characterized as a function of the stress-intensity-factor range, and scanning electron microscopy was used to examine hydrogen-induced changes in fracture mechanisms. The results demonstrate that FCG accelerates as hydrogen concentration increases, with a strong dependence on steel grade. X70 exhibited substantially greater hydrogen-induced FCG acceleration than X52, despite showing better fatigue resistance under hydrogen-free conditions. Fatigue life reductions approached 60% for X70 at 100% hydrogen, compared with approximately 30% for X52 under the same conditions. Significant early-life sensitivity was observed in X70 even at low hydrogen concentrations, whereas X52 showed more pronounced acceleration during later stages of crack growth. The influence of hydrogen was nonlinear and tended to stabilize at elevated blend fractions, indicating a saturation-type response once hydrogen-assisted crack growth became dominant. Fractographic analyses revealed a transition from ductile tearing in natural gas environments to terrace- and facet-controlled crack propagation in hydrogen-rich environments, accompanied by secondary cracking and river-pattern features. These findings demonstrate that hydrogen–natural gas blending can significantly alter fatigue crack growth behavior and relative material performance in pipeline steels, highlighting the need for grade-specific integrity assessment of existing pipeline infrastructure.

This study investigates hydrogen-assisted fatigue crack growth (FCG) in vintage pipeline steels to quantify grade-dependent degradation and develop a FCG model that accounts for the hydrogen-degraded material state. Fatigue behavior was evaluated using compact-tension specimens extracted from API pipeline steels and tested in natural gas–hydrogen mixtures at a total pressure of 6.9 MPa and ambient temperature, with hydrogen concentrations ranging from 0% to 100%. Fatigue crack propagation was characterized through crack-growth-rate analyses as a function of the stress-intensity-factor range, and scanning electron microscopy was employed to elucidate the underlying hydrogen-induced fracture mechanisms. Concurrently, a machine-learning framework was developed in which hydrogen-degraded reduction of area and fracture toughness were first inferred from independent mechanical-property datasets and subsequently incorporated as material-state descriptors for fatigue-crack-growth prediction.The results demonstrate that FCG accelerates as hydrogen concentration increases. Hydrogen embrittlement shows a strong dependence on steel grade, with X70 exhibiting substantially greater degradation than X52. Fatigue life reductions approached 60% for X70 at 100% hydrogen, compared with approximately 30% for X52 under the same conditions. Significant early-life sensitivity was observed even at low hydrogen concentrations. The influence of hydrogen was nonlinear and tended to stabilize at elevated blend fractions. Fractographic analyses revealed a transition from ductile tearing in natural gas environments to terrace- and facet-controlled crack propagation, accompanied by secondary cracking and river-pattern features, in hydrogen-rich environments. The proposed modeling framework accurately captures hydrogen- and grade-dependent fatigue crack growth behavior across different regimes and provides predictive capabilities for pipeline integrity assessment and screening.