This study investigates the fatigue crack growth rate (FCGR) behavior of pipeline and low-alloy pressure vessel steels in high-pressure gaseous hydrogen. Despite a broad range of yield strengths and microstructures ranging from ferrite/pearlite, acicular ferrite, bainite, and martensite, the FCGR in gaseous hydrogen remained consistent (falling within a factor of 2–3). Steels with higher fractions of pearlite, typical of older vintage pipeline steels, exhibited modestly lower crack growth rates in gaseous hydrogen compared to steels with lower fractions of pearlite. Crack growth rates in these materials exhibit a systematic dependence on stress ratio and partial pressure of hydrogen, as captured in the recently published fatigue design curves in ASME B31 code case 220 for pipeline steels and ASME BPVC code case 2938 for pressure-vessel steels.
Existing natural gas (NG) pipeline infrastructure can be used to transport gaseous hydrogen (GH2) or blends of NG and hydrogen as low carbon alternatives to NG. Pipeline steels exhibit accelerated fatigue crack growth rates and reduced fracture resistance in the presence of GH2. The hydrogen-assisted fatigue crack growth (HAFCG) rates and hydrogen assisted fracture (HAF) resistance for pipeline steels depend on the hydrogen gas pressure. This study aims to correlate and compare the HAFCG rates of pipeline steels tested in two different gaseous environments at different pressures; high-purity hydrogen (99.9999 % H2) and a blend of nitrogen with 3% hydrogen gas (N2+3%H2). K-controlled FCG tests were performed using compact tension (CT) samples extracted from a vintage X52 (installed in 1962) and a modern X70 (2021) pipeline steel in the different gaseous environments. Subsequently, monotonic fracture tests were performed in the GH2 environment. The HAFCG rates increased with increasing GH2 pressure for both steels, in the ΔK range explored in this study. Nearly identical HAFCG rates were observed for the steels tested in different environments with equivalent fugacity (34.5 bar pure GH2 and 731 bar Blend with 3%H2). The fracture resistance of pipeline steels was significantly reduced in the presence of GH2, even at pressure as low as 1 bar. The reduction in HAF resistance tends to saturate with increasing GH2 pressure. While the fracture resistance of modern steel is substantially higher than vintage steel in air, in high pressure GH2, the HAF resistance is comparable. Similar HAF resistance values were obtained for the respective steels in the pure and blended GH2 environment with similar fugacity. This study confirms that fugacity parameter can be used to correlate HAFCG and HAF behavior of different hydrogen blends. The fracture surface features of the pipeline steels, tested in the different environments are compared to rationalize the observed behavior in GH2.
To decarbonize the energy sector, there are international efforts to displace carbon-based fuels with renewable alternatives, such as hydrogen. Storage and transportation of gaseous hydrogen are key components of large-scale deployment of carbon-neutral energy technologies, especially storage at scale and transportation over long distances. Due to the high cost of deploying large-scale infrastructure, the existing pipeline network is a potential means of transporting blended natural gas-hydrogen fuels in the near term and carbon-free hydrogen in the future. Much of the existing infrastructure in North America was deployed prior to 1970 when greater variability existed in steel processing and joining techniques often leading to microstructural inhomogeneities and hard spots, which are local regions of elevated hardness relative to the pipe or weld. Hard spots, particularly in older pipes and welds, are a known threat to structural integrity in the presence of hydrogen. High-strength materials are susceptible to hydrogen-assisted fracture, but the susceptibility of hard spots in otherwise low-strength materials (such as vintage pipelines) has not been systematically examined. Assessment of fracture performance of pipeline steels in gaseous hydrogen is a necessary step to establish an approach for structural integrity assessment of pipeline infrastructure for hydrogen service. This approach must include comprehensive understanding of microstructural anomalies (such as hard spots), especially in vintage materials. In this study, fracture resistance of pipeline steels is measured in gaseous hydrogen with a focus on high strength materials and hardness limits established in common practice and in current pipeline codes (such as ASME B31.12). Elastic-plastic fracture toughness measurements were compared for several steel grades to identify the relationship between hardness and fracture resistance in gaseous hydrogen.
Natural gas pipelines could be an important pathway to transport gaseous hydrogen (GH2) as a cleaner alternative to fossil fuels. However, a comprehensive understanding of hydrogen-assisted fatigue and fracture resistance in pipeline steels is needed, including an assessment of the diverse microstructures present in natural gas infrastructure. In thus study, we focus on modern steel pipe and consider both welded pipe and seamless pipe. In-situ fatigue crack growth (FCG) and fracture tests were conducted on compact tension samples extracted from the base metal, seam-weld, and heat affected zone of an X70 pipe steel in high-purity GH2 (210 bar pressure). Additionally, a seamless X65 pipeline microstructure (with comparable strength) was evaluated to compare the different microstructure of seamless pipe. The different microstructures had comparable FCG rates in GH2, with crack growth rates up to 30 times faster in hydrogen compared to air. In contrast, the fracture resistance in GH2 depended on the characteristics of the microstructure varying in the range of approximately 80 to 110 MPa√m.
There is a global interest in decarbonizing the existing natural gas infrastructure by blending the natural gas with hydrogen. However, hydrogen is known to embrittle pipeline and pressure vessel steels used in gas transportation and storage applications. Thus, assessing the structural integrity of vintage pipeline (pre-1970s) in the presence of gaseous hydrogen is a critical step towards successful implementation of hydrogen blending into existing infrastructure. To this end, fatigue crack growth (FCG) behavior and fracture resistance of several vintage X52 pipeline steels were evaluated in high purity gaseous hydrogen environments at pressure of 210 bar (3,000 psi) and 34 bar (500 psi). The base metal and seam weld microstructures were characterized using optical microscopy, scanning electron microscopy (SEM) and Vickers hardness mapping. The base metals consisted of ferrite-pearlite banded microstructures, whereas the weld regions contained ferrite and martensite. In one case, a hook-like crack was observed in an electric resistance (seam) weld; whereas hard spots were observed near the bond line of a double-submerged arc (seam) weld. For a given hydrogen gas pressure, comparable FCG rates were observed for the different base metal and weld microstructures. Generally, the higher strength microstructures had lower fracture resistance in hydrogen. In particular, lower fracture resistance was measured when local hard spots were observed in the approximate region of the crack plane of the weld. Samples tested in lower H2 pressure (34 bar) exhibited lower FCG rates (in the lower ∆K regime) and greater fracture resistance when compared to the respective high-pressure (210 bar) hydrogen tests. The hydrogen-assisted fatigue and fracture surfaces were qualitatively characterized using SEM to rationalize the influence of microstructure on the dominant fracture mechanisms in gaseous hydrogen environment.