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.
Hydrogen is known to embrittle austenitic stainless steels, which are widely used in high-pressure hydrogen storage and delivery systems, but the mechanisms that lead to such material degradation are still being elucidated. The current work investigates the deformation behavior of single crystal austenitic stainless steel 316L through combined uniaxial tensile testing, characterization and atomistic simulations. Thermally precharged hydrogen is shown to increase the critical resolved shear stress (CRSS) without previously reported deviations from Schmid's law. Molecular dynamics simulations further expose the statistical nature of the hydrogen and vacancy contributions to the CRSS in the presence of alloying. Slip distribution quantification over large in-plane distances (>1 mm), achieved via atomic force microscopy (AFM), highlights the role of hydrogen increasing the degree of slip localization in both single and multiple slip configurations. The most active slip bands accumulate significantly more deformation in hydrogen precharged specimens, with potential implications for damage nucleation. For 〈110〉 tensile loading, slip localization further enhances the activity of secondary slip, increases the density of geometrically necessary dislocations and leads to a distinct lattice rotation behavior compared to hydrogen-free specimens, as evidenced by electron backscatter diffraction (EBSD) maps. The results of this study provide a more comprehensive picture of the deformation aspect of hydrogen embrittlement in austenitic stainless 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.
Structural materials used in combustion or power generation systems need to have both environmental and temperature resistance to ensure long-term performance. As the energy sector transitions to hydrogen, there is a need to ensure compatibility of highly-alloyed austenitic steels and nickel-based alloys with hydrogen over a range of temperatures. Hydrogen embrittlement of these alloy systems is often considered most detrimental near ambient temperatures and low temperatures, although there is some evidence in the literature that hydrogen can affect creep behavior at elevated temperature. In the intermediate temperature range (e.g., 100-400C), it is uncertain whether hydrogen degradation of mechanical properties will be of concern. In this study, three alloys (304L, IN625, Hastelloy X) commonly used in power generation systems were thermally precharged with hydrogen and subsequently tensile tested to failure in air at temperatures ranging from 20°C to 200°C. At 20°C, the hydrogen-precharged condition for all materials exhibited loss in ductility with relative reduction of area ranging between 32% and 57%. The three alloys exhibited different trends with temperature but, in general, the relative reduction of area improved with increasing temperature tending towards noncharged behavior. Tests were performed at a nominal strain rate of 2 x 10-3 s-1 in order to minimize loss of hydrogen during elevated temperature testing. Hydrogen contents from the grip sections were measured both before and after testing and remained within 10% of starting content for 100°C tests and within 8-23% for 200°C tests.
Gaseous hydrogen is known to embrittle most steels, including the steels used in natural gas pipelines. As injection of hydrogen into the existing natural gas infrastructure is considered globally by the pipeline industry, the structural integrity of pipelines transporting gaseous hydrogen must be investigated. Hydrogen Extremely Low Probability of Rupture (HELPR) is a publicly available and open-source probabilistic fatigue and fracture mechanics toolkit recently developed at Sandia National Laboratories. HELPR is intended to incorporate the influence of hydrogen into structural integrity assessments of natural gas transmission and distribution infrastructure. HELPR utilizes engineering models, such as those specified in ASME B31.12 and API 579, with relatively low computational costs to perform large sample ensembles, enabling estimation of performance distributions including low probability tail estimates. Leveraging the probabilistic capabilities built into HELPR, the sensitivity of fatigue and fracture calculations to specific modeling parameters on performance margins can be quantified. Through applying HELPR’s probabilistic capabilities to realistic scenarios, the impact of uncertainty in specific model parameter descriptions on performance margins, such as cycles to unstable crack growth or rupture in gaseous hydrogen, can be characterized; this same approach can then be used to assess the impact of reducing uncertainty sources on the resulting performance metrics, margins, and associated risks. A few industry-motivated scenarios are used to demonstrate this approach.
Full-scale testing of pipes is costly and requires significant infrastructure investments. Subscale testing offers the potential to substantially reduce experimental costs and provides testing flexibility when transferrable test conditions and specimens can be established. To this end, a subscale pipe testing platform was developed to pressure cycle 60 mm diameter pipes (Nominal Pipe Size 2) to failure with gaseous hydrogen. Engineered defects were machined into the inner surface or outer surface to represent pre-existing flaws. The pipes were pressure cycled to failure with gaseous hydrogen at pressures to match operating stresses in large diameter pipes (e.g., stresses comparable to similar fractions of the specified minimum yield stress in transmission pipelines). Additionally, the pipe specimens were instrumented to identify crack initiation, such that crack growth could be compared to fracture mechanics predictions. Predictions leverage an extensive body of materials testing in gaseous hydrogen (e.g., ASME B31.12 Code Case 220) and the recently developed probabilistic fracture mechanics framework for hydrogen (Hydrogen Extremely Low Probability of Rupture, HELPR). In this work, we evaluate the failure response of these subscale pipe specimens and assess the conservatism of fracture mechanics-based design strategies (e.g., API 579/ASME FFS). This paper describes the subscale hydrogen testing capability, compares experimental outcomes to predictions from the probabilistic hydrogen fracture framework (HELPR), and discusses the complement to full-scale testing.
Emerging hydrogen technologies span a diverse range of operating environments. High-pressure storage for mobility applications has become commonplace up to about 1,000 bar, whereas transmission of gaseous hydrogen can occur at hydrogen partial pressure of a few bar when blended into natural gas. In the former case, cascade storage is utilized to manage hydrogen-assisted fatigue and the Boiler and Pressure Vessel Code, Section VIII, Division 3 includes fatigue design curves for fracture mechanics design of hydrogen vessels at pressure of 1,030 bar (using a Paris Law formulation). Recent research on hydrogen-assisted fatigue crack growth has shown that a diverse range of ferritic steels show similar fatigue crack growth behavior in gaseous hydrogen environments, including low-carbon steels (e.g., pipeline steels) as well as quench and tempered Cr-Mo and Ni-Cr-Mo pressure vessel steels with tensile strength less than 915 MPa. However, measured fatigue crack growth is sensitive to hydrogen partial pressure and fatigue crack growth can be accelerated in hydrogen at pressure as low as 1 bar. The effect of hydrogen partial pressure from 1 to 1,000 bar can be quantified through a simple semi-empirical correction factor to the fatigue crack growth design curves. This paper documents the technical basis for the pressure-sensitive fatigue crack growth rules for gaseous hydrogen service in ASME B31.12 Code Case 220 and for revision of ASME VIII-3 Code Case 2938-1, including the range of applicability of these fatigue design curves in terms of environmental, materials and mechanics variables.
Decarbonization efforts highlight hydrogen as an attractive alternative to fossil fuels, but its tendency to embrittle structural metals demands careful consideration when designing hydrogen infrastructure. Moreover, the mechanisms by which hydrogen degrades these materials are still being elucidated. The current work develops new computational tools to quantify the different contributions of hydrogen to the energy barrier of cross-slip, a key deformation mechanism. Novel features are implemented to a line tension model, which include the use of non-singular dislocation interactions, character-dependent dislocation energies and simulations of the constriction configurations. A new molecular dynamics technique is developed to calculate the interaction energy between the partials of a dissociated dislocation via fixing the centers of mass of the regions below and above the Shockley partials and performing time-averaged calculations. Hydrogen is found to impact the stacking fault width of dislocations in different ways depending on their characters: it decreases for dislocations with a character θ > 30°, remains unchanged for θ = 30° and increases for θ < 30°. The latter regime is a newly identified mechanism by which hydrogen inhibits cross-slip. Moreover, formation of nano-hydrides is predicted to occur around screw dislocations for high hydrogen concentrations, a phenomenon previously identified only in dislocations with an edge component. If nano-hydrides develop, their influence extending the equilibrium stacking fault width and increasing both the constriction and cross-slip energy barriers dominate over all other hydrogen contributions. The theory and tools developed will pave the way towards a comprehensive understanding of hydrogen-dislocation interactions in structural metals.
In this work, we investigate the potential of liquid hydrogen storage (LH2) on-board Class-8 heavy duty trucks to resolve many of the range, weight, volume, refueling time and cost issues associated with 350 or 700-bar compressed H2 storage in Type-3 or Type-4 composite tanks. We present and discuss conceptual storage system configurations capable of supplying H2 to fuel cells at 5-bar with or without on-board LH2 pumps. Structural aspects of storing LH2 in double walled, vacuum insulated, and low-pressure Type-1 tanks are investigated. Structural materials and insulation methods are discussed for service at cryogenic temperatures and mitigation of heat leak to prevent LH2 boiloff. Failure modes of the liner and shell are identified and analyzed using the regulatory codes and detailed finite element (FE) methods. The conceptual systems are subjected to a Failure modes and effects analysis (FMEA) and a safety, codes, and standards (SCS) review to rank failures and identify safety gaps. The results indicate that the conceptual systems can reach 19.6% usable gravimetric capacity, 40.9 g-H2/L usable volumetric capacity and $174-183/kg-H2 cost (2016 USD) when manufactured 100,000 systems annually.
Austenitic stainless steels have been extensively tested in hydrogen environments; however, limited information exists for the effects of hydrogen on the fatigue life of high-strength grades of austenitic stainless steels. Moreover, fatigue life testing of finished product forms (such as tubing and welds) is challenging. A novel test method for evaluating the influence of internal hydrogen on fatigue of orbital tube welds was reported, where a cross hole in a tubing specimen is used to establish a stress concentration analogous to circumferentially notched bar fatigue specimens for constant-load, axial fatigue testing. In that study (Kagay et al, ASME PVP2020-8576), annealed 316L tubing with a cross hole displayed similar fatigue performance as more conventional materials test specimens. A similar cross-hole tubing geometry is adopted here to evaluate the fatigue crack initiation and fatigue life of XM-19 austenitic stainless steel with high concentration of internal hydrogen. XM-19 is a nitrogen-strengthened Fe-Cr-Ni-Mn austenitic stainless steel that offers higher strength than conventional 3XX series stainless steels. A uniform hydrogen concentration in the test specimen is achieved by thermal precharging (exposure to high-pressure hydrogen at elevated temperature for two weeks) prior to testing in air to simulate the equilibrium hydrogen concentration near a stress concentration in gaseous hydrogen service. Specimens are also instrumented for direct current potential difference measurements to identify crack initiation. After accounting for the strengthening associated with thermal precharging, the fatigue crack initiation and fatigue life of XM-19 tubing were virtually unchanged by internal hydrogen.
Austenitic stainless steels are used in high-pressure hydrogen containment infrastructure for their resistance to hydrogen embrittlement. Applications for the use of austenitic stainless steels include pressure vessels, tubing, piping, valves, fittings and other piping components. Despite their resistance to brittle behavior in the presence of hydrogen, austenitic stainless steels can exhibit degraded fracture performance. The mechanisms of hydrogen-assisted fracture, however, remain elusive, which has motivated continued research on these alloys. There are two principal approaches to evaluate the influence of gaseous hydrogen on mechanical properties: internal and external hydrogen, respectively. The austenite phase has high solubility and low diffusivity of hydrogen at room temperature, which enables introduction of hydrogen into the material through thermal precharging at elevated temperature and pressure; a condition referred to as internal hydrogen. H-precharged material can subsequently be tested in ambient conditions. Alternatively, mechanical testing can be performed while test coupons are immersed in gaseous hydrogen thereby evaluating the effects of external hydrogen on property degradation. The slow diffusivity of hydrogen in austenite at room temperature can often be a limiting factor in external hydrogen tests and may not properly characterize lower bound fracture behavior in components exposed to hydrogen for long time periods. In this study, the differences between internal and external hydrogen environments are evaluated in the context of fracture resistance measurements. Fracture testing was performed on two different forged austenitic stainless steel alloys (304L and XM-11) in three different environments: 1) non-charged and tested in gaseous hydrogen at pressure of 1,000 bar (external H2), 2) hydrogen precharged and tested in air (internal H), 3) hydrogen precharged and tested in 1,000 bar H2 (internal H + external H2). For all environments, elastic-plastic fracture measurements were conducted to establish J-R curves following the methods of ASTM E1820. Following fracture testing, fracture surfaces were examined to reveal predominant fracture mechanisms for the different conditions and to characterize differences (and similarities) in the macroscale fracture processes associated with these environmental conditions.
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.
Austenitic stainless steels have been extensively tested in hydrogen environments; however, limited information exists for the effects of hydrogen on the fatigue life of high-strength grades of austenitic stainless steels. Moreover, fatigue life testing of finished product forms (such as tubing and welds) is challenging. A novel test method for evaluating the influence of internal hydrogen on fatigue of orbital tube welds was reported, where a cross hole in a tubing specimen is used to establish a stress concentration analogous to circumferentially notched bar fatigue specimens for constant-load, axial fatigue testing. In that study (Kagay et al, ASME PVP2020-8576), annealed 316L tubing with a cross hole displayed similar fatigue performance as more conventional materials test specimens. A similar cross-hole tubing geometry is adopted here to evaluate the fatigue crack initiation and fatigue life of XM-19 austenitic stainless steel with high concentration of internal hydrogen. XM-19 is a nitrogen-strengthened Fe-Cr-Ni-Mn austenitic stainless steel that offers higher strength than conventional 3XX series stainless steels. A uniform hydrogen concentration in the test specimen is achieved by thermal precharging (exposure to high-pressure hydrogen at elevated temperature for two weeks) prior to testing in air to simulate the equilibrium hydrogen concentration near a stress concentration in gaseous hydrogen service. Specimens are also instrumented for direct current potential difference measurements to identify crack initiation. After accounting for the strengthening associated with thermal precharging, the fatigue crack initiation and fatigue life of XM-19 tubing were virtually unchanged by internal hydrogen.
Numerous projects are looking into distributing blends of natural gas and different amounts of gaseous hydrogen through the existing natural gas distribution system, which is widely composed of medium density polyethylene (MDPE) line pipes. The mechanical behavior of MDPE with hydrogen is not well understood; therefore, the effect of gaseous H2 on the mechanical properties of MDPE needs to be examined. In the current study, we investigate the effects of gaseous H2 on fatigue life and fracture resistance of MDPE in the presence of 3.4 MPa gaseous H2. Fatigue life tests were also conducted at a pressure of 21 MPa to investigate the effect of gas pressure on the fatigue behavior of MDPE. Results showed that the presence of gaseous H2 did not degrade the fatigue life nor the fracture resistance of MDPE. Additionally, based on the value of fracture resistance calculated, a failure assessment diagram was constructed to determine the applicability of using MDPE pipeline for distribution of gaseous H2. Even in the presence of a large internal crack, the failure assessment evaluation indicated that the MDPE pipes lie within the safe region under typical service conditions of natural gas distribution pipeline system.
High pressure hydrogen storage vessels are frequently retired upon reaching their designed number of pressure cycles, even in cases where the in-use pressure cycles are significantly less severe than the design pressure cycle. One method for extending the life of hydrogen vessels is recertification through non-destructive evaluation (NDE); however, NDE techniques are frequently evaluated with machined defects in test samples rather than fatigue cracks which occur during pressure cycling and may be more difficult to detect. In this paper, 50 mm wide ring sections (called C-rings, since they represent slightly more than half the circumference) were extracted from pressure vessels and mechanically cycled to establish fatigue cracks. Sub-millimeter starter notches were machined, via plunge electrical discharge machining (EDM), to control the location of crack initiation. Crack growth was monitored via direct current potential difference (DCPD) and backface strain gauges, both of which were shown to be good indicators for crack propagation. The C-ring geometry and fatigue crack growth were modeled to demonstrate the ability to monitor/control the crack length and area, which can be used to develop calibration samples of varying crack depth for NDE techniques. Additionally, this sample is intended to evaluate the influence of residual stresses on the sensitivity of NDE techniques, such as the design stresses in autofrettaged vessels.
Type 2 high-pressure hydrogen vessels for storage at hydrogen refueling stations are designed assuming a predefined operational pressure cycle and targeted autofrettage conditions. However, the resulting finite life depends significantly on variables associated with the autofrettage process and the pressure cycles actually realized during service, which many times are not to the full range of the design. Clear guidance for cycle counting is lacking, therefore industry often defaults to counting every repressurization as a full range pressure cycle, which is an overly conservative approach. In-service pressure cycles used to predict the growth of cracks in operational pressure vessels results in significantly longer life, since most in-service pressure cycles are only a fraction of the full design pressure range. Fatigue crack growth rates can vary widely for a given pressure range depending on the details of the residual strains imparted during the autofrettage process because of their influence on crack driving forces. Small changes in variables associated with the autofrettage process, e.g., the target autofrettage overburden pressure, can result in large changes in the residual stress profile leading to possibly degraded fatigue life. In this paper, computational simulation was used for sensitivity studies to evaluate the effect of both operating conditions and autofrettage conditions on fatigue life for Type 2 highpressure hydrogen vessels. The analysis in this paper explores these sensitivities, and the results are used to provide guidance on cycle counting. In particular, we identify the pressure cycle ranges that can be ignored over the life of the vessel as having negligible effect on fatigue life. This study also examines the sensitivity of design life to the autofrettage process and the impact on life if the targeted residual strain is not achieved during manufacturing.
Decarbonizing natural gas networks is a challenging enterprise. Replacing natural gas with renewable hydrogen is one option under global consideration to decarbonize heating, power and residential uses of natural gas. Hydrogen is known to degrade fatigue and fracture properties of structural steels, including pipeline steels. In this study, we describe environmental testing strategies aimed at generating baseline fatigue and fracture trends with efficient use of testing resources. For example, by controlling the stress intensity factor (K) in both K-increasing and K-decreasing modes, fatigue crack growth can be measured for multiple load ratios with a single specimen. Additionally, tests can be designed such that fracture tests can be performed at the conclusion of the fatigue crack growth test, further reducing the resources needed to evaluate the fracture mechanics parameters utilized in design. These testing strategies are employed to establish the fatigue crack growth behavior and fracture resistance of API grade steels in gaseous hydrogen environments. In particular, we explore the effects of load ratio and hydrogen partial pressure on the baseline fatigue and fracture trends of line pipe steels in gaseous hydrogen. These data are then used to test the applicability of a simple, universal fatigue crack growth model that accounts for both load ratio and hydrogen partial pressure. The appropriateness of this model for use as an upper bound fatigue crack growth is discussed.
Despite their susceptibility to hydrogen-assisted fracture, ferritic steels make up a large portion of the hydrogen infrastructure. It is impractical and too costly to build large scale components such as pipelines and pressure vessels out of more hydrogen-resistant materials such as austenitic stainless steels. Therefore, it is necessary to understand the fracture behavior of ferritic steels in high-pressure hydrogen environments to manage design margins and reduce costs. Quenched and tempered (Q&T) martensite is the predominant microstructure of high-pressure hydrogen pressure vessels, and higher strength grades of this steel type are more susceptible to hydrogen degradation than lower strength grades. In this study, a single heat of 4340 alloy was heat treated to develop alternative microstructures for evaluation of fracture resistance in hydrogen gas. Fracture tests of several microstructures, such as lower bainite and upper bainite with similar strength to the baseline Q&T martensite, were tested at 21 and 105 MPa H2. Despite a higher MnS inclusion content in the tested 4340 alloy which reduced the fracture toughness in air, the fracture behavior in hydrogen gas fit a similar trend to other previously tested Q&T martensitic steels. The lower bainite microstructure performed similar to the Q&T martensite, whereas the upper bainite microstructure performed slightly worse. In this paper, we extend the range of high-strength microstructures evaluated for hydrogen-assisted fracture beyond conventional Q&T martensitic steels.
High strength austenite-ferrite duplex stainless steels are a potential alternative to austenitic stainless steels for components in hydrogen gas storage systems. Since these components experience cyclic loading from frequent pressurization and depressurization, the effect of hydrogen on the fatigue behavior of duplex stainless steel must be understood. To determine the influence of hydrogen on fatigue crack initiation and fatigue life of a 255 super duplex stainless steel, circumferentially notched tensile (CNT) specimens were fatigue tested in the as-received condition in air, with pre-charged internal hydrogen in air, and in the as-received condition in high pressure hydrogen gas. The direct current potential difference (DCPD) method was used to detect crack initiation so that S-N curves could be produced for both (i) cycles to crack initiation and (ii) cycles to failure. An electropolished CNT specimen was also cycled in the as-received and hydrogen pre-charged conditions but interrupted just after crack initiation. The microstructural locations of small fatigue cracks were then identified with scanning electron microscopy and electron backscatter diffraction (EBSD). High pressure hydrogen gas and pre-charged hydrogen decreased the fatigue life of 255 duplex stainless steel by a nearly identical amount. The effects of hydrogen on fatigue crack initiation and fatigue life of 255 duplex stainless steel are discussed and compared to austenitic stainless steels.
Austenitic stainless steels are the standard materials for containment of hydrogen and tritium because of their resistance to mechanical property degradation in those environments. The mechanical performance of the primary containment material is critical for tritium handling, processing, and storage, thus comprehensive understanding of the processes of tritium embrittlement is an enabling capability for fusion energy. This work describes the investigation of the effects of low levels of tritium-decay-helium ingrowth on 304 L tubes. Long-term aging with tritium leads to high helium contents in austenitic stainless steels and can reduce fracture toughness by 95 %, but the details of behavior at low helium contents are not as well characterized. Here, we present results from tensile testing of tritium pre-charged 304 L tube specimens with a variety of starting microstructures that all contain a low level of helium. The results of the tritium exposed-and-aged materials are compared to previously reported results on similar specimens tested in an unexposed condition as well as the hydrogen precharged condition. Tritium precharging and aging for a short duration resulted in increased yield strengths, ultimate tensile strengths and slightly increased elongation to failure, comparable to higher concentrations of hydrogen precharging.
This report serves as the proceedings of the Hydrogen Compatible Materials Workshop held virtually by Sandia National Laboratories on December 2-3, 2020. The purpose of the workshop was to assemble subject matter experts at Sandia and its national laboratory partners within the U.S. Department of Energy's (DOE) Hydrogen Materials Compatibility (H-Mat) Consortium with public and private stakeholders in the research, development and deployment of hydrogen technologies to discuss the topic of hydrogen compatible materials. This workshop was designed to build on past events and current research and development (R&D) efforts to develop a forward-looking vision that identifies gaps and challenges for the next decade. In particular, the workshop organizers sought to expand their understanding of hydrogen compatible materials needs for power, manufacturing and other industrial uses to enable deeper impact and widespread use of hydrogen while continuing to address open questions in hydrogen-powered transportation of concern to Original Equipment Manufacturers, hydrogen producers, materials & component suppliers and other private entities. The workshop was primarily organized as a series of panel-led discussions on the topics of hydrogen-enabled transportation, heating and power, and industrial uses. Each panel consisted of 2-3 subject matter experts who relayed their perspectives on a set of framing questions developed to facilitate discussion by the broader group of workshop participants. By the workshop's conclusion, the participants identified and prioritized a list of technical challenges for each panel topic where further R&D is warranted.
Austenitic stainless steels are used extensively in harsh environments, including for high-pressure gaseous hydrogen service. However, the tensile ductility of this class of materials is very sensitive to materials and environmental variables. While tensile ductility is generally insufficient to qualify a material for hydrogen service, ductility is an effective tool to explore microstructural and environmental variables and their effects on hydrogen susceptibility, to inform understanding of the mechanisms of hydrogen effects in metals, and to provide insight to microstructural variables that may improve relative performance. In this study, hydrogen precharging was used to simulate high-pressure hydrogen environments to evaluate hydrogen effects on tensile properties. Several austenitic stainless steels were considered, including both metastable and stable alloys. Room temperature and subambient temperature tensile properties were evaluated with three different internal hydrogen contents for type 304L and 316L austenitic stainless steels and one hydrogen content for XM-11. Significant ductility loss was observed for both metastable and stable alloys, suggesting the stability of the austenitic phase is not sufficient to characterize the effects of hydrogen. Internal hydrogen does influence the character of deformation, which drives local damage accumulation and ultimately fracture for both metastable and stable alloys. While a quantitative description of hydrogen-assisted fracture in austenitic stainless steels remains elusive, these observations underscore the importance of the hydrogen-defect interactions and the accumulation of damage at deformation length scales.
The effects of internal hydrogen on the deformation microstructures of 304L austenitic stainless steel have been characterized using electron backscattered diffraction (EBSD), transmission Kikuchi diffraction (TKD), high-resolution scanning transmission electron microscopy (HRSTEM), and nanoprobe diffraction. Samples, both thermally precharged with hydrogen and without thermal precharging, were subjected to tensile deformation of 5 and 20 pct true strain followed by multiple microscopic interrogations. Internal hydrogen produced widespread stacking faults within the as-forged initially unstrained material. While planar deformation bands developed with tensile strain in both the hydrogen-precharged and non-precharged material, the character of these bands changed with the presence of internal hydrogen. As shown by nanobeam diffraction and HRSTEM observations, in the absence of internal hydrogen, the bands were predominantly composed of twins, whereas for samples deformed in the presence of internal hydrogen, ε-martensite became more pronounced and the density of deformation bands increased. For the 20 pct strain condition, α'-martensite was observed at the intersection of ε-martensite bands in hydrogen-precharged samples, whereas in non-precharged samples α'-martensite was only observed along grain boundaries. We hypothesize that the increased prevalence of α'-martensite is a secondary effect of increased ε-martensite and deformation band density due to internal hydrogen and is not a signature of internal hydrogen itself.
Fracture resistance of pipeline welds from a range of strength grades and welding techniques was measured in air and 21 MPa hydrogen gas, including electric resistance weld of X52, friction stir weld of X100 and gas metal arc welds (GMAW) of X52, X65 and X100. Welds exhibited a decrease in fracture resistance in hydrogen compared to complementary tests in air. A general trend was observed that fracture resistance in 21 MPa hydrogen gas decreased with increasing yield strength. To accommodate material constraints, two different fracture coupon geometries were used in this study, which were shown to yield similar fracture resistance values in air and 21 MPa hydrogen gas; values using different coupons resulted in less than 15% difference. In addition, fracture coupons were removed from controlled locations in select welds to examine the potential influence of orientation and residual stress. The two orientations examined in the X100 GMAW exhibited negligible differences in fracture resistance in air and, similarly, negligible differences in hydrogen. Residual stress exhibited a modest influence on fracture resistance; however, a consistent trend was not observed between tests in air and hydrogen, suggesting further studies are necessary to better understand the influence of residual stress. A comparison of welds and base metals tested in hydrogen gas showed similar susceptibility to hydrogen-assisted fracture. The overall dominant factor in determining the susceptibility to fracture resistance in hydrogen is the yield strength.
Hydrogen additions to natural gas are being considered around the globe as a means to utilize existing infrastructure to distribute hydrogen. Hydrogen is known to enhance fatigue crack growth and reduce fracture resistance of structural steels used for pressure vessels, piping and pipelines. Most research has focused on high-pressure hydrogen environments for applications of storage (>100 MPa) and delivery (10-20 MPa) in the context of hydrogen fuel cell vehicles, which typically store hydrogen onboard at pressure of 70 MPa. In applications of blending hydrogen into natural gas, a wide range of hydrogen contents are being considered, typically in the range of 2-20%. In natural gas infrastructure, the pressure differs depending on location in the system (i.e., transmission systems are relatively high pressure compared to low-pressure distribution systems), thus the anticipated partial pressure of hydrogen can be less than an atmosphere or more than 10 MPa. In this report, it is shown that low partial pressure hydrogen has a very strong effect on fatigue and fracture behavior of infrastructure steels. While it is acknowledged that materials compatibility with hydrogen will be important for systems operating with high stresses, the effects of hydrogen do not seem to be a significant threat for systems operating at low pressure as in distribution infrastructure. In any case, system operators considering the addition of hydrogen to their network must carefully consider the structural performance of their system and the significant effects of hydrogen on structural integrity, as fatigue and fracture properties of all steels in the natural gas infrastructure will be degraded by hydrogen, even for partial pressure of hydrogen less than 0.1 MPa.
High pressure Type 2 hoop-wrapped, thick-walled vessels are commonly used at hydrogen refueling stations. Vessels installed at stations circa 2010 are now reaching their design cycle limit and are being retired, which is the motivation for exploring life extension opportunities. The number of design cycles is based on a fatigue life calculation using a fracture mechanics assessment according to ASME Section VIII, Division 3, which assumes each cycle is the full pressure range identified in the User's Design Specification for a given pressure vessel design; however, assessment of service data reveals that the actual pressure cycles are more conservative than the design specification. A case study was performed in which in-service pressure cycles were used to re-calculate the design cycles. It was found that less than 1% of the allowable crack extension was consumed when crack growth was assessed using in-service design pressures compared to the original design fatigue life from 2010. Additionally, design cycles were assessed on the 2010 era vessels based on design curves from the recently approved ASME Code Case 2938, which were based on fatigue crack growth rate relationships over a broader range of K. Using the Code Case 2938 design curves yielded nearly 2.7 times greater design cycles compared to the 2010 vessel original design basis. The benefits of using inservice pressure cycles to assess the design life and the implications of using the design curves in Code Case 2938 are discussed in detail in this paper.
Objectives of the project include: Enable the use of high strength steel hydrogen pipelines, as significant cost savings can result by implementing high strength steels as compared to lower strength pipes. Demonstrate that girth welds in high-strength steel pipe exhibit fatigue performance similar to lower-strength steels in high-pressure hydrogen gas. Identify pathways for developing high-strength pipeline steels by establishing the relationship between microstructure constituents and hydrogen-accelerated fatigue crack growth (HA-FCG)
The fatigue crack growth behavior of Ti–10V–2Fe–3Al in gaseous hydrogen (H2) was assessed through comparative experiments conducted in laboratory air and 8.3 MPa H2. The measured fatigue crack growth rate (da/dN) versus applied stress intensity factor range (ΔK) relationships and observed fracture morphologies for laboratory air and H2 were comparable up to ΔK ≈ 6.9 MPa√m, when tested at a load ratio of 0.1 and frequency of 10 Hz. At higher ΔK values, significant crack deflection and subsequent catastrophic failure occurred in the specimen tested in H2. This degradation was not observed in a specimen pre-exposed to 8.3 MPa H2 for 96 h and then immediately tested in laboratory air. X-ray diffraction of the failed H2-tested specimen revealed that the material remnants were predominantly composed of TiH2, suggesting that hydride formation was the catalyst for catastrophic failure in H2. The mechanistic implications of these results and their impact on current material compatibility assessments for Ti alloys in hydrogen service are then discussed.