Publications

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Austenitic stainless steels such as 304L are frequently used for hydrogen service applications due to their excellent resistance to hydrogen embrittlement. However, welds in austenitic stainless steels often contain microstructures that are more susceptible to the presence of hydrogen. This study examines the tensile strength and ductility of a multi-pass gas tungsten arc weld made on 304L cross-rolled plate using 308L weld filler wire. Sub-sized tensile specimens were used to ensure the entire gage section of each tensile specimen consisted of weld metal. Specimens were extracted in both axial and transverse orientations, and at three different depths within the weld (root, center, and top). Yield strength decreased and ductility increased moving from the root to the top of the weld. A subset of specimens was precharged with hydrogen at 138 MPa (20,000 psi) and 300oC prior to testing, resulting in a uniform hydrogen concentration of 7700 appm. The presence of hydrogen resulted in a slight increase in yield and tensile strength and a roughly 50% decrease in tensile elongation and reduction in area, compared to the hydrogen-free properties.

Palladium and its alloys are model systems for studying the solid-state storage of hydrogen. Mechanical milling is commonly used to process complex powder systems for solid-state hydrogen storage; however, milling can also be used to evolve nanostructured powder to modify hydrogen sorption characteristics. In the present study, cryomilling (mechanical attrition milling in a cryogenic liquid) is used to produce nanostructured palladium-rhodium alloy powder. Characterization of the cryomilled Pd-10Rh using electron microscopy, X-ray diffraction and surface area analysis reveal that (i) particle morphology evolves from spherical to flattened disk-like particles; while (ii) crystallite size decreases from several microns to less than 100 nm; and (iii) dislocation density increases with increased cryomilling time. Hydrogen absorption and desorption isotherms as well as the time scales for absorption were measured for cryomilled Pd-10Rh, and correlated with observed microstructural changes induced by the cryomilling process. In short, as the microstructure of the Pd-10Rh alloy is refined by cryomilling: (i) the maximum hydrogen concentration in the α-phase increases, (ii) the pressure plateau becomes flatter and (iii) the equilibrium hydrogen capacity increases at pressure of 101.3 kPa. Additionally, the rate of hydrogen absorption was reduced by an order of magnitude compared to non-cryomilled (atomized) powder.

In forged, welded, and machined components, residual stresses can form during the fabrication process. These residual stresses can significantly alter the fatigue and fracture properties compared to an equivalent component containing no residual stress. When performing lifetime assessment, the residual stress state must be incorporated into the analysis to most accurately reflect the initial condition of the component. The focus of this work is to present the computational and experimental tools that we are developing to predict and measure the residual stresses in stainless steel for use in pressure vessels. The contour method was used to measure the residual stress in stainless steel forgings. These results are compared to the residual stresses predicted using coupled thermo-mechanical simulations that track the evolution of microstructure, strength and residual stress during processing.

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Refueling infrastructure for use in gaseous hydrogen powered vehicles requires extensive manifolding for delivering the hydrogen from the stationary fuel storage at the refueling station to the vehicle as well as from the mobile storage on the vehicle to the fuel cell or combustion engine. Manifolds for gas handling often use welded construction (as opposed to compression fittings) to minimize gas leaks. Therefore, it is important to understand the effects of hydrogen on tubing and tubing welds. This paper provides a brief overview of on-going studies on the effects of hydrogen precharging on the tensile properties of austenitic stainless tubing and orbital tube welds.

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A principal challenge to the widespread adoption of hydrogen infrastructure is the lack of quantifiable data on its safety envelope and concerns about additional risk from hydrogen. To convince regulatory officials, local fire marshals, fuel suppliers, and the public at large that hydrogen refueling is safe for consumer use, the risk to personnel and bystanders must be quantified and minimized to an acceptable level. Such a task requires strong confidence in the safety performance of high pressure hydrogen systems. Developing meaningful materials characterization and qualification methodologies in addition to enhancing understanding of performance of materials is critical to eliminating barriers to the development of safe, low-cost, high-performance high-pressure hydrogen systems for the consumer environment.

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The wedge geometry is a simple geometry for establishing a relatively constant gradient of strain in a forged part. The geometry is used to establish gradients in microstructure and strength as a function of strain, forging temperature, and quenching time after forging. This geometry has previously been used to benchmark predictions of strength and recrystallization using Sandias materials model for type 304L austenitic stainless steel. In this report, the processing conditions, in particular the times to forge and quench the forged parts, are summarized based on information recorded during forging on June 18, 2013 of the so-called wedge geometry from type 316L and 21Cr-6Ni-9Mn austenitic stainless steels.

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Automakers and fuel providers have made public commitments to commercialize light duty fuel cell electric vehicles and fueling infrastructure in select US regions beginning in 2014. The development, implementation, and advancement of meaningful codes and standards is critical to enable the effective deployment of clean and efficient fuel cell and hydrogen solutions in the energy technology marketplace. Metrics pertaining to the development and implementation of safety knowledge, codes, and standards are important to communicate progress and inform future R&D investments. This document describes the development and benchmarking of metrics specific to the development of hydrogen specific codes relevant for hydrogen refueling stations. These metrics will be most useful as the hydrogen fuel market transitions from pre-commercial to early-commercial phases. The target regions in California will serve as benchmarking case studies to quantify the success of past investments in research and development supporting safety codes and standards R&D.

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The US Department of Energy (DOE) Energy Efficiency and Renewable Energy (EERE) Office of Fuel Cell Technologies Office (FCTO) is establishing the Hydrogen Fueling Infrastructure Research and Station Technology (H2FIRST) partnership, led by the National Renewable Energy Laboratory (NREL) and Sandia National Laboratories (SNL). FCTO is establishing this partnership and the associated capabilities in support of H2USA, the public/private partnership launched in 2013. The H2FIRST partnership provides the research and technology acceleration support to enable the widespread deployment of hydrogen infrastructure for the robust fueling of light-duty fuel cell electric vehicles (FCEV). H2FIRST will focus on improving private-sector economics, safety, availability and reliability, and consumer confidence for hydrogen fueling. This whitepaper outlines the goals, scope, activities associated with the H2FIRST partnership.

Type 316/316L austenitic stainless steels are considered the benchmark for resistance to hydrogen embrittlement in gaseous hydrogen environments. Type 316/316L alloys are used extensively in handling systems for gaseous hydrogen, which has created engineering basis for its use. This material class, however, is relatively expensive compared to other structural metals including other austenitic stainless steels, thus the hydrogen fuel cell community seeks lower-cost alternatives. Nickel content is an important driver of cost and hydrogen-embrittlement resistance; the cost of austenitic stainless steels is largely determined by nickel content, while high nickel content generally improves resistance to hydrogen embrittlement. These circumstances create the perception that less-expensive grades of austenitic stainless steels are not appropriate for hydrogen service. While other grades of austenitic stainless steels are generally more susceptible to hydrogen embrittlement, in many cases the hydrogen-affected properties are superior to the properties of materials that are considered acceptable, such as aluminum alloys and A-286 austenitic stainless steel. In this paper, the properties of a variety of austenitic stainless steels are compared with the aim of promoting the consideration of a wider range of austenitic stainless steels to reduce cost and reduce weight of high-pressure components for hydrogen service.

Effects of low temperature on hydrogen-assisted cracking in 304L/308L austenitic stainless steel welds were investigated using elastic-plastic fracture mechanics methods. Thermally precharged hydrogen (140. wppm) decreased fracture toughness and altered fracture mechanisms at 293 and 223. K relative to hydrogen-free welds. At 293. K, hydrogen increased planar deformation in austenite, and microcracking of δ-ferrite governed crack paths. At 223. K, low temperature enabled hydrogen to exacerbate localized deformation, and microvoid formation, at austenite deformation band intersections near phase boundaries, dominated damage initiation; microcracking of ferrite did not contribute to crack growth. © 2013 Elsevier Ltd.

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The objective of this work is to enable the safe design of hydrogen pressure vessels by measuring the fatigue crack growth rates of ASME code-qualified steels in high-pressure hydrogen gas. While a design-life calculation framework has recently been established for high-pressure hydrogen vessels, a material property database does not exist to support the analysis. This study addresses such voids in the database by measuring the fatigue crack growth rates for three heats of ASME SA-372 Grade J steel in 100 MPa hydrogen gas at two different load ratios (R). Results show that fatigue crack growth rates are similar for all three steel heats and are only a mild function of R. Hydrogen accelerates the fatigue crack growth rates of the steels by at least an order of magnitude relative to crack growth rates in inert environments. Despite such dramatic effects of hydrogen on the fatigue crack growth rates, measurement of these properties enables reliable definition of the design life of steel hydrogen containment vessels. Copyright © 2013 by ASME.

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This document addresses polymer materials for use in hydrogen service. Section 1 summarizes the applications of polymers in hydrogen infrastructure and vehicle fuel systems and identifies polymers used in these applications. Section 2 reviews the properties of polymer materials exposed to hydrogen and/or high-pressure environments, using information obtained from published, peer-reviewed literature. The effect of high pressure on physical and mechanical properties of polymers is emphasized in this section along with a summary of hydrogen transport through polymers. Section 3 identifies areas in which fuller characterization is needed in order to assess material suitability for hydrogen service.

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Threshold stress intensity factors were measured in high-pressure hydrogen gas for a variety of low alloy ferritic steels using both constant crack opening displacement and rising crack opening displacement procedures. Thresholds for crack extension under rising displacement, K THi, for crack extension under constant displacement, KTHi*, and for crack arrest under constant displacement K THa, were identified. These values were not found to be equivalent, i.e. K THi < K THa < K THi*. The hydrogen assisted fracture mechanism was determined to be strain controlled for all of the alloys in this study, and the micromechanics of strain controlled fracture are used to explain the observed disparities between the different threshold measurements. K THa and K THi differ because the strain singularity of a stationary crack is stronger than that of a propagating crack; K THa must be larger than K THi to achieve equivalent crack tip strain at the same distance from the crack tip. Hydrogen interacts with deformation mechanisms, enhancing strain localization and consequently altering both the nucleation and growth stages of strain controlled fracture mechanisms. The timing of load application and hydrogen exposure, i.e., sequential for constant displacement tests and concurrent for rising displacement tests, leads to differences in the strain history relative to the environmental exposure history and promotes the disparity between K THi* and K THi. K THi is the only conservative measurement of fracture threshold among the methods presented here. © 2012 The Minerals, Metals & Materials Society and ASM International.

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This report summarizes the investigation of the release of approximately 300kg of hydrogen at the AC Transit Facility in Emeryville, CA. The hydrogen release was avoidable in both the root cause and contributing factors. The report highlights the need for communication in all phases of project planning and implementation. Apart from the failed valve, the hydrogen system functioned as designed, venting the hydrogen gas a safe distance above surrounding structures and keeping the subsequent fire away from personnel and equipment. The Emeryville Fire Department responded appropriately given the information provided to the Incident Commander. No injuries or fatalities resulted from the incident.

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Gaseous hydrogen is an alternative to petroleum-based fuels, but it is known to significantly reduce the fatigue and fracture resistance of steels. Steels are commonly used for containment and distribution of gaseous hydrogen, albeit under conservative operating conditions (i.e., large safety factors) to mitigate so-called gaseous hydrogen embrittlement. Economical methods of distributing gaseous hydrogen (such as using existing pipeline infrastructure) are necessary to make hydrogen fuel competitive with alternatives. the effects of gaseous hydrogen on fracture resistance and fatigue resistance of pipeline steels, however, has not been comprehensively evaluated and this data is necessary for structural integrity assessment in gaseous hydrogen environments. In addition, existing standardized test methods for environment assisted cracking under sustained load appear to be inadequate to characterize low-strength steels (such as pipeline steels) exposed to relevant gaseous hydrogen environments. In this study, the principles of fracture mechanics are used to compare the fracture and fatigue performance of two pipeline steels in high-purity gaseous hydrogen at two pressures: 5.5 MPa and 21 MPa. In particular, elastic-plastic fracture toughness and fatigue crack growth rates were measured using the compact tension geometry and a pressure vessel designed for testing materials while exposed to gaseous hydrogen. Copyright © 2010 by ASME.

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Fatigue crack growth rates and rising displacement fracture thresholds have been measured for a 4130X steel in 45 MPa hydrogen gas. the ratio of minimum to maximum load (R-ratio) and cyclic frequency was varied to assess the effects of these variables on fatigue crack growth rates. Decreasing frequency and increasing R were both found to increase crack growth rate, however, these variables are not independent of each other. Changing frequency from 0.1 Hz to 1 Hz reduced crack growth rates at R = 0.5, but had no effect at R = 0.1. When applied to a design life calculation for a steel pressure vessel consistent with a typical hydrogen trailer tube, the measured fatigue and fracture data predicted a re-inspection interval of nearly 29 years, consistent with the excellent service history of such vessels which have been in use for many years. Copyright © 2010 by ASME.

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Threshold stress intensity factors were measured in high-pressure hydrogen gas for a variety of low alloy ferritic steels using both constant crack opening displacement and rising crack opening displacement procedures. The sustained load cracking procedures are generally consistent with those in ASME Article KD-10 of Section VIII Division 3 of the Boiler and Pressure Vessel Code, which was recently published to guide design of high-pressure hydrogen vessels. Three definitions of threshold were established for the two test methods: K{sub THi}* is the maximum applied stress intensity factor for which no crack extension was observed under constant displacement; K{sub THa} is the stress intensity factor at the arrest position for a crack that extended under constant displacement; and K{sub JH} is the stress intensity factor at the onset of crack extension under rising displacement. The apparent crack initiation threshold under constant displacement, K{sub THi}*, and the crack arrest threshold, K{sub THa}, were both found to be non-conservative due to the hydrogen exposure and crack-tip deformation histories associated with typical procedures for sustained-load cracking tests under constant displacement. In contrast, K{sub JH}, which is measured under concurrent rising displacement and hydrogen gas exposure, provides a more conservative hydrogen-assisted fracture threshold that is relevant to structural components in which sub-critical crack extension is driven by internal hydrogen gas pressure.

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Applications requiring the containment and transportation of hydrogen gas at pressures greater than 70 MPa are anticipated in the evolving hydrogen economy infrastructure. Since hydrogen is known to alter the mechanical properties of materials, data are needed to guide the selection of materials for structural components. The objective of this study is to characterize the role of yield strength, microstructural orientation, and small concentrations of ferrite on hydrogen-assisted fracture in two austenitic stainless steels: 21Cr-6Ni-9Mn (21-6-9) and 22Cr-13Ni-SMn (22-13-5). The testing methodology involves exposure of tensile specimens to high-pressure hydrogen gas at elevated temperature in order to precharge the specimens with hydrogen, and subsequently testing the specimens in laboratory air to measure strength and ductility. In all cases, the alloys remain ductile despite precharging to hydrogen concentrations of ∼1 at. %, as demonstrated by reduction in area values between 30% and 60% and fracture modes dominated by microvoid processes. Low concentrations of ferrite and moderate increases in yield strength do not exacerbate hydrogen-assisted fracture in 21-6-9 and 22-13-5, respectively. Microstructural orientation has a pronounced effect on ductility in 22-13-5 due to the presence of aligned second-phase particles. Copyright © 2008 by ASME.

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