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A system model of a proton exchange membrane green hydrogen generation system is developed with the intent of demonstrating the dynamic interactions of the system with varying inputs such as electrolyzer current density and hydrogen pressure. The model includes physics-based modules that are validated with available experimental data for the power electronics, electrolyzer stack, pressure swing adsorption dryer, hydrogen compressor, and cooling circuit components of the system. A single electrolyzer system and multi-electrolyzer system are evaluated with varying operating conditions and multi-stack system operating strategies. Increasing electrolyzer cathode pressure results in an overall increase in specific energy consumption but may allow the use of low-pressure storage without the need for a hydrogen compressor. For a theoretical case study of excess renewable energy generation in Texas, the system achieves an overall efficiency of 46.5 % defined as the efficiency of converting electricity into an equivalent energy content of the produced hydrogen. [Display omitted] • The system rapidly loses efficiency below 40 % of the rated electrolyzer power. • Hydrogen loss is the primary reason for low efficiency at low electrolyzer power. • Increasing hydrogen pressure reduces system efficiency and operating range. • Large-scale systems can increase efficiency with power management strategies. • System modeling can be used to find the optimal installed electrolyzer capacity. [ABSTRACT FROM AUTHOR]

Abstract In recent years, the extensive participation of turbine generator in deep peak regulation has caused significant damage to the rotor windings, which is rooted in the frequent and significant changes in the electrical load of the rotor, accompanied by thermal expansion and contraction, causing harm to the rotor insulation and stress fatigue of the metal conductor. Hydrogen, as the cooling medium for the rotor windings of turbine generators, has a direct impact on the windings temperature through its parameters. Starting from suppressing the harm of deep peak regulation on the rotor windings, the inhibitory effect of hydrogen parameter changes on the temperature of the gap‐pickup diagonal‐flow rotor is studied. A calculation model for the rotor temperature field of the turbo‐generator with gap‐pickup diagonal‐flow ventilation is established. The finite volume method is used to iteratively calculate the control equations in the fluid‐thermal‐solid coupling heat transfer problem, and the temperature field distribution of various components of the rotor under different excitation loads is obtained. On this basis, the effects of individual adjustment of hydrogen temperature and pressure, as well as the combined adjustment, on the temperature of the rotor windings are studied. Finally, a variable hydrogen parameter adjustment strategy is proposed for the operation of turbo‐generators under deep peak regulation conditions. To a certain extent, the winding temperature fluctuation can be suppressed, which provides a potential solution for the thermal stable operation of the rotor.

In recent years, the extensive participation of turbine generator in deep peak regulation has caused significant damage to the rotor windings, which is rooted in the frequent and significant changes in the electrical load of the rotor, accompanied by thermal expansion and contraction, causing harm to the rotor insulation and stress fatigue of the metal conductor. Hydrogen, as the cooling medium for the rotor windings of turbine generators, has a direct impact on the windings temperature through its parameters. Starting from suppressing the harm of deep peak regulation on the rotor windings, the inhibitory effect of hydrogen parameter changes on the temperature of the gap‐pickup diagonal‐flow rotor is studied. A calculation model for the rotor temperature field of the turbo‐generator with gap‐pickup diagonal‐flow ventilation is established. The finite volume method is used to iteratively calculate the control equations in the fluid‐thermal‐solid coupling heat transfer problem, and the temperature field distribution of various components of the rotor under different excitation loads is obtained. On this basis, the effects of individual adjustment of hydrogen temperature and pressure, as well as the combined adjustment, on the temperature of the rotor windings are studied. Finally, a variable hydrogen parameter adjustment strategy is proposed for the operation of turbo‐generators under deep peak regulation conditions. To a certain extent, the winding temperature fluctuation can be suppressed, which provides a potential solution for the thermal stable operation of the rotor. [ABSTRACT FROM AUTHOR]

The in situ small punch (SP) test method is a simple screening technology developed to assess the hydrogen embrittlement (HE) characteristics of structural steels. This method can easily adjust the influencing parameters such as test temperature, gas pressure, and punch velocity depending on the hydrogen service environment. With increased hydrogen consumption, using pipelines for mass hydrogen transportation is being considered. This study evaluated the HE susceptibility of API-X52 and API-X70 steels, considering the hydrogen usage environment. The study investigated the effects of hydrogen pressure and punch velocity on the HE behaviors of each pipe steel at room temperature using the SP energy and relative reduction in thickness (RRT) to determine their effect on HE susceptibility quantitatively. The study found that hydrogen pressure produced a different HE effect; the lower the hydrogen pressure, the more HE was relieved. Particularly, when the punch velocity was high, such as 1 mm/min, the HE effect was significantly relaxed. However, when the punch velocity was below 0.01 mm/min, HE occurred even at low hydrogen pressure conditions, meaning hydrogen diffusion within the specimen during the SP testing reached a critical hydrogen concentration to create a brittle fracture. Both pipeline steels showed similar HE behaviors under a wide range of H2 pressures and punch velocities, showing an inverse S-curve for quantitative factors of SP energy and RRT against the H2 pressure at 1.0 mm/min punch velocity. The study classified the observed HE behaviors into four types based on quantitative and qualitative aspects. These findings confirm that the in situ SP test is a useful screening technique, and the factor RRT can be effectively applied to the HE screening of pipeline steels in low and high-pressure hydrogen environments. [ABSTRACT FROM AUTHOR]

In recent years, to reduce manufacturing costs and alleviate the rare-earth supply/demand imbalance in the N-Fe-B magnet market, researchers have actively pursued the development of (Nd,Ce)-Fe-B magnets, in which Nd is replaced by Ce. In the present study, we focused on magnetic powders treated by the hydrogenation-disproportionation-desorption-recombination (HDDR) process and explored the relationship between the hydrogen pressure and temperature (PH2-T curves) in the Ce-Fe-B system, which is essential for the development of (Nd,Ce)-Fe-B magnets. It was confirmed that the disproportionation and recombination reactions of both Ce2Fe14B and CeFe2 take place in the Ce-Fe-B system. Furthermore, compared with Nd2Fe14B, the PH2-T curve of Ce2Fe14B was found to be shifted to higher pressure and lower temperature, suggesting a corresponding shift in the HDDR conditions under which good magnetic properties can be obtained. [ABSTRACT FROM AUTHOR]

The authors have investigated hydro-liquefaction of Lemna minor (duckweed) aquatic biomass and a hydrogen-donor solvent, methanol, at temperatures of 180 °C, 200 °C and 220 °C and final reactor pressure values of 70, 80 and 90 bar. The rise in reactor pressure for all reaction temperatures led to an increase in biocrude yield with a maximum higher heating value of 22.19 MJ/kg, while the biochar also depicted its potential to be used in varied applications and as a bio-refinery feedstock. Fourier Transform Infrared Spectroscopy and Gas Chromatography Mass Spectrometry analysis of biocrude indicated the presence of esters and long chain alkanes, ethers, phenols, etc. The main finding of this work disclosed that fuel properties of the present biocrude (without any catalyst) were better than their non-catalytic and hydro-catalytic bio-oil counterparts obtained from similar biomass at a pyrolysis temperature of 550 °C.

The in situ small punch (SP) test method is a simple screening technology developed to assess the hydrogen embrittlement (HE) characteristics of structural steels. This method can easily adjust the influencing parameters such as test temperature, gas pressure, and punch velocity depending on the hydrogen service environment. With increased hydrogen consumption, using pipelines for mass hydrogen transportation is being considered. This study evaluated the HE susceptibility of API-X52 and API-X70 steels, considering the hydrogen usage environment. The study investigated the effects of hydrogen pressure and punch velocity on the HE behaviors of each pipe steel at room temperature using the SP energy and relative reduction in thickness (RRT) to determine their effect on HE susceptibility quantitatively. The study found that hydrogen pressure produced a different HE effect; the lower the hydrogen pressure, the more HE was relieved. Particularly, when the punch velocity was high, such as 1 mm/min, the HE effect was significantly relaxed. However, when the punch velocity was below 0.01 mm/min, HE occurred even at low hydrogen pressure conditions, meaning hydrogen diffusion within the specimen during the SP testing reached a critical hydrogen concentration to create a brittle fracture. Both pipeline steels showed similar HE behaviors under a wide range of H2 pressures and punch velocities, showing an inverse S-curve for quantitative factors of SP energy and RRT against the H2 pressure at 1.0 mm/min punch velocity. The study classified the observed HE behaviors into four types based on quantitative and qualitative aspects. These findings confirm that the in situ SP test is a useful screening technique, and the factor RRT can be effectively applied to the HE screening of pipeline steels in low and high-pressure hydrogen environments.

The authors have investigated hydro-liquefaction of Lemna minor (duckweed) aquatic biomass and a hydrogen-donor solvent, methanol, at temperatures of 180 °C, 200 °C and 220 °C and final reactor pressure values of 70, 80 and 90 bar. The rise in reactor pressure for all reaction temperatures led to an increase in biocrude yield with a maximum higher heating value of 22.19 MJ/kg, while the biochar also depicted its potential to be used in varied applications and as a bio-refinery feedstock. Fourier Transform Infrared Spectroscopy and Gas Chromatography Mass Spectrometry analysis of biocrude indicated the presence of esters and long chain alkanes, ethers, phenols, etc. The main finding of this work disclosed that fuel properties of the present biocrude (without any catalyst) were better than their non-catalytic and hydro-catalytic bio-oil counterparts obtained from similar biomass at a pyrolysis temperature of 550 °C. [ABSTRACT FROM AUTHOR]

The hydrogen embrittlement sensitivity of L80 steel under different hydrogen pressure at room temperature was evaluated this work. The variation rules of hydrogen embrittlement sensitivity of L80 steel at hydrogen pressure from 3 MPa to 12 MPa were studied by notch slow strain tensile test combined with fracture analysis. Results showed that L80 steel had no obvious brittleness at room temperature under hydrogen pressure of lower than 5 MPa, and yet had obvious brittleness under hydrogen pressure of more than 8 MPa. Under hydrogen pressure from 3 MPa to 12 MPa, the micro morpholgy at the center of the main section of L80 steel tensile sample changed from dimple morphology to coexistence of dimple morphology and cleavage morphology. The micro morphology at the edge gradually changed from dimple morphology to cleavage. In addition, the change rate of section shrinkage gradually increased from 16.19% to 46.79%. With the increase of hydrogen pressure, the plastic loss of L80 steel increased, the fracture showed obvious embrittlement characteristics, and the hydrogen embrittlement sensitivity gradually increased.

The utilization of fluidized bed reactors for the biomass hydropyrolysis vapor upgrading is essential in the conversion of biomass into liquid fuels. Conducting pyrolysis experiments in fluidized bed reactors with a N 2 -H 2 mixed atmosphere is advantageous for system operation as it allows for lower minimum operating velocities, enhances pyrolysis efficiency, and reduces heat loss. This research examines the impact of hydrogen pressure in a bubbling fluidized bed reactor using a N 2 -H 2 mixed carrier gas, with poplar wood as the feedstock and Ni-Moγ/Al 2 O 3 as the catalyst. Through experimental and correlation analyses, significant relationships are observed between hydrogen pressure and the production of liquid and gas-phase products, with varying effects attributed to changes in hydrogen ratio and reaction pressure. Elevated hydrogen ratios facilitate secondary reactions such as the CO 2 hydrogenation, resulting in a decline in CO 2 production and an elevation in the yields of light hydrocarbons and aqueous phase products. Augmenting the reaction pressure to enhance hydrogen pressure aids in promoting hydrogen saturation and cracking, and amplifying the hydrogen pressure through an increase in the reaction pressure at a specific hydrogen ratio proves more efficacious in enhancing both the quantity and quality of bio-oil produced. In the context of poplar hydropyrolysis vapor upgrading utilizing a Ni-Moγ/Al 2 O 3 catalyst, a hydrogen ratio of 50 % at a reaction pressure of 1.5 MPa emerged as the preferred option for achieving a 12 % bio-oil yield with a moderate average bio-oil carbon number and satisfactory hydrogenation levels. This comprehensive investigation examines the influence of hydrogen pressure on biomass hydropyrolysis, highlighting variations in the impacts of changes in hydrogen ratio and reaction pressure on pyrolysis results. These results offer a theoretical framework for discerning distinctions between hydropyrolysis investigations carried out in pure hydrogen atmospheres versus mixed N 2 -H 2 atmospheres. [Display omitted] • N 2 -H 2 mixed carrier gas reduces operating velocities of fluidized bed systems. • The hydrogen pressure increased by increasing hydrogen ratio promotes the carbon dioxide hydrogenation reaction. • The hydrogen pressure increased by increasing reaction pressure promotes hydrogen saturation and cracking reactions. • Hydrogen pressure, hydrogen ratio and reaction pressure intricately influence product distribution. [ABSTRACT FROM AUTHOR]

The effects of hydrogen gas pressure and prior austenite grain size (PAGS) on the susceptibility of a 22MnB5 press-hardened martensitic steel (PHS) to hydrogen embrittlement were studied. The hydrogen test apparatus at NIST-Boulder was modified for tensile testing of plate-type and sheet-type specimens in gaseous hydrogen. This modification made it possible to evaluate the slow strain rate tensile (SSRT) properties of the PHS with three different PAGS at various hydrogen pressures (0.21 MPa–5.5 MPa). SSRT testing in gaseous hydrogen resulted in significant reductions of both the tensile strength and ductility, as compared to those measured in air. In addition, the presence of gaseous hydrogen resulted in a transition in fracture morphology from the near-45° slant fracture to a more brittle fracture along a plane perpendicular to the tensile axis. The hydrogen-affected fracture zones were connected to the sheet specimen free surfaces, signifying the effect of external hydrogen. The fracture surfaces of the hydrogen-embrittled specimens contained relatively flat, "cleavage-like" facets, the size of which depended on the PAGS or packet size. The PHS having the largest PAGS represented generally larger secondary cracks and straighter crack paths in addition to a greater area fraction of the "cleavage-like" facets, likely indicative of a lower frequency of crack deflections. Compared to the largest PAGS condition, the two PHS with smaller PAGS were more resistant to the hydrogen-induced fracture especially at relatively low hydrogen gas pressures (<0.52 MPa). In contrast, with an increase in hydrogen pressure, all PHS specimens exhibited significant decreases in tensile strength and ductility. The positive effect of refining martensitic microstructure, at the low hydrogen pressures, is likely associated with improved toughness of the smaller grain-sized specimens. • Gaseous H embrittlement (HE) in ultra-high strength martensitic steel is studied. • Test apparatus for sheet-type specimen testing in gaseous H is developed. • Martensitic constituent sizes affect HE susceptibility and fracture morphologies. • Martensite with smaller prior austenite grain size is more resistant to gaseous HE. • Various metallurgical factors related to martensite HE sensitivity are discussed. [ABSTRACT FROM AUTHOR]

This study develops a hydrogen fueling station (HFS) thermodynamic model that simulates the actual fueling process in which hydrogen is supplied from a high-pressure (HP) storage tank into a fuel cell electric vehicle (FCEV) tank. To make the model as accurate as possible, we use the same components and specifications as in actual HFSs, such as a pressure control valve, a pre-cooling system, and an FCEV tank. After the components and their specifications are set, pressure and temperature profiles are set as the HP tank supply conditions. Based on the pressure and temperature profiles, the model solves for the temperature, pressure, and mass flow rate of hydrogen at each downstream position, including the inside of the vehicle tank. The values predicted by the model are compared with experimental data, and we show that the developed model makes it possible to accurately simulate those values at any position during the fueling process. • Thermodynamic modeling to simulate the real world hydrogen fueling process. • Holistic fueling system from high-pressure storage to vehicle tank is modeled. • Hydrogen temperature, pressure and mass flow is precisely predicted at any location. [ABSTRACT FROM AUTHOR]

Although start-up is a critical procedure in biofilm-based processes, systematic research aimed at improving start-up efficiency in hydrogen-based membrane biofilm reactors (H 2 -MBfRs) is lacking. In this study, we investigated the effects of H 2 pressure on the stabilized denitrification performance of H 2 -MBfRs during start-up. Following inoculation with anoxic sludge, the biomass was acclimated to various H 2 pressures. The increase in denitrifying microbial activity under high H 2 pressures (i.e., H 2 flux) shortened the denitrification lag time and reduced NO 3 –N removal flux, indicating an improved acclimation efficiency. The acclimated biomass was attached to membranes under different H 2 pressures. High H 2 pressures impeded biomass attachment, possibly because of bubble-less evaporation. Reducing the H 2 pressure appeared to be necessary for improving the biomass attachment. However, this measure decreased previously acclimated denitrification activity. Therefore, the loss of both acclimation and biomass adhesion must be considered during the attachment phase. The highest removal flux (0.63 e- eq/m2-d) was obtained after just 3.5 d of acclimated biomass attachment (i.e., activated sludge floc) at 1.6 bar. During Phase 3, the initial performance was further improved to 0.89 e- eq/m2-d by gradually increasing the H 2 pressure. These results provide a start-up strategy for improving the initial performance of H 2 -MBfRs. [Display omitted] • The stabilization performance of H 2 -MBfRs was improved by adjusting the H 2 pressure. • High H 2 pressures can increase the acclimation efficiency of the inoculum. • Biomass can be hindered from membrane attachment by high H 2 pressures. • A gradual increase in H 2 pressure can further improve the initial performance. [ABSTRACT FROM AUTHOR]

The fatigue crack growth rates (FCGRs) of 4130X steel in 45, 70, 87.5 and 100 MPa hydrogen were measured, and the hydrogen concentrations at the crack tip of test specimens were analyzed by establishing a hydrogen transport model. This model can describe the hydrogen behaviors in the process of environmental embrittlement. Results show that the FCGR increases with the increase in hydrogen pressure. However, the acceleration reaches the threshold under 87.5 MPa. Hydrogen pressure has the same influence on hydrogen concentration, and the threshold of the effect is probably related to the fact that the hydrogen absorption on the material surface reaches the limit. [ABSTRACT FROM AUTHOR]

In a vehicular fuel cell system, alternative load and frequent purge action can lead to anode pressure varies with the hydrogen mass flow fluctuation. It's crucial to control the pressure difference between anode and cathode within a reasonable range to avoid adverse phenomena such as membrane failure, reactant starvation, or even water management fault. In this paper, an improved proportional integrative (PI) controller by the fuzzy logic technique that considers the engineer experience and knowledge on the hydrogen supply system behavior is proposed for hydrogen pressure control, in which the PI parameters are tuned by a fuzzy decision process. Furthermore, load current and purge action regarded as input disturbances are applied for feedforward compensation to reduce the pressure response hysteresis. A hydrogen supply subsystem based on the proportional valve is modeled, and corresponding parameters are determined by analyzing the response time and steady pressure fluctuation. The performance of the conventional PI controller, the fuzzy logic PI (FLPI) controller and fuzzy logic PI with feedforward (FLPIF) controller is validated. The presented results indicated that the FLPI controller significantly improves the dynamic response of hydrogen pressure compared to the PI controller, and the FLPIF controller can further reduce overshoot caused by disturbance. Finally, the proposed FLPIF controller is implemented on a rapid prototype platform of the hydrogen supply subsystem and an actual fuel cell system, exhibiting satisfactory performance. • A proportional valve controls anode pressure to track cathode pressure. • Hydrogen pressure regulation is developed by feedforward fuzzy logic PI controller. • The feedforward compensation reduces pressure fluctuations of 0.8 kPa. • Pressure fluctuation was within 2 kPa in the test bench and real fuel cell system. [ABSTRACT FROM AUTHOR]

Road transportation consists of a significant contributor to total greenhouse gas emissions in developed countries. New alternative technologies in transportation such as electric vehicles aim to reduce substantially vehicle emissions, particularly in urban areas. Incentives of using two-wheel electric vehicles such as bicycles in big cities centres are promoted by local governments, and in fact, some countries are already trying to adopt this transition. An interesting case consists of the use of hydrogen fuel cells in such vehicles to increase their driving range under short refuelling times. To this end, this paper investigated the social and financial prospects of hydrogen infrastructure for city-oriented fuel cell electric vehicles such as bicycles. The results of the research indicated that a light mobility urban hydrogen refuelling station able to provide refuelling processes at pressures of 30 bar with a hydrogen fuel cost of 34.7 €/kgH 2 is more favourable compared to larger stations. • Operation and multiple-pressure refuelling processes of a hydrogen-fuelled bicycle. • Estimation of the autonomy of a fuel cell electric bicycle. • Hydrogen price calculated tο range between 34.7 and 36.9 €/kgH 2. • Hydrogen fuel cost per km calculated to approximately 0.019 €/km. [ABSTRACT FROM AUTHOR]

This review summarizes the thermodynamics of hydrogen (H2) in mixed gases of nitrogen (N2), methane (CH4) and natural gas, with a special focus on hydrogen fugacity. A compilation and interpretation of literature results for mechanical properties of steels as a function of hydrogen fugacity implies that test results obtained in gas mixtures and in pure hydrogen, both at the same fugacity, are equivalent. However, this needs to be verified experimentally. Among the test methods reviewed here, fatigue crack growth testing is the most sensitive method to measure hydrogen effects in pipeline steels followed by fracture toughness testing and tensile testing.

This paper is an analysis of driving characteristics of a metal-hydrogen (MH) actuator for driving a rehabilitative system using environmentally friendly hydrogen. The temperature–pressure characteristics of the MH actuator according to the temperature control of the Peltier element were analyzed to apply to the driving of the rehabilitative system to assist the daily activities of the patient. In this experiment, Peltier element is used to control the temperature of MH module which contains hydrogen storage alloy with on/off control scheme and fan on/off. To improve the thermal conductivity of the hydrogen-absorbing alloy, an assembly of MH module based on copper has been used. The control currents of the thermoelectric elements were 1 A and 2 A, and the temperature control range was between 35° and 70°. As the result of experiments, it is proper to act fan only while cooling duration and there exist a proper cooling current to drop temperature rapidly. It takes about 100 s to increase to 70 °C and drop to 35 °C of MH module temperature and about 90 s to increase to 70 °C and drop to 40 °C in ambient temperature 30 °C while fan is on only in cooling duration. The desirable characteristics of MH actuator, which makes it suitable for the uses in medical and rehabilitation applications, have been also studied. [ABSTRACT FROM AUTHOR]

Abstract Precise control of hydrogen pressure is crucial for the performance and durability of fuel cell systems. With the widely used common-rail injection system, traditional PID controller still dominates. For a long time, the hydrogen pressure fluctuates acutely when hydrogen purge valve switches or load sharply changes with using PID controller. In recent studies, several new control strategies are presented. However, mostly of them are theoretical and experimental. In this study, an improved common-rail injection system, hydrogen injector/ejector assembly is introduced. Based on a real fuel cell system, a Mamdani fuzzy controller is designed to regulate the hydrogen pressure. The algorithm of fuzzy controller is explained in detail. A comparative study is carried out between fuzzy controller and PID controller. According to the results, the stability of hydrogen pressure with using fuzzy controller is better than using PID controller. This research could be useful for the control of fuel cell system. Highlights • The characters of hydrogen pressure's fluctuation was studied. • A Mamdani-type fuzzy control system was designed to controller the hydrogen pressure in fuel cell system. • The algorithm of the fuzzy controller was introduced in detail. • The controller was proved to be valid and effective for real fuel cell systems. • A comparative study is carried out between fuzzy controller and PID controller. [ABSTRACT FROM AUTHOR]

Magnetic properties of hydrides of the intermetallic compound Nd 2 Fe 14 BH x are investigated in the temperature range covering the Curie temperatures ( T C ) of the compounds (up to 670 K). The temperature dependencies of magnetization are measured under continuous control of hydrogen content in the investigated samples. The dependencies of Curie and spin-reorientation transition ( T SR ) temperatures on the hydrogen concentration are studied in detail. The dependence of hydrogen concentration on pressure at a constant temperature (near T C ) and on the temperature at various pressures are obtained. We attempted to estimate the contributions of the unit cell volume increase upon hydrogenation and the electronic structure change in the variation of T C of the hydrogenated Nd 2 Fe 14 B . [ABSTRACT FROM AUTHOR]

A dynamic simulation approach to investigate an optimal hydrogen refueling method is proposed. The proposed approach simulates a transient temperature, pressure and mass flow rate of hydrogen flowing inside filling equipment in an actual station during the refueling process to an Fuel Cell Vehicle (FCV) tank. The simulation model is the same as in an actual hydrogen refueling station (HRS), and consists of a Break-Away, a hose, a nozzle, pipes and an FCV tank. Therefore, we can set actual configurations and thermal properties to the simulation model, and then simulate the temperature, pressure and mass flow rate of hydrogen passing through each position based on the supply conditions (temperature and pressure) at the Break-Away. In this study, the simulated temperature, pressure and mass flow rate are compared with the corresponding experimental data. Therefore, we show that the dynamic simulation approach can accurately obtain those values at each position during the refueling process and is an effective step in proposing the optimal refueling method. [ABSTRACT FROM AUTHOR]

In the present work, the susceptibility to hydrogen embrittlement under hydrogen pressure was evaluated for an experimental X-120 microalloyed steel with four quenching treatments to determine the effect of hydrogen on the hardness and toughness behaviour, for its use in gas and oil transport. The steel was reheated at 820° and 900 °C and then quenched in four different cooling media. The microstructure obtained by quenching in spray water showed a martensite-bainite type microstructure, and the pressurized air condition showed a microstructure composed of bainite and acicular ferrite. In contrast, a microstructure composed mainly of finer lath martensite-bainite and acicular ferrite was obtained in the water-oil emulsion. In the oil quenching condition, a microstructure consisting of banded martensite, polygonal ferrite, and acicular ferrite was obtained. Susceptibility to hydrogen embrittlement was determined by hydrogen gas pressure testing at 1, 4, and 7 MPa. The results showed a clear trend where the steel decreases its impact toughness due to the effect of hydrogen pressure, with a maximum decrease of 29%, 24%, and 39%, respectively, for each hydrogen pressure condition (1, 4, 7 MPa). • The susceptibility to HE under hydrogen gas pressure was evaluated for an experimental X-120 microalloyed steel. • Four quenching treatments were carried out to determine the effect of hydrogen on the behaviour of hardness and toughness. • The microhardness decreased progressively due to the accumulation of hydrogen with respect to the applied pressure. • The toughness shows a clear tendency to decrease due to the effect of hydrogen gas pressure. • A decrease in maximum impact toughness of 29%, 24%, and 39% is observed for each pressure. [ABSTRACT FROM AUTHOR]

A new criterion for hydrogen-induced cracking (HIC) that includes both the embrittlement effect and the loading effect of hydrogen was obtained theoretically. The surface cohesive energy and plastic deformation energy are reduced by hydrogen atoms at the interface; thus, the fracture toughness is reduced according to fracture mechanics theory. Both the pressure effect and the embrittlement effect mitigate the critical condition required for crack instability extension. During the crack instability expansion, the hydrogen in the material can be divided into two categories: hydrogen atoms surrounding the crack and hydrogen molecules in the crack cavity. The loading effect of hydrogen was verified by experiments, and the characterization methods for the stress intensity factor under hydrogen pressure in a linear elastic model and an elastoplastic model were analyzed using the finite-element simulation method. The hydrogen pressure due to the aggregation of hydrogen molecules inside the crack cavity regularly contributed to the stress intensity factor. The embrittlement of hydrogen was verified by electrolytic charging hydrogen experiments. According to the change in the atomic distribution during crack propagation in a molecular dynamics simulation, the transition from ductile to brittle fracture and the reduction in the fracture toughness were due to the formation of crack tip dislocation regions suppressed by hydrogen. The HIC formation mechanism is both the driving force of crack propagation due to the hydrogen gas pressure and the resisting force reduced by hydrogen atoms.