The influence of intergranular and interphase boundaries and δ-ferrite volume fraction on hydrogen embrittlement of high-nitrogen steel

We study the effect of grain size of austenitic and ferritic phases and volume fraction of δ-ferrite, which were obtained in different solution-treatment regimes (at 1050, 1100, 1150 and 1200 °C), on hydrogen embrittlement of high-nitrogen steel (HNS). The amount of dissolved hydrogen is similar for the specimens with different densities of interphase (γ-austenite/δ-ferrite) and intergranular (γ-austenite/γ-austenite, δ-ferrite/δ-ferrite) boundaries. Despite, the susceptibility of the specimens to hydrogen embrittlement, depth of the hydrogen-assisted surface layers, hydrogen transport during tensile tests and mechanisms of the hydrogen-induced brittle fracture all depend on grain size and ferrite content. The highest hydrogen embrittlement index I_H = 32%, the widest hydrogen-affected layer and a pronounced solid-solution hardening by hydrogen atoms is typical of the specimens with the lowest fraction of the boundaries. Even though fast hydrogen transport via coarse ferritic grains provides longer diffusion paths during H-changing, the width of the H-affected surface layer in the dual-phase structure of the HNS specimens is mainly determined by the hydrogen diffusivity in austenite. In tension, hydrogen transport with dislocations increases with the decrease in density of boundaries due to the longer dislocation free path, but stress-assisted diffusion transport does not depend on grain size and ferrite fraction. The contribution from intergranular fracture increases with an increase in the density of intergranular and interphase boundaries.     

Effect of high temperature deformation on the microstructure,mechanical properties and hydrogen embrittlement of 2.25Cr–1Mo-0.25 V steel

In this paper, the effects of high temperature deformation on the microstructure, mechanical properties and hydrogen embrittlement (HE) of the 2.25Cr–1Mo-0.25 V steel was investigated by a scanning electron microscope (SEM), a transmission electron microscope (TEM) and tensile tests. The SEM and TEM images demonstrated that high temperature plastic deformation (HTPD) led to the coarsening of carbides and the dislocation density increase. The tensile tests displayed that the HTPD resulted in the cracking susceptibility increase, as indicated by the increased numbers and sizes of cracks at the fractured surface. This was attributed to the coarsening of carbides during high temperature deformation. In contrast, the HTPD highly decreased the loss of ductility compared to the un-deformed specimens, although the amount of ductility losses (elongation or reduction of area) did not change significantly as the HTPD increased. The correlations among carbides, hydrogen and cracks were discussed.     

Effect of microstructure on the electrocatalytic activity for hydrogen evolution of amorphous and nanocrystalline Zr–Ni alloys

Rapidly quenched Zr₂Ni amorphous and nanocrystalline ribbons were studied as electrocatalysts for hydrogen evolution in 6 M KOH. Linear polarization, potentiostatic hydrogen charge/discharge and EIS measurements at various potentials were carried out for the Zr alloys with different microstructure with the aim to extract information about the mechanism of hydrogen evolution and absorption and estimate the kinetic parameters of the hydrogen evolution reaction (HER). Though the melt-spun Zr₆₇Ni₃₃ alloys with varying microstructure do not show substantially different catalytic activity for HER, it could be clearly demonstrated that the nanocrystalline material reveals better catalytic performance than the entirely amorphous and nano-/amorphous alloys with the same chemical composition. It was found that all studied Zr–Ni alloys absorb hydrogen under the conditions of the hydrogen evolution experiments, as the amount of the absorbed hydrogen depends to a large degree on the alloys microstructure as well as on the applied potential during the HER experiment. The diffusion coefficient of hydrogen into the amorphous Zr₆₇Ni₃₃ alloy, as well as the thickness of the hydrided layer were found to be noticeably larger than those of the nanocrystalline alloy at the same conditions of hydrogen charging. Therefore the improved electrocatalytic properties of the nanocrystalline alloy could only be explained by its favorable microstructure (e.g. higher density of defects) and weaker hydrogen absorption into the nanostructured material under the conditions of the HER.     

Effect of boron on the hydrogen-induced grain boundary embrittlement in α-Fe

The influence of interstitial impurities such as B and C on the H-induced Fe Σ5(310) symmetrical tilt grain boundary embrittlement was investigated using the projector augmented-wave method. It was shown that in contrast to hydrogen, both boron and carbon decrease the grain boundary energy more significantly than the surface one. This results in an increase in the Griffith work, i.e. the grain boundary strengthening. The strengthening of grain boundary is more pronounced with increased number of B atoms whereas the increase of H concentration makes the process of intergranular brittle cleavage fracture easier. The grain boundary energy is lowered with an increased number of B atoms indicating a strong driving force for segregation. Our estimations of the Griffith work for the Fe Σ5(310) grain boundary containing both B and H atoms show an increase in comparison with the undoped grain boundary. It is revealed that improved cohesion of Fe Σ5(310) grain boundary due to B is mainly a chemical effect, whereas both elastic and chemical contributions to the Griffith work in case of H are negative, i.e. they are embrittling contributions.     

Effect of S on H-induced grain-boundary embrittlement in γ-Fe by first-principles calculations

The effect of interstitial impurities (H and S) on the atomic, electronic structure, and mechanical properties of the γ-Fe Σ5 (021) grain boundary (GB) was investigated via first-principles calculations. H atoms act as an intergranular embrittler in the Σ5 GB due to a reduction in the charge density between the Fe atoms connected to the grains, whereas H and S co-segregation produces more pronounced embrittlement behavior, resulting in intergranular fracture. The S-induced embrittlement plays a crucial role in the H and S segregation, due to a combination of the structural and chemical effects. The fracturing of Σ5 GB due to S and H segregation is a two-step process. The first step is the breaking of Fe–Fe bonds in the GB, followed by the breaking of the remaining Fe–S bonds in the second step, resulting in the complete separation of the two grains. Moreover, the S atom can slightly compensate for the embrittlement induced by H, because some of the Fe atoms that obtain electrons from the S atoms can provide more electrons to the H atoms, and thus, they can compensate for the electrons that must be acquired from other Fe atoms. We call this “the electrons compensating effect” and this effect is helpful in the design and alloying of steels that are resistant to H embrittlement.     

Effects of surface oxide films on hydrogen permeation and susceptibility to embrittlement of X80 steel under hydrogen atmosphere

Hydrogen embrittlement (HE) induced by hydrogen permeation is a serious threat to the hydrogen transmission pipeline. In this study, oxide films were prepared on X80 steel by applying high-temperature oxidation, blackening treatment and passivation in concentrated H₂SO₄, and their effects on hydrogen permeation and HE susceptibility of X80 substrate were studied by conducting hydrogen permeation tests and slow strain rate tension (SSRT) tests. A numerical diffusion model was established to quantitatively determine the resistance of these oxide films to hydrogen permeation. Results showed that the oxide film prepared by high-temperature oxidation presented the highest resistance to hydrogen permeation with the ?_m/?_f value of 3828, and the corresponding HE index decreased from 38.07% for bare X80 steel to only 4.00% for that covered with oxide film. The characteristic of the corresponding fracture surfaces changed from brittle features such as quasi cleavage facets and secondary cracks to typical ductile dimple feature.     

Influence of microstructure-driven hydrogen distribution on environmental hydrogen embrittlement of an Al–Cu–Mg alloy

The hydrogen trap sites and corresponding hydrogen binding energies in an Al–Cu–Mg alloy with the different microstructures were investigated to unravel the environmental hydrogen embrittlement (HE) behavior of the alloy. The results showed that hydrogen can reside at interstitial lattices, dislocations, S′-phase, and vacancies. In the aged specimen with the highest hydrogen content, it was firstly reported that hydrogen resided at S′-phase particles with relatively high binding energy, which is a determinant factor on HE resistance of the alloy. In the cold-rolled specimen, high content of hydrogen trapped at dislocations with a reversible nature leads to intergranular hydrogen-assisted cracking. In the solution-treated specimen, hydrogen migration to the surface due to low trap density results in low hydrogen content and prevents the GBs from reaching critical hydrogen concentration. The obtained results clearly reveal that trap site density, and the nature of trap sites can determine environmental HE susceptibility of the alloy.     

Effect of oxygen on the microstructure and hydrogen storage properties of V–Ti–Cr–Fe quaternary solid solutions

The effect of low (<300 ppm O) and high (10,000 ppm O) residual oxygen concentration in vanadium raw metals on the microstructure and hydrogenation properties of V₄₀Fe₈Ti₂₆Cr₂₆, was investigated by means of XRD, SEM, TEM and pressure-composition isotherms. A high oxygen concentration in the vanadium raw metal led to the formation of an oxygen-rich secondary phase isostructural with α-Ti. The lattice parameter of the BCC main phase of the high-oxygen sample was reduced to 3.0141 (3) Å compared to 3.0308 (2) Å for the low-oxygen sample. As a result of the high oxygen content the equilibrium hydrogen pressure of the material was increased from 1 MPa to 4 MPa. Deoxidization through the addition of 1 at% rare earth metal could be achieved. The lattice constant of the deoxidized sample was 3.0297 (3) Å, and the thermodynamic properties were also the same as in case of the low-oxygen sample.     

Strain rate sensitivity of hydrogen-assisted ε-martensitic transformation and associated hydrogen embrittlement in high-Mn steel

This study presents the degree of promotion of deformation-induced γ?ε martensitic transformation by hydrogen charging and the associated fracture behavior at various strain rates ranging from 10^?5 to 10^?2 s^?1. A decrease in the strain rate from 10^?2 to 10^?5 s^?1 promotes the deformation-induced γ?ε martensitic transformation regardless of hydrogen charging (65%→82% in area fraction for uncharged specimens, 68%→84% for hydrogen-charged specimens). Hydrogen charging, which provides 11.7 mass ppm hydrogen concentration, further promotes the γ?ε martensitic transformation. However, the degree of promotion of the transformation by hydrogen is insensitive to the strain rate. Corresponding to the promotion of the γ?ε martensitic transformation, the hydrogen embrittlement susceptibility increases with a decrease in the strain rate. For instance, the elongation of the hydrogen-charged specimens decreases from 36 to 32% by decreasing strain rate from 10^?2 to 10^?5 s^?1. Hydrogen uptake deteriorates the resistance to crack initiation and propagation. Furthermore, the primary effect of the decrease in strain rate on hydrogen embrittlement is the acceleration of the crack propagation. In addition to the promotion of γ?ε martensitic transformation, a decrease in the strain rate in the presence of hydrogen may cause hydrogen localization at the crack tip, which assists brittle-like martensite cracking.     

Effect of hydrogen on butanol–biodiesel blends in compression ignition engines

Research suggests that there is a dramatic reduction in CO and particulate matter (PM) emissions when butanol is blended with biodiesel derived from rapeseed oil (RME), but a small increase in THC emissions. The addition of hydrogen as a combustion enhancer can be used to counteract the increase in THC emissions seen with the butanol fuel blends and further reduce CO and PM emissions. The emission benefits with hydrogen addition were shown to be further improved for RME-butanol fuel blends. The penalty for using hydrogen is an increase in NO_x emissions due to the increase in NO₂ formation during combustion, but this is expected to have significant benefits in the function of aftertreatment systems. In this study, it is shown that the increase in engine-out NO_x emissions can be effectively controlled through exhaust gas recirculation (EGR) without an excessive PM penalty thanks to the low PM concentration in the EGR (with an impeding PM recirculation penalty).     

Effect of Ti on the microstructure and Al–water reactivity of Al-rich alloy

Ti-bearing Al alloys (0.1–1 wt.%) were prepared using arc melting techniques. Their microstructures were investigated using XRD and SEM/EDX, and found to depend strongly on Ti contents. Al grains are columnar as Ti contents are lower, but they are refined and turn into equiaxed ones when Ti contents are higher. The particle sizes of Ga–In–Sn phase decrease with Al grain refinement. Al–water reactivities were also investigated under different water temperatures. Kinetic measurements found that Ti prohibits Al–water reaction and reduces hydrogen yields when alloys contain little Ti. However, Al reacts with water fast and hydrogen yields rise with the increase of Ti contents of alloys. Reasons concerning the variations of microstructures and Al–water reactivities with Ti additions are discussed.     

Effect of hydrogen on hydrogen–methane turbulent non-premixed flame under MILD condition

Energy crises and the preservation of the global environment are placed man in a dilemma. To deal with these problems, finding new sources of fuel and developing efficient and environmentally friendly energy utilization technologies are essential. Hydrogen containing fuels and combustion under condition of the moderate or intense low-oxygen dilution (MILD) are good choices to replace the traditional ones. In this numerical study, the turbulent non-premixed CH₄+H₂ jet flame issuing into a hot and diluted co-flow air is considered to emulate the combustion of hydrogen containing fuels under MILD conditions. This flame is related to the experimental condition of Dally et al. [Proc. Combust. Inst. 29 (2002) 1147–1154]. In general, the modelling is carried out using the EDC model, to describe turbulence–chemistry interaction, and the DRM-22 reduced mechanism and the GRI2.11 full mechanism to represent the chemical reactions of H₂/methane jet flame. The effect of hydrogen content of fuel on flame structure for two co-flow oxygen levels is studied by considering three fuel mixtures, 5%H₂+95%CH₄, 10%H₂+90%CH₄ and 20% H₂+80%CH₄(by mass). In this study, distribution of species concentrations, mixture fraction, strain rate, flame entrainment, turbulent kinetic energy decay and temperature are investigated. Results show that the hydrogen addition to methane leads to improve mixing, increase in turbulent kinetic energy decay along the flame axis, increase in flame entrainment, higher reaction intensities and increase in mixture ignitability and rate of heat release.     

Permeation barriers for hydrogen embrittlement prevention in metals – A review on mechanisms,materials suitability and efficiency

To develop large-scale use of hydrogen as an environmentally sensible alternative to fossil energy sources, the design of safe and innovative storage and transportation infrastructure is a crucial issue. In direct contact with the high-pressure hydrogen, structural materials, especially traditional alloys, are indeed susceptible to degradation of their mechanical properties due to the diffusion of hydrogen atoms into their atomic lattice structure. This phenomenon leads to materials' embrittlement and results in severe damage to the employed components. Therefore, the prevention of hydrogen atom diffusion is one key consideration to avoid its adverse effects on materials' mechanical properties. This paper aims to review the mechanisms and factors responsible for the hydrogen embrittlement phenomenon. The main specifications to fulfill while selecting appropriate materials are hence considered for hydrogen energy uses. Finally, the effective surface modification solutions are reviewed for implementation as a permeation barrier to protect the structural materials from hydrogen degradation.     

Effect of temperature on microstructure and texture of rolled Ni–9·3 at-%W alloy

Abstract

In the present paper, the microstructure and texture of cold rolling and warm rolling Ni–9·3 at-%W alloy were investigated by electron backscatter diffraction technique. It is found that warm rolling will reduce the deformation twins and transfer the rolling texture from brass type to copper type. The forming mechanism of rolling microstructure and texture of Ni–9·3 at-%W alloys as well as the temperature effects was discussed.     

Effect of sludge recirculation on characteristics of hydrogen production in a two-stage hydrogen–methane fermentation process treating food wastes

The two-stage hydrogen–methane fermentation process with different patterns of recirculation was investigated. Operations with the circulation of heat-treated sludge performed considerably better than those with the recirculation of raw sludge with respect to both the hydrogen production rate and yield. In addition, the results of the batch tests demonstrated that circulated sludge was capable of consuming hydrogen under acidogenic pH while the heat-treated sludge was not. These results suggest that the recirculation of active methanogenic sludge had an inhibitive effect on the hydrogen production, which can likely be attributed to the high hydrogen-consuming activity of microorganisms present in the circulated sludge. On the other hand, operations without any sludge recirculation did not perform well in terms of hydrogen production or carbohydrates degradation compared to those with recirculation, perhaps due to a shortage of available nitrogen. This suggests that sludge recirculation in effect supplemented the NH₄⁺ in the hydrogen reactor.     

Hydrogen embrittlement of super duplex stainless steel – Towards understanding the effects of microstructure and strain

The effects of austenite spacing, hydrogen charging, and applied tensile strain on the local Volta potential evolution and micro-deformation behaviour of grade 2507 (UNS S32750) super duplex stainless steel were studied. A novel in-situ methodological approach using Digital Image Correlation (DIC) and Scanning Kelvin Probe Force Microscopy (SKPFM) was employed. The microstructure with small austenite spacing showed load partitioning of tensile micro-strains to the austenite during elastic loading, with the ferrite then taking up most tensile strain at large plastic deformation. The opposite trend was seen when the microstructure was pre-charged with hydrogen, with more intense strain localisation formed due to local hydrogen hardening. The hydrogen-charged microstructure with large austenite spacing showed a contrasting micro-mechanical response, resulting in heterogeneous strain localisation with high strain intensities in both phases in the elastic regime. The austenite was hydrogen-hardened, whereas the ferrite became more strain-hardened. SKPFM measured Volta potentials revealed the development of local cathodic sites in the ferrite associated with hydrogen damage (blister), with anodic sites related to trapped hydrogen and/or micro voids in the microstructure with small austenite spacing. Discrete cathodic sites with large Volta potential variations across the ferrite were seen in the coarse-grained microstructure, indicating enhanced susceptibility to micro-galvanic activity. Microstructures with large austenite spacing were more susceptible to hydrogen embrittlement, related to the development of tensile strains in the ferrite.     

Effect of hydrogen concentration on vented explosion overpressures from lean hydrogen–air deflagrations

Experimental data from vented explosion tests using lean hydrogen–air mixtures with concentrations from 12 to 19% vol. are presented. A 63.7-m³ chamber was used for the tests with a vent size of either 2.7 or 5.4 m². The tests were focused on the effect of hydrogen concentration, ignition location, vent size, and obstacles on the pressure development of a propagating flame in a vented enclosure. The dependence of the maximum pressure generated on the experimental parameters was analyzed. It was confirmed that the pressure maxima are caused by pressure transients controlled by the interplay of the maximum flame area, the burning velocity, and the overpressure generated outside of the chamber by an external explosion. A model proposed earlier to estimate the maximum pressure for each of the main pressure transients was evaluated for the various hydrogen concentrations. The effect of the Lewis number on the vented explosion overpressure is discussed.     

Effect of composition and temperature on the burning velocity of the nitric oxide—hydrogen flame

The burning velocity of the nitric oxide-hydrogen flame has been measured at 1 atm, over the composition range 35–70% NO, for two initial temperatures, 298°K and 1023°K. In addition, the burning velocity of the 50% NO mixture was determined as a function of initial temperature over the temperature range 300 to 1000°K. A free radical mechanism is proposed in which the principal nitro-oxide destruction reaction is: H + NO = OH + N.     

Effect of the relative position of oxygen–hydrogen plate channels and inlets on a PEMFC

A numerical study of a PEMFC (proton exchange membrane fuel cell) bipolar plate has been made to study the influence of the relative entry positions of hydrogen and oxygen on the distribution of gases. Various 3D configurations, under simplified working conditions, have been considered. As the main result, it is shown that the flow of the gas in stoichiometric defect (usually hydrogen) forces the pattern of the other reactant gas (oxygen).     

Relationship between hydrogen permeation and microstructure in Nb–TiNi two-phase alloys

The relationship between the microstructure and hydrogen permeability of the as-cast two-phase Nb–TiNi alloys is investigated and discussed on the basis of the mixing rule. The alloy compositions consisted of the bcc-(Nb, Ti) and B2–TiNi phases are expressed as (Nb₄Ti₄₆Ni₅₀)_1−x(Nb₈₅Ti₁₃Ni₂)_x. The alloy for x = 0.19 has a fully eutectic structure of the (Nb, Ti) and TiNi phases in the as-cast state. The hydrogen permeability of this alloy corresponds to that of a model alloy in which these two phases are distributed randomly. The primary TiNi and (Nb, Ti) phases are formed in the alloys for x < 0.19 and x > 0.19, respectively. Their volume fractions decrease and increase with x, respectively. According to the mixing rule, the hydrogen permeability of alloys having a primary (Nb, Ti) phase can be expressed as the model alloy in which the primary phase is isolated in the eutectic structure. However, the hydrogen permeability of alloys having a primary TiNi phase is higher than that expected for the above model alloy.