Hydrogen-charged Specimens Research Articles

Hydrogen embrittlement of a V+Nb-microalloyed medium-carbon bolt steel subjected to different tempering temperatures

The effect of different tempering temperatures on the hydrogen embrittlement (HE) resistance of a V + Nb-microalloyed medium-carbon bolt steel was studied by slow strain rate tensile testing and hydrogen thermal analysis. The HE resistance represented by notch tensile strength σbN of hydrogen-charged specimens increases with increasing tempering temperature Ttem, and the σbN of the in situ hydrogen-charged sample is considerably lower than that of the pre-hydrogen-charged sample. Both the diffusible hydrogen content Cdiff and hydrogen-trapping capability of the precipitates show a significant increase-decrease change trend with Ttemp. The ratio of Cdiff primarily trapped by nanoscale V-rich MC precipitates increases linearly with increasing Ttemp. The control of V-rich MC through suitable increasing Ttemp is vital for obtaining the best HE resistance although at the cost of decreased strength. It is thus suggested that a compromise between strength and HE resistance should be considered for the practical application of this type of steel.

Evaluating hydrogen embrittlement susceptibility of operated natural gas pipeline steel intended for hydrogen service

Green hydrogen produced by renewable energy sources is expected to play an important role as a decarbonized energy carrier. It can be transported cost-effectively over long distances by pipelines. Hydrogen transport through existing natural gas pipeline networks is supposed to be prospective. However, a possible degradation of gas pipeline steels as a result of their long-term operation should be taken into account in assessing the prospects of using the existing gas network for hydrogen transportation safely. In this study, hydrogen embrittlement susceptibility of low-carbon pipeline steel in the as-received state and after long-term operation is investigated. The influence of long-term operation on the microstructure, mechanical properties, and fracture mechanism of steel is assessed, considering the direction of cutting the specimens relative to the pipe axis. The electrolytic hydrogen pre-charging process with different intensities is used to analyze the influence of absorbed hydrogen on the mechanical behaviour of steel by tensile tests of uncharged and hydrogen-charged specimens. The as-delivered steel is not prone to hydrogen embrittlement after moderate hydrogen charging, while the operated one is sensitive to it. Metallographic and fractographic analyses revealed a crucial role of non-metallic inclusions in the hydrogen embrittlement of steel, which depended on its technical state and hydrogen charging conditions. Moderate hydrogen charging led to intergranular embrittlement of steel, while intensive − to transgranular embrittlement, indicating different mechanisms of transport of hydrogen absorbed by the metal.

Impact of the Delay Period between Electrochemical Hydrogen Charging and Tensile Testing on the Mechanical Properties of Mild Steel

With escalating global regulatory pressure for countries to adhere to emission laws, repurposing existing natural gas pipelines for hydrogen-based commodities stands to be an economical solution. However, the effects of hydrogen embrittlement must be thoroughly considered for this application to avoid the unexpected catastrophic failure of these pipelines. The literature proposes several physicochemical embrittlement models. This paper reports one aspect of hydrogen embrittlement that remains to be quantified: the recovery of ductility (embrittlement) of mild steel specimens subjected to artificially accelerated hydrogen absorption via electrochemical charging as a function of time. The effects of charging duration and particularly the delay period between charging and mechanical tensile testing were investigated. Unsurprisingly, longer charging time shows a greater loss of elongation; however, a more extensive recovery of ductility correlated with longer charging time in the first few days after charging. The data also show that while the uncharged mild steel met all minimum required values for strength and elongation for the specified grade, there was a substantial variability in the elongation to failure. The same trends in variability of elongation translated to the hydrogen-charged specimens. Due to this extensive variability, failure to meet the elongation specification of the grade is reported based on the worst-case scenario obtained for a given set of samples for each exposure condition. These results have practical implications for the monitoring and testing of infrastructure exposed to hydrogen, particularly as this relates to industry planned operational shutdown schedules.

Enhanced susceptibility of high-strength fastener nuts to hydrogen-induced stress corrosion cracking

Abstract High-strength HV-fastener sets of dimensions M48 and M64 with property class 10.9 were employed in offshore wind turbine frameworks. The M64 were used in coupling flanges within monopiles. The M48 were employed in another offshore wind farm and encountered natural weathering. In both installations, time-delayed fractures of the nuts were observed. Owing to the presence of macroscopically visible corrosion products, hydrogen-induced stress corrosion cracking (Hi-SCC) was established as the probable cause of failure. However, a nut fracture in a properly pre-tensioned bolt assembly is atypical since the stresses in the bolt threads are higher than those in the nuts. Based on the Hi-SCC theory, the fracture should occur at the most stressed component, which is the bolt. During the root cause analysis, extensive examinations were conducted to determine the cause of the nut fractures. The focus was on investigating whether the nut material was more prone to Hi-SCC than the bolt material. The examination program included scanning electron microscopy and energy dispersive x-ray spectroscopy (SEM-EDS) analysis of the fracture surfaces, optical microscopy of microspecimens, mechanical tests, and stress rupture tests of hydrogen-charged specimens. While the results suggest that the tested nuts comply with the requirements of the applicable standards regarding material properties, they also reveal that the nut material is, despite its lower tensile strength, significantly more susceptible to Hi-SCC than the bolt material. Therefore, a direct relationship between material susceptibility to Hi-SCC and the tensile strength, as standards and guidelines imply, is not given.

Experiments and DFT calculations on the effects of interstitial hydrogen on Ti corrosion products in high temperature water

The effects of the interstitial hydrogen on the characteristics of corrosion product of pure Ti in high temperature water was studied by a double exposure corrosion experiment and DFT calculations. A double-layer corrosion product was found in both internally hydrogen charged specimen and internally vacuumed specimen. The main difference in corrosion product was the thickness of outer layer. The adsorption energies of Fe, Ti and O atoms on three surfaces (Ti, TiO2 and FeTiO3) with and without an interstitial hydrogen were considered in the DFT calculation. The correlations between the experimental results and DFT calculations were also discussed.

The effect of hydrogen and notch orientation in SENT specimens on the fracture toughness of an API 5L X70 pipeline steel

Transporting hydrogen gas through pipelines is an important topic within the context of energy transition. An essential challenge hereto is the phenomenon of hydrogen embrittlement of pipeline steels which includes a reduction in fracture toughness. As the theories accounting for hydrogen embrittlement are still debated, improving fundamental insights into the underlying mechanisms is crucial. Fracture toughness tests on single edge notched tension (SENT) specimens have been performed to examine the influence of hydrogen on the fracture behavior of a pipeline steel. An API 5L X70 steel was selected for this purpose, characterized by a distinctive banded microstructure commonly seen in pipeline steels. The test specimens were extracted from the steel pipe, and a notch in two different orientations was machined. By using a combination of fractography and X-ray micro-CT (Computed Tomography), the fracture mechanisms are revealed. The tearing resistance characteristics of the steel, with and without the presence of hydrogen, are demonstrated through tearing resistance curves. The experiments show that the banded microstructure plays a key role in the results through the development of splits at the microstructural bands. While these splits are also present in uncharged toughness test specimens, they are increasingly present in hydrogen charged specimens suggesting a hydrogen related weakening of the banded microstructure. In addition, it is demonstrated that hydrogen induces an increasingly zigzagged crack path. This is attributed to a potential combination of accelerated void nucleation at the microstructural bands, and accelerated shear-driven fracture.

Study on ductile fracture behavior of hydrogen-charged API pipeline steel

With the increasing focus on reducing carbon emissions, hydrogen has emerged as a promising alternative energy source. However, the safe transportation of hydrogen poses challenges due to its potential impact on the integrity of pipeline materials. This study aims to investigate the effect of hydrogen charging time on the plastic behavior and ductile fracture characteristics of API 5L X42 pipelines, which are commonly used for transporting hydrocarbons. Tensile tests were conducted on various types of hydrogen-charged specimens at room temperature to assess different fracture modes. Hydrogen was introduced into the specimens using a cathodic electrolytic method, and 24-h charging time durations were considered. Finite element analyses were performed on standard dog-bone and notched tension specimens to evaluate the plastic behavior, employing the Swift hardening law to model the flow stress. Numerical analyses were conducted to determine the loading path leading to fracture initiation. A damage framework based on the Hosford–Coulomb model was utilized to predict ductile fracture under non-proportional loading conditions. The findings of this study provide insights into the fracture behavior of pipelines under hydrogen exposure, aiding in the design and assessment of safe and reliable hydrogen transport systems.

Pre-strain and hydrogen charging effect on the plastic and fracture behavior of quenching and partitioning (Q&P) steel

This study investigates the fracture behavior of plastically deformed quenching and partitioning (Q&P) steel under hydrogen conditions. The experimental results from slow strain rate tensile (SSRT) tests reveal a considerable decrease in elongation with increase of hydrogen traps induced by pre-strains. The effect of pre-strain on hydrogen susceptibility is analyzed through fractography and microstructure measurements. The relative reduction of area decreased from 0.389 at 0 % pre-strain to 0.181 at 12 % pre-strain with hydrogen. For hydrogen-free case, fracture surfaces show typical ductile fracture with shallow dimples by micro-void coalescence induced by phase transformation. For hydrogen-charged specimens, the fractography indicates quasi-cleavage and transgranular fracture at specimen center and brittle-like fracture can be analyzed by the hydrogen-enhanced decohesion. Additionally, the brittle fracture area is enlarged with pre-strain. The mixed fracture between the edge and center of cross-section can be explained using finite element modeling, which calculates the distributions of dislocation density and flux, hydrostatic stress during SSRT at different pre-strains. As an effect of pre-strain in the hydrogen-charged Q&P steel, micro-cracks initiate at transformed martensite with low level of pre-strain, whereas their propagation is blunted at ferrite interface. For specimens with pre-strains exceeding 8 %, the cracks initiate predominantly at the martensite, which propagate through deformed ferrite matrix with high dislocations and trapped hydrogens. Our findings shed light on a new metallurgical design for improving the resistance to hydrogen embrittlement in Q&P steel when pre-strain, hydrogen transport, and phase transformation are properly controlled.

A novel strategy for estimating the diffusion behavior of hydrogen in metallic materials with the combined effect of stress corrosion and hydrogenation: Case study of 2.25Cr-1Mo-0.25V high-strength steel

Hydrogen equipment for long-term service in complex hydrogen environment is threatened by the phenomenon of hydrogen-induced damage, and the combined effect of stress corrosion and hydrogen attack can intensify the formation and development of hydrogen damage. In this study, a new strategy to indirectly estimate the hydrogen diffusivity of metallic materials under tensile stress was proposed by combining the electrochemical hydrogen permeation test (EHPT), the hydrogen diffusion descriptive equation based on Fick's law, and hydrogen pre-charged tensile test. The results revealed that the hydrogen permeation curve obtained after considering the effect of the residual hydrogen extraction stage was highly approximate to the theoretical trend. The hydrogen embrittlement (HE) susceptibility of the pre-tensioned tensile and hydrogen-charged specimens increased with increasing stress. At tensile stress (0–400 MPa) the effective hydrogen diffusivity of 2.25Cr-1Mo-0.25 V steel was: Deff=1.1686+0.4842×exp(σ/277.869). The micrograph from the fracture surfaces also reflected the aggravated brittle fracture characteristics of the steel.

Effect of stress triaxiality on the hydrogen embrittlement micromechanisms in a pipeline steel evaluated by fractographic analysis

Hydrogen gas, as an energy carrier, is expected to play an important role in the storage and transport of energy produced by renewable sources. The transport of hydrogen gas can be managed through the use of existing pipeline networks. However, the uptake of hydrogen in steel structures is known to cause a degradation of mechanical properties, a phenomenon called hydrogen embrittlement (HE). In this study, the hydrogen-assisted ductility loss of an API 5L X70 pipeline steel is investigated. The influence of hydrogen on mechanical behavior is evaluated by tensile tests on uncharged and hydrogen charged specimens. These tests are performed on notched round bar specimens with different notch radii, allowing for a range of positive stress triaxialities to be examined. Notched samples are more embrittled than unnotched specimens after hydrogen charging. Hydrogen signatures in the form of fisheyes and quasi-cleavage are detected on the fracture surface. Fisheyes initiate primarily at segregation bands in the steel for all specimen geometries. Fisheyes can also be linked to different inclusion types present in the steel, depending on notch geometry. For unnotched as well as the least sharply notched specimens tested in this study, the embrittlement can be correlated with the area fraction of fisheyes on the fracture surface.

Temperature dependence of hydrogen-assisted damage evolution and fracture behavior in TRIP-aided bainitic ferrite steel

We systematically investigated the effects of hydrogen and deformation temperature on the micro-damage evolution and fracture behavior of the TRIP-aided bainitic ferrite steel using slow strain rate tensile tests and microstructure examinations. For hydrogen-uncharged condition, the micro-damage evolution behavior was not dependent on temperature, except for −100 °C. The exceptional behavior at −100 °C was attributed to the occurrence of brittle fracture. Hydrogen uptake significantly increased the number density of micro-damage in all deformation temperatures. The fracture mode of hydrogen-charged specimens changed from quasi-cleavage and ductile fractures to cleavage, quasi-cleavage, and intergranular fractures with decreasing deformation temperature. Quasi-cleavage fracture features were attributed to the crack growth process consisting of repetition of small crack initiation/blunting (or void initiation) and crack coalescence, whereas the cleavage and intergranular surfaces with smooth facets were due to the occurrence of hydrogen-reduction cohesive strength of cleavage planes and prior austenite grain boundaries.

A local stress criterion to assess the effects of hydrogen embrittlement on the fracture strength of notched tensile specimens

This work addresses the applicability of a local criterion incorporating the coupling of critical stress and a critical hydrogen concentration to predict hydrogen embrittlement effects on the fracture strength of high strength steels using notched round specimens with different notch root radii. The numerical simulations incorporating a relatively simple hydrogen transport model provide strong support for the adoption of a failure criterion in terms of achieving a critical level of tensile stress coupled to the local hydrogen concentration, which, in turn, enable the construction of a failure locus for the degraded material. For the cases analyzed here, construction of such a failure locus based on a critical combination of maximum principal stress and hydrogen concentration enabled predictions of fracture strength for hydrogen-charged tensile specimens which are in very good agreement with experimental data. Overall, the results presented here lend additional support for further developments of a local stress-based criterion to predict hydrogen embrittlement effects on the fracture strength of high strength steels.

The role of precipitation in hydrogen diffusivity and mechanical properties of a high entropy alloy

In the present work, we investigate the interaction of hydrogen with the microstructure of the novel Fe38Mn25Ni24Co5Cr2Al4Ti2 high entropy alloy (HEA), under annealed and aged conditions. For this purpose, hydrogen gas permeation tests, slow strain rate tests in previously hydrogen charged specimens and thermal desorption analyzes were carried out. Under both conditions, low hydrogen diffusivity values were calculated at room temperature, comparable to that of nickel-based superalloys. Hydrogen diffusion coefficient for the alloy under annealed condition can be expressed by D=1.58x10−7m2.s−1exp(−43.7RT)kJ.mol−1 and, for the aged condition, by D=1.25x10−7m2.s−1exp(−41.9RT)kJ.mol−1. Furthermore, mechanical characterization and fracture surface analyzes demonstrate that the aging treatment improves the mechanical properties and does not impact the hydrogen embrittlement susceptibility of the alloy. Our findings contribute to the investigation of hydrogen embrittlement mechanism and provide data for the design of HEAs with suitable properties for structural applications in severe environments.

Hydrogen embrittlement prompt fracture in Ni-based single crystal superalloy

Hydrogen-fueled and hydrogen-hybridized aircraft engines are a new trend in the aviation industry for environmental reasons. Single crystalline Ni-based superalloys are the most commonly used engine materials and their hydrogen embrittlement properties need urgent investigation. In this study, the hydrogen embrittlement behavior and underlying fracture mechanism of a second-generation Ni-based single crystal superalloy with electrochemical hydrogen pre-charge were investigated. The superalloy showed tremendous susceptibility to hydrogen embrittlement with reduced strength and ductility. A large number of micropores and cracks on the fracture surface are found in hydrogen-charged specimens, leading to embrittlement and ultimate cracking. More dislocations, stacking faults and DSBs are observed in specimens with hydrogen uptake. Hydrogen-induced micropores first form at the γ/γ′ interface and then propagate into the γ′ phase, leading to cracking, which was analyzed using in situ environmental studies with a transmission electron microscope. Hydrogen reduces the cohesive strength between the γ- and γ′-phase and accelerates crack propagation along the voids. Hydrogen embrittlement fracture in Ni-based single crystal superalloys is due to synergistic hydrogen-enhanced local plasticity, strain-induced vacancies and decohesion in the hydrogen-induced cracking process.

Three-dimensional propagation behavior of hydrogen-related intergranular cracks in high-strength martensitic steel

Using X-ray computed tomography and focused ion beam-scanning electron microscopy (FIB-SEM) serial sectioning, this study investigated the three-dimensional propagation behavior of hydrogen-related intergranular cracks in martensitic steel with a tensile strength of 1.2 GPa, focusing on segments of prior austenite grain boundaries (PAGB segments). X-ray computed tomography revealed that the crack morphology was more continuous in the hydrogen-charged specimens. Through FIB-SEM serial sectioning, we found that crack-tip blunting and ductile rupture of un-cracked ligaments were associated with specific PAGB segments in the uncharged specimen. In the case of hydrogen-related intergranular crack propagation, even very fine, low-angle PAGB segments (sub-micrometer size) can act as obstacles for crack propagation. Based on these results, we propose that the misorientation of each PAGB segment has a large influence on the local arrestability of intergranular crack propagation.

An experimental and numerical study on strain controlled fatigue response of TRIP steel under influence of hydrogen

The present work aims to study the influence of hydrogen charging on the transformation induced plasticity steel under strain-controlled low cycle fatigue (LCF). Hydrogen charging had detrimental effect on strain-controlled LCF lives. Cyclic hardening in un-charged specimen due to strain-induced martensitic transformation (SIMT) dominated over cyclic softening. In contrast, generation of large fraction of low angle grain boundaries in hydrogen charged specimen leads to cyclic softening and supresses cyclic hardening due to SIMT. Additionally, Cyclic plasticity modelling using Ohno-Wang kinematic hardening model is validated by simulating cyclic stress–strain hysteresis loop and cyclic softening curve of uncharged and hydrogen charged specimens.

Effect of high-pressure hydrogen environment in elastic and plastic deformation regions on slow strain rate tensile tests for iron-based superalloy A286

To investigate the effects of a high-pressure hydrogen environment in the elastic and plastic deformation regions, we performed slow strain-rate tensile tests of iron-based superalloy A286 at 150 °C by switching the atmosphere from 70 MPa hydrogen to air during the tests. The relationship between the nominal strain exposed to a hydrogen environment and the relative reduction in area (RRA) revealed that in the plastic deformation region, the RRA value decreased gradually depending on the nominal strain exposed to hydrogen, but in the elastic deformation region, the RRA value decreased rapidly. The RRA value further decreased when the stress cycle was applid in the elastic region. The fracture surface exhibited an intragranular slip plane fracture similar to that of the hydrogen-charged specimen. These phenomena suggest that the lattice decohesion theory is dominant in the elastic region, where hydrogen embrittlement occurs owing to an increase in the content of dissolved hydrogen.

The effect of friction stir processing on the hydrogen susceptibility of AA5083 specimens after hydrogen cathodic charging

The present study investigated the effect of hydrogen on the mechanical degradation of friction stir processed (FSPed) 5083 aluminum alloy by intense hydrogen cathodic charging (HCC). The effect of different numbers of FSP passes was investigated: 3 and 8 passes, respectively. Hydrogen-charged and uncharged specimens were subjected to tensile testing and microhardness evaluation analysis, and were examined through optical microscopy, focus variation microscopy (FVM), and scanning electron microscopy (SEM) both on the microstructure and fracture zone. The results showed that the FSP process introduced a refined microstructure with finer grains. This led to an improved mechanical response during tension tests of the uncharged specimens; the energy absorption increased from 85 MJ/m3 of the base material to 94 MJ/m3 and 97 MJ/m3 for the 3 and 8 FSP passes, respectively. The introduction of hydrogen through the HCC process led to a more brittle mechanical response with a decrease in the energy absorption capability for all the charged specimens. The more prone specimen was the 8 FSP passes specimen where the energy absorption dropped by 20% and 71% for the two different charging current densities. The 3 FSP passes specimen presented a reduction of energy absorption of 4% and 18%, respectively, where the base material presented a reduction of 8% and 14%, respectively. This brittle response is also evident from the microhardness testing where the hydrogen charging led to increased surface hardness values. The 3 FSP passes specimen presented a better mechanical response with respect to the base material specimen (and the 8 FSP passes specimen) for all the charging conditions, and this led to the conclusion that a small number FSP surface modification could be a beneficial surface modification process as it improves the mechanical response of the material and is not significantly affected by hydrogen charging environments.

A systematic study on the influence of electrochemical charging conditions on the hydrogen embrittlement behaviour of a pipeline steel

Although hydrogen embrittlement (HE) has been the subject of extensive research over the past century, a systematic study on the HE susceptibility of steels under different electrochemical charging conditions has been lacking. This study specifically targets this knowledge gap by evaluating the HE behaviour of a typical pipeline steel X65 after hydrogen-charging in acidic, neutral, and alkaline electrolytes that simulate various industrial environments. Results from a series of experiments show that the HE susceptibility of X65 steel varied significantly with hydrogen-charging electrolytes and, to a smaller extent, with electrochemical charging variables. The highest and lowest HE susceptibilities were found from specimens charged in acidic and alkaline electrolytes, respectively. An increase in yield strength was observed for almost all hydrogen-charged specimens, regardless of the charging conditions. Under severe electrochemical charging conditions, blistering was detected and mechanical properties were substantially decreased. Discussion has been made in comprehending these relationships.

Hydrogen-assisted intergranular fatigue crack initiation in metals: Role of grain boundaries and triple junctions

In this work, small-scale, low-cycle fatigue experiments on hydrogen charged nickel specimens are performed that highlight particular grain boundaries (GBs) and triple junctions as potential intergranular crack initiation sites. To understand the micromechanics and underlying physics, a dislocation density-based crystal plasticity model coupled with slip-rate based hydrogen transport model is developed. A fatigue indicator parameter (FIP) is also developed that models the crack initiation process by considering the contributions of accumulated plastic slip, GB normal stress, and local hydrogen concentration. Depending on the diffusivity, hydrogen binding energy, and misorientation, GBs are categorized as ‘special’ or ‘random’, and their role on hydrogen distribution is analysed using a model microstructure. Special GBs are ones with low diffusivity and high hydrogen binding energy whereas the random GBs have high diffusivity but low hydrogen binding energy. Complying with the experimental observations, the evolution of FIP with load cycles suggests certain triple junction configurations in the microstructure involving in the crack initiation process. For the case of uniform initial hydrogen concentration, special GBs are found to retain more hydrogen with load cycles primarily due to their low hydrogen diffusivity whereas the random GBs diffuse hydrogen out quickly to the bulk. The high hydrogen concentration and favourable stress state in the form of high hydrostatic stress spots generally found at triple junction of special/random GBs fulfil the necessary condition leading to an intergranular crack initiation.