Mechanical load induced hydrogen charging of retained austenite in quenching and partitioning (Q&P) steel

This work investigates the effect of a constant load on hydrogen diffusion through a Q&P steel containing metastable retained austenite by combining electrochemical hydrogen permeation and thermal desorption spectroscopy. Material samples are placed under different external loading conditions, ranging from 50% to 125% of the yield stress. The permeation transients indicate that hydrogen diffusion is delayed under all stressed conditions, even at stresses in the elastic regime, with the delay increasing with the applied load. From thermal desorption spectroscopy performed on the same specimens after the permeation test, it appears that the samples tested under load show a high temperature peak, which is not present in the unloaded sample. Further differential scanning calorimetry analysis confirms that the high temperature peak is related to retained austenite and is a result of hydrogen effusion and hydrogen release due to transformation of the retained austenite.     

Hydrogen embrittlement behavior of high strength low carbon medium manganese steel under different heat treatments

Hydrogen embrittlement (HE) behavior was investigated in a low carbon medium Mn steel with three different volume fraction of retained austenite (RA), which was obtained after different heat treatments. The hydrogen permeation test showed a higher permeability for directly water quenched specimen compared to quench-tempered specimens. Melt extraction test showed hydrogen concentration increased with hydrogen charging current density in the order of directly quenched specimen, QLA, quenched with low-temperature annealed specimens and QHA quenched with high-temperature annealed specimens. Slow strain-rate tensile test was employed to examine the HE behavior, the HE indices decreased with the increase of RA irrespective of increased hydrogen concentration. HE susceptibility can be suppressed by raising intercritical annealing temperature because Mn enrichment increases the stability of RA.     

The effect of quench cracks and retained austenite on the hydrogen trapping capacity of high carbon martensitic steels

Hypereutectoid martensitic steels possess excellent hardness levels which make them attractive materials for specific industrial applications. However, they can contain cracks and/or retained austenite after quenching which show a particular interaction with hydrogen (H). Hence, this work evaluates the interaction between H and a martensitic Fe-1.1C alloy by combining a wide variety of (H) characterization techniques with a systematic approach for specially designed H charged and heat treated samples. A detailed analysis of the microstructure for every condition serves as the basis for the interpretation. The results show that the presence of H leads to additional cracking and branching or growth of pre-existing quench cracks. Moreover, it is shown that when the temperature exceeds the retained austenite decomposition temperature while the austenitic grains contain H, additional cracking occurs which increases the amount of reversible H trapping sites thus raising the HE susceptibility.     

Hydrogen diffusion and trapping in X70 pipeline steel

In this investigation, the hydrogen permeation experiment was used to determine the parameters for diffusion, and the hydrogen microprint technique was used to visualize the diffusion path in X70 pipeline steels. The samples in mid-layer at the segregation zone and top-layer of the steel were used in this study. The diffusion parameters from the hydrogen permeation experiment enabled us to determine that the calculated density of total, reversible, and irreversible hydrogen trapping sites in the top layer of the steel decreases for larger grains. However, the irreversible and total trapping sites of the mid-layer showed an initial growth and subsequent decay with grain growth due to the inclusions in mid-layer. The observations from the hydrogen microprint experiment allowed us to conclude that the preferential hydrogen diffusion increases in the order of grains, grain boundaries, triple junctions and cementites with cementites being the easiest path for hydrogen diffusion.     

Effective hydrogen diffusion coefficient for CoCrFeMnNi high-entropy alloy and microstructural behaviors after hydrogen permeation

Herein, the first observation of the effective hydrogen diffusion coefficient of CoCrFeMnNi high-entropy alloy (HEA) was performed using electrochemical hydrogen permeation; further, it was compared with those of stainless steels (SS) 304 and 316L. HEA and SS 316L showed similar effective hydrogen diffusion coefficient of 1.75 × 10⁻¹¹ m²/s and 1.91 × 10⁻¹¹ m²/s, respectively. SS 304 showed the smallest that of 0.58 × 10⁻¹¹ m²/s in the study. Hydrogen diffusion through the grain boundary was dominant in face-centered cubic metals. Hydrogen permeation resulted in no change in the microstructure of HEA and SS 316L; however, it caused a martensitic transformation in SS 304.     

FEM simulation supported evaluation of a hydrogen grain boundary diffusion coefficient in MgH2

The diffusion coefficient data of hydrogen in the Magnesium-hydrogen system shows a large scatter, their trends extrapolations vary at room temperature between 10^?12 m²/s and 10^?29 m²/s. At room temperature the hydrogen diffusion coefficient in MgH₂ is, thus, uncertain by about 17 orders of magnitude. This may be partially attributed to grain boundaries contributing to the measured diffusion coefficient. In this paper we use finite-element (FEM) simulations to evaluate the influence of the grain boundary diffusion on the measured total diffusion depending on the difference of the grain boundary (D_GB) and volume (D_V) diffusion coefficients, as well as on the grain size. These results will be compared to Harrisson's analytical solutions. When the diffusion coefficients differ by more than D_V < 10^?3·D_GB, Harrison's diffusion regime C becomes the best way to describe the total diffusion. The results are used to re-interpret literature data on hydrogen diffusion in MgH₂ from this grain boundary contribution point of view. At 300 K, a hydrogen grain boundary diffusion coefficient ranging from D_GB = 10^?17 m²/s to D_GB = 10^?20 m²/s, depending on the individual type of sample in MgH₂, results from the data evaluation.     

Understanding microstructural influences on hydrogen diffusion characteristics in martensitic steels using finite element analysis (FEA)

A two-fold approach is considered to study hydrogen (H) diffusion characteristics in martensitic steels. Initially, a multi-trap stress coupled H diffusion finite element model was developed to investigate the role of various trap states on effective H trapping during a four point bend test. The calculations show that high angle boundaries are more influential in controlling H diffusion in presence of low initial (bulk) H concentrations, while dislocations can have more pronounced impact, when the bulk H concentration is higher. A microstructural model comprising of prior austenite grains and packets was further developed. The study highlights the importance of packet boundaries (PBs) moderating H diffusion in martensite microstructure. The presence of retained austenite content affecting H diffusion paths was also studied. Overall, this parametric study presents complementary techniques in numerical modeling, as well as implications on the role of various microstructural entities affecting H diffusion.     

Hydrogen effect on a low carbon ferritic-bainitic pipeline steel

Hydrogen effect on an API 5L X65 low carbon ferritic-bainitic steel is investigated, by evaluating the fracture toughness parameters in air and in hydrogen environment. The hydrogen environment is manifested by in situ hydrogen charging of the X65 steel, using the electrolytic solution NS4, which simulates the electrolyte trapped between the pipeline steel and the coating in a buried pipeline. The fracture toughness results of the X65 are compared to two other pipeline steels with different microstructures, namely an X52 and an X70, possessing a banded ferritic-pearlitic and banded ferritic-mixed bainitic-pearlitic microstructure, respectively. The X65 steel exhibits significant reduction of fracture toughness parameter J₀ integral due to hydrogen charging and insignificant variation of fracture toughness parameter K_Q. Comparing the three steels, the lowest reduction of J₀ integral due to hydrogen charging, is met on the X52 and the highest in the X65.     

Evolution of microstructure and texture in pipeline steels at different TMCP procedures with regard to hydrogen induced cracking

SEM and EBSD techniques are used to evaluate hydrogen induced cracking susceptibility in API X70 pipeline steels produced by thermo-mechanical controlled process (TMCP) in laboratory scale. Based on the observations, there is no dominant texture in the specimens and the grains are randomly distributed. Different TMCP parameters and rolling processes generates different grain size, and grains are often elongated along the rolling direction. The results also show that cooling rate is another factor affecting the grain size. A high cooling rate does not allow the grains to grow. The reason for the transgranular type of cracking might be the strong grain boundaries in ambient temperatures which prevents the intergranular cracking. Based on experiments, the hydrogen environment does not have permanent effects on the mechanical properties of the investigated specimens. The electrochemical hydrogen charging experiment shows that the grain refinement improves the resistance to hydrogen embrittlement.     

Role of microstructure on electrochemical hydrogen permeation properties in advanced high strength steels

The hydrogen permeation process in steels is closely associated with the microstructure of steels that greatly affect hydrogen trapping and hydrogen diffusion behaviors. In this study, the electrochemical hydrogen permeation experiment using a modified Devanathan-Stachurski (D-S) cells was employed to evaluate the hydrogen permeation properties in advanced high strength steels with four types of microstructures (from single phase, dual phase to complex phase). Results showed that both phase interfaces and retained austenite (RA) could act as the trapping sites for hydrogen and consequently reduced the hydrogen diffusion coefficient in steels. Furthermore, it was suggested that the role of RA on hydrogen trapping behaviors depended on its morphology. Finally, the lattice diffusion coefficient (D_L) in each steel was determined and the correlations between the microstructure in steels and hydrogen evolution reaction (HER) kinetics were also investigated.     

Hydrogen diffusivity in the Sr-doped LaScO3 proton-conducting oxides

The hydrogen diffusivity in the La_0.91Sr_0.09ScO_3–δ oxide was investigated by the hydrogen isotope exchange and the high-temperature thermogravimetry in the temperature range of 500–800°С and pressure ranges of hydrogen and water vapour of 0.2–4.1 kPa and 8.1–24.3 kPa, respectively. The values of hydrogen diffusion coefficients were obtained. The electrical conductivity of La_0.91Sr_0.09ScO_3–δ was studied using the electrochemical impedance spectroscopy in the temperature range of 300–750 °C and molecular hydrogen (protium and deuterium) pressure of 0.2 kPa. The H/D-isotope effect in conductivity and diffusivity was observed. The structural positions of protonic defects, determining transport in water- and hydrogen-containing atmospheres, were determined by the neutron powder diffraction.     

Susceptibility of different crystal orientations and grain boundaries of polycrystalline Ni to hydrogen blister formation

The effects of crystallographic orientations, grain boundaries (GBs) and the possible contribution of dislocations to the diffusion of hydrogen were studied. Our experimental results show that the (001) crystal orientation has the minimum number of hydrogen induced blisters compared to other crystal orientations. We observed formation of blisters along the slip traces after plastic deformation and along special GBs. It shows that the interaction between dissolved hydrogen and lattice defects (e.g. dislocations and GBs) could cause void formation and ultimately induce intergranular and/or transgranular cracks in nickel. In this work we analyzed hundreds of GBs mostly with a misorientation of less than 60°. It was observed that the random GBs with a misorientation between 30 and 40° have the fastest hydrogen diffusion rate and are very sensitive to hydrogen segregation. In contrast, random GBs with a misorientation below 25°, low angle GBs and the coincidence site lattice (CSL) GBs are much less prone to blister formation.     

Hydrogen generation from small-scale wind-powered electrolysis system in different power matching modes

This study presents a techno-economic evaluation on hydrogen generation from a small-scale wind-powered electrolysis system in different power matching modes. For the analysis, wind speed data, which measured as hourly time series in Kirklareli, Turkey, were used to predict the electrical energy and hydrogen produced by the wind–hydrogen energy system and their variation according to the height of the wind turbine. The system considered in this study is primarily consisted of a 6 kW wind-energy conversion system and a 2 kW PEM electrolyzer. The calculation of energy production was made by means of the levelized cost method by considering two different systems that are the grid-independent system and the grid-integrated system. Annual production of electrical energy and hydrogen was calculated as 15,148.26 kWh/year and 102.37 kg/year, respectively. The highest hydrogen production is obtained in January. The analyses showed that both electrical energy and hydrogen production depend strongly on the hub height of wind turbine in addition to the economic indicators. In the grid-integrated system, the calculated levelized cost of hydrogen changes in the range of 0.3485–4.4849 US$/kg for 36 m hub height related to the specific turbine cost. The grid-integrated system can be considered as profitable when the excess electrical energy delivered by system sold to the grid.     

Hydrogen embrittlement and associated surface crack growth in fine-grained equiatomic CoCrFeMnNi high-entropy alloys with different annealing temperatures evaluated by tensile testing under in situ hydrogen charging

The effect of the annealing temperature after cold rolling on hydrogen embrittlement resistance was investigated with a face-centered cubic (FCC) equiatomic CoCrFeMnNi high-entropy alloy using tensile testing under electrochemical hydrogen charging. Decreasing annealing temperature from 800 °C to 750 °C decreased grain sizes from 3.2 to 2.1 μm, and resulted in the σ phase formation. Interestingly, the specimen annealed at 800 °C, which had coarser grains, showed a lower hydrogen embrittlement susceptibility than the specimen annealed at 750 °C, although hydrogen-assisted intergranular fracture was observed in both annealing conditions. Because the interface between the FCC matrix and σ was more susceptible to hydrogen than the grain boundary, the presence of the matrix/σ interface significantly assisted hydrogen-induced mechanical degradation. In terms of intergranular cracking, crack growth occurred via small crack initiation near a larger crack tip and subsequent crack coalescence, which has been observed in various steels and FCC alloys that contained hydrogen.     

Hydrogen induced cracking susceptibility in different layers of a hot rolled X70 pipeline steel

Hydrogen induced cracking (HIC) behaviour was investigated in three layers of RD–ND plane. HIC test showed that all cracks initiated from the mid-thickness of the RD–ND plane and propagated in the rolling direction of the steel plate. Hydrogen permeation test results showed a lower permeability and diffusivity coefficient for the third layer resulting in the highest density of traps and consequently HIC susceptibility. Considering the HIC test and crystallographic texture measurements, cracks initiated from the grain boundaries associated with {100} grain orientation and arrested in regions with some strong texture components, such as {110}//ND, {112}//ND, and possibly {332}//ND. The role of HABs and CSLs boundaries was important in crack propagation.     

Influence of grain boundary misorientation on hydrogen embrittlement in bi-crystal nickel

Computational techniques and tools have been developed to understand hydrogen embrittlement and hydrogen induced intergranular cracking based on grain boundary (GB) engineering with the help of computational materials engineering. This study can help to optimize GB misorientation configurations by identifying the cases that would improve the material properties increasing resistance to hydrogen embrittlement. In order to understand and optimize, it is important to understand the influence of misorientation angle on the atomic clustered hydrogen distribution under the impact of dilatational stress distributions. In this study, a number of bi-crystal models with tilt grain boundary (TGB) misorientation angles (θ) ranging between 0°≤ θ ≤ 90° were developed, with rotation performed about the [001] axis, using numerical microstructural finite element analysis. Subsequently, local stress and strain concentrations generated along the TGB (due to the difference in individual neighbouring crystals elastic anisotropy response as functions of misorientation angles) were evaluated when bi-crystals were subjected to overall uniform applied traction. Finally, the hydrogen distribution and segregations as a function of misorientation angles were studied. In real nickel, as opposed to the numerical model, geometrically necessary dislocations are generated due to GB misorientation. The generated dislocation motion along TGBs in response to dilatational mismatch varies depending on the misorientation angles. These generated dislocation motions affect the stress, strain and hydrogen distribution. Hydrogen segregates along these dislocations acting as traps and since the dislocation distribution varies depending on misorientation angles the hydrogen traps are also influenced by misorientation angles. From the results of numerical modelling it has been observed that the local stress, strain and hydrogen distributions are inhomogeneous, affected by the misorientation angles, orientations of neighbouring crystal and boundary conditions. In real material, as opposed to the numerical model, the clustered atomic hydrogens are segregated in traps near to the TGB due to the influence of dislocations developed under the effects of applied mechanical stress. The numerical model predicts maximum hydrogen concentrations are accumulated on the TGB with misorientation angles ranging between 15°< θ < 45°. This investigation reinforces the importance of GB engineering for designing and optimizing these materials to decrease hydrogen segregation arising from TGB misorientation angles.     

Total Site Hydrogen Integration with fresh hydrogen of multiple quality and waste hydrogen recovery in refineries

This paper proposes a novel method combining Pinch Methodology and waste hydrogen recovery, aiming to minimise fresh hydrogen consumption and waste hydrogen discharge. The method of multiple-level resource Pinch Analysis is extended to the level of Total Site Hydrogen Integration by considering fresh hydrogen sources with various quality. Waste hydrogen after Total Site Integration is further regenerated. The technical feasibility and economy of the various purification approaches are considered, demonstrated with a case study of a refinery hydrogen network in a petrochemical industrial park. The results showed that fresh hydrogen usage and waste hydrogen discharge could be reduced by 21.3% and 67.6%. The hydrogen recovery ratio is 95.2%. It has significant economic benefits and a short payback period for Total Site Hydrogen Integration with waste hydrogen purification. The proposed method facilitates the reuse of waste hydrogen before the purification process that incurs an additional environmental footprint. In line with the Circular Economy principles, hydrogen resource is retained in the system as long as possible before discharge.     

Evaluation of electrochemical hydrogen absorption in welded pipe with steel API X52

The assessment of ability to absorb hydrogen by welds components of API grade pipeline steel X52 has been done. The factors of cathodic hydrogen charging, time of exposure on hydrogen concentration in base metal, heat affected zone and metal of weld were taken into account. It has been shown that all components of weld demonstrate the sensitivity to hydrogenating in deoxygenated, near-neutral pH NS4 solution under relatively “soft” cathodic polarisation, although the efficiency of hydrogen permeation in metal is relatively low and depends on time of exposure. The ability to absorb hydrogen decreases in the following sequence: heat affected zone – base metal – weld. The sensitivity to hydrogenation is higher for heat affected zone in comparison with base metal and weld.     

Hydrogen production from water gas shift reactions in association with separation using a palladium membrane tube

Hydrogen production from water gas shift reactions (WGSRs) of synthesis gas (syngas) followed by separation via a Pd membrane was studied experimentally. In the reactions, a variety of combinations of a high‐temperature shift reaction (HTSR), a low‐temperature shift reaction (LTSR) and a palladium (Pd) membrane tube were considered. The results indicated that the CO conversion from the LTSR was close to that of the HTSR and LTSR in series; however, the latter with the Pd membrane could provide a much low CO concentration at the permeate side. On the other hand, while the produced hydrogen diffused through the membrane, methane was also found at the both sides of the membrane due to the methanation reaction activated by the Pd membrane. In the present system, increasing the steam/CO ratio enhanced the forward reaction of the WGSRs and elongated the residence time of the reactants in the catalyst beds, resulting in the increases of CO conversion and hydrogen recovery. As a whole, the concentration of CO in the separated hydrogen was lower than 50 ppm from the combination of the HTSR and the LTSR with the membrane, whereby the produced hydrogen could be applied in proton exchange membrane fuel cells. Copyright © 2010 John Wiley & Sons, Ltd.