Improvement of hydrogen embrittlement resistance by intense pulsed ion beams for a martensitic steel

Intense pulsed ion beams (IPIB) have been applied on the surface of a lath martensitic steel with aim to improve its hydrogen embrittlement resistance and reveal the key cmaterial factors leading to failure. Hydrogen charging slow strain rate tensile tests show that IPIB can increase the ultimate fracture strength. The main fracture mode changes from intergranular fracture (untreated) to quasi-cleavage fracture (treated). Atomic probe tomography reveals that C atoms segregate at prior austenite grain boundaries for the untreated steel. After IPIB treatment, the C content at the PAGBs is reduced and high-carbon martensite forms in the treated layer, which improves the HE resistance. This study suggests that C segregation at grain boundaries is one of the main factors to cause the high HE susceptibility for the investigated lath martensitic steel and C segregation should be avoided when developing high strength steels with high HE resistance.     

Different augmentations of absorbed hydrogen under elastic straining in high-pressure gaseous hydrogen environment by as-quenched and as-tempered martensitic steels: combined experimental and simulation study

Hydrogen embrittlement (HE) behavior of as-quenched and as-tempered martensitic steels under elastic straining in a high-pressure gaseous hydrogen environment was studied for the first time. The difference in the total content of defects acting as H trap sites has the highest contribution to the increase in absorbed hydrogen content under the elastic loading. The hydrogen-induced fracture occurred in the as-quenched specimen during elastic straining in hydrogen environment, while the tempered specimen did not fracture under the elastic loading. Higher accumulation of the dislocations near the main crack initiation sites (prior austenite grain boundaries (PAGBs)) in the as-quenched specimen will be the main source of the hydrogen to the potential flaw to crack to initiate. H accumulated near PAGBs enhances dislocation slip along {011} planes and helps transgranular crack propagation. Due to less possibility to provide the critical local amount of hydrogen at PAGBs for the crack to initiate, the as-tempered specimen remained unfractured.     

Effect of δ-ferrite in welded ER308 and ER316 microstructure on hydrogen embrittlement

In this study, the hydrogen embrittlement (HE) behavior confined to focusing the weld microstructure of austenitic stainless steels (ER308 and ER316) was investigated. To investigate the microstructural influence on HE, microstructural changes were induced by different heat-treatment times at 1050 °C. These changes were prepared from microstructures containing δ-ferrite and microstructures where δ-ferrite was removed at welded parts. The results of the slow strain rate test (SSRT) indicated that the HE index, calculated from relative reduction width (RRW), was greatly reduced due to microstructure changes. To demonstrate the correlation between the deformation mechanism and HE index, deformation behavior was analyzed and verified through chemical composition, microstructure, fracture morphology, and modified Curssard–Jaoul (C–J) analysis. In all heat-treatment conditions, more mechanical twinning and a more drastic change in the HE index were observed for ER 308 compared to that of ER 316. The longest heat-treatment time conditions in a greatest decrease in HE index for ER308 and ER316. Hydrogen decohesion effects due to accumulation with dislocation were verified observing a parallel twin plane or microvoid along the twin plane in a transgranular fracture. In additional, as the heat-treatment time increased, the removal of δ-ferrite decreased the HE index for both stainless steels. Because, δ-ferrite was assisted in hindering mechanical twinning by misostrain concentrated at δ-ferrite sites in the early state of SSRT, it was enhanced to the resistance of HE.     

Mechanical behavior of electrochemically hydrogenated electron beam melting (EBM) and wrought Ti–6Al–4V using small punch test

The influence of electrochemical charging of hydrogen at j = ?5 mA/cm² for 6, 12, 48 and 96 h on the structural and the mechanical behavior of wrought and electron beam melting (EBM) Ti–6Al–4V alloys containing 6 wt% β and similar impurities level was investigated. The length of the α/β interphase boundaries in the EBM alloy was larger by 34% compared to that in the wrought alloy. The small punch test (SPT) technique was used to characterize the mechanical behavior of the non-hydrogenated and hydrogenated specimens. It was found that the maximum load and the displacement at maximum load of the wrought alloy remained nearly stable after 6 h of charging, showing a maximum decrease of ～32% and 11%, respectively. Similarly, hydrogenation of the EBM alloy resulted in a gradual degradation in mechanical properties with charging time, up to ～81% and 86% in pop-in load and displacement at the “pop-in” load, respectively. The mode of fracture of the wrought alloy changed from ductile to semi-brittle with mud-cracking in all hydrogenated specimens. In contrast, the mode of fracture of the EBM alloy changed from a mixed mode ductile-brittle fracture to brittle fracture with star-like morphology. The degraded mechanical properties of the EBM alloy are attributed to its α/β lamellar microstructure which acted as a short-circuit path and enhanced hydrogen diffusion into the bulk as well as δ_a and δ_b hydride formation on the surface. In contrast, a surface layer with higher concentration of δ_a and δ_b hydrides in the wrought alloy served as a barrier to hydrogen uptake into the bulk and increased the alloy resistivity to hydrogen embrittlement (HE). This study shows that EBM Ti–6Al–4V alloy is more susceptible to mechanical degradation due to HE than wrought Ti–6Al–4V alloy.     

Unraveling the effect of dislocations and deformation-induced boundaries on environmental hydrogen embrittlement behavior of a cold-rolled Al–Zn–Mg–Cu alloy

The effect of dislocation substructure, and deformation-induced boundaries on the hydrogen embrittlement (HE) behavior and the fracture mechanism of a 7xxx series aluminum alloy was investigated using X-ray diffraction line-profile analysis, electron backscatter diffraction, transmission electron microscopy, thermal desorption spectroscopy, and visualization of hydrogen. Hydrogen resides at interstitial lattice sites, statistically-stored dislocations (SSDs), and high-angle boundaries (HABs). SSDs are not the main trap site affecting HE behavior of the alloy. However, the HABs with the high desorption energy act as an almost irreversible trap site, which strongly absorbs hydrogen. It was firstly reported that the higher density of HABs as a strong trap site in a deformed 7xxx series aluminum alloy leads to decreasing the possibility of building up a critical hydrogen concentration required for crack initiation in a typical HAB, resulting in an excellent hydrogen embrittlement resistance.     

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.     

Improvement of hydrogen embrittlement resistance of 2205 duplex stainless steel by laser peening

The hydrogen embrittlement (HE) resistance of 2205 duplex stainless steel (DSS) treated with laser peening (LP) with different laser power densities was studied. The results show that LP changes the morphologies and distribution of ferrite phase and austenitic phase, thus changes the path of hydrogen transportation and diffusing. LP-induced grain refinement provides more tortuous grain boundaries that increases the difficulty of hydrogen atoms to penetrate them. The beneficial LP-induced microstructures interacts (e.g. dislocation entanglements, dislocation walls, mechanical twins) and helps to trap the hydrogen atoms, reducing their mobility ability. The hydrogen determination test provides direct evidence that LP reduced the amount of hydrogen penetration into the material. In addition, the tensile fracture exhibits that the average depth of the brittle region was inversely proportional to the laser power density, suggesting that an increase in laser power density can reduce the HE sensitivity of 2205 DSS.     

Hydrogen embrittlement characteristics of hot-stamped 22MnB5 steel

Hydrogen embrittlement (HE) characteristics of 22MnB5 steel (U-bent specimen) manufactured using hot-stamping process at various temperatures were experimentally and numerically investigated. Steel resistance to HE was examined through delayed failure tests under static and cyclic loading during hydrogen charging. First, the low cyclic loading caused severe HE, in which a clear difference in the extent of HE was obtained depending on the hot-stamped sample, which directly affected the microstructural characteristics and stress–strain distribution. The hot-stamped samples with large martensite phase showed low resistance to HE compared with those with small martensite phase because of the high concentration of hydrogen trapped in the phase boundaries. Moreover, the dual phase (ferrite and martensite) of the hot-stamped samples reduced their resistance to HE, which is caused by the hydrogen trapped in the laminar-shaped pearlite phase. The resistance to HE was improved by low-temperature heating at 200 °C for 1 h because of the generation of ε-carbides as trap sites as they render the hydrogen non-diffusible. Furthermore, the internal strain in the U-bent sample could accelerate HE because of the high concentration of hydrogen. These results were verified by experimental and numerical analyses. Thus, the hydrogen trapping mechanism was proposed as a valid mechanism for HE in 22MnB5.     

Solute hydrogen effects on plastic deformation mechanisms of α-Fe with twist grain boundary

The deformation mechanisms in the α-Fe twist bi-crystals (TBCs) containing differently angled twist grain boundaries (TGBs) are investigated carefully using the molecular dynamics modeling, with especial concerns on how solute hydrogen affects them. The results show that there are three main deformations in the TBCs, i.e. the dislocation glide-dominated mechanism, the twining-dominated mechanism, the dislocation glide and twining co-dominated mechanism, depending upon both the twist angle and the loading direction. In the dislocation glide-dominated TBCs, solute hydrogen increases the dislocation nucleation strength, dislocation mobility and dislocation density, further increases the vacancies concentration due to frequent interactions of solute hydrogen atoms with dislocations. In the dislocation glide and twining co-dominated TBCs, the solute hydrogen has weaker effect on the increase of dislocations density and the decrease of twins fraction with increasing tensile strain. However, in the twining-dominated TBCs, solute hydrogen assists the deformation twinning but doesn't increase significantly the vacancies concentration. So, it seems that twinning deformation is beneficial to resist hydrogen embrittlement (HE). These knowledge is helpful for us to understand the HE mechanism and develop new hydrogen-resistant high-strength materials.     

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.     

Microstructure evolution in HAZ and suppression of Type IV fracture in advanced ferritic power plant steels

The effect of boron and nitrogen on the microstructure evolution in heat affected zone (HAZ) of 9Cr steel during simulated heating and on the Type IV fracture in welded joints has been investigated at 650 °C. Gr.92 exhibits a significant decrease in time to rupture after thermal cycle to a peak temperature near A_C3, while the creep life of Gr.92N, subjected to only normalizing but no tempering, and 9Cr-boron steel is substantially the same as that of the base metals. In Gr.92 after A_C3 thermal cycle, very few precipitates are formed along PAGBs in the fine-grained microstructure. In the P92N and 9Cr-boron steel after A_C3 heat cycle, on the other hand, not only PAGBs but also lath and block boundaries are covered by M₂₃C₆ carbides in the coarse-grained microstructure. It is concluded that the degradation in creep life in Gr.92 after the A_C3 thermal cycle is not caused by grain refinement but that the reduction of boundary and sub-boundary hardening is the most important. Soluble boron is essential for the change in α/γ transformation behavior during heating and also for the suppression of Type IV fracture in welded joints. Newly alloy-designed 9Cr steel with 160 ppm boron and 85 ppm nitrogen exhibits much higher creep rupture strength of base metal than P92 and also no Type IV fracture in welded joints at 650 °C.     

The influence of the processing route on the fragmentation of (Nb,Ti)C stringers and its role on mechanical properties and hydrogen embrittlement of nickel based alloy 718

The nickel-base superalloy 718 is a precipitation hardened alloy widely used in the nuclear fuel assembly of pressurized water reactors (PWR). However, the alloy can experience failure due to hydrogen embrittlement (HE). The processing route can influence the microstructure of the material and, therefore, the HE degree. In particular, the size and distribution of the (Nb,Ti)C particles can be affected by the processing. In this regard, the objective of this work was to analyze the influence of cold and hot deformation processing routes on the development of the microstructure, and the consequences on mechanical properties and hydrogen embrittlement. Tensile samples were hydrogenated through gaseous charging and compared to non-hydrogenated samples. Characterization was performed via scanning and transmission electron microscopies, as well as electron backscattered diffraction. The processing was effective to promote significant variations in average grain size and length fraction of special Σ3ⁿ boundaries, as well as reduction of average (Nb,Ti)C particle size, being these changes more intense for the cold-rolled route. For the mechanical properties, on one side, the cold-rolled route presented the highest increase in ductility for non-hydrogenated samples, while, on the other side, had the highest degree of embrittlement under hydrogen. This dual behavior was attributed to the interaction of hydrogen with the (Nb,Ti)C particles and stringers and its ensuing influence on the fracture processes.     

Investigation for enhancing the resistance of H-induced delayed cracking for martensitic steel by quenching tempering and quenching partitioning: Influence of microstructure on hydrogen embrittlement and cracking behavior

The objective of the present study is to enhance the hydrogen embrittlement (HE) of the commercial martensitic steel (QT220). For this purpose, the heat treatments of quenching tempering and quenching partitioning are conducted, labeled as QT400 and Q&QP400, respectively. Compared to QT220, the mechanical properties of the both heat-treated specimens are reduced, nevertheless, the HE resistance is extremely promoted, resulting from the lesser dislocations, the more Mo_yC_x, and the existence of the strained interface of cementite. Besides the above favorable factors, the presence of the ferrite is another important factor which contributes to the lowest HE susceptibility in Q&QP400, resulting from the propagation's inhibition of hydrogen induced cracks (HICs) by ferrite. The HICs behavior of QT220, QT400 and Q&QP400 are mainly influenced by the dislocation glide, the cementite at the high angle boundaries and ferrite, respectively, mainly resulting in the fractographs of quasi-cleavage, intergranular and finely fragmented quasi-cleavage, respectively. In addition, HICs always deflect when propagating to the RD//<112～114> orientations, providing a valuable direction for research to enhance the HE resistance in the future.     

Hydrogen effects on the mechanical behaviour and deformation mechanisms of inclined twin boundaries

It has been observed that coherent twin boundaries (CTBs) are resistant to hydrogen embrittlement (HE). However, little is known about the role of inclined twin boundaries in the H-related deformation and failure. Here we comprehensively investigate H segregation and its influence on the mechanical behaviour and deformation mechanisms of inclined Σ3 twin boundaries at inclination 0°≤Φ ≤ 90° using molecular dynamics simulations. Our results demonstrate that for Φ = 0° CTB and Φ = 90° symmetric incoherent twin boundary (SITB), the presence of H reduces the yield stress required for dislocation nucleation under uniaxial tension, while for inclined twin boundaries (0°<Φ < 90°), the yield stress increases with increasing H concentration. Under shear deformation, solute H increases the critical shear stress for the SITB and inclined twin boundaries (0°<Φ < 90°). The underlying deformation mechanisms are directly associated with H-modified atomic structure and GB motion. These findings deepen our understanding of the HE mechanisms of inclined twin boundaries, and provide a pathway for designing materials with high HE resistance.     

The effect of the Ni/Cu ratio on H-induced ductility loss and its mechanism in Cu–Ni binary alloy system

The effect of the Ni/Cu ratio on hydrogen embrittlement (HE) behavior is studied in the context of the Cu–Ni binary alloy system. When classifying HE sensitivity as a function of the Ni fraction, two regimes are obtained: Regime 1 (wherein Ni fraction is less than 80 wt%) with moderate HE and Regime 2 (where Ni fraction exceeds 80 wt%) with more severe HE, although hydrogen- (H-)induced intergranular (IG) cracking is noted to be the common primary cause of H-induced degradation. Necessities of H-transportation towards grain boundaries (GBs) via H–dislocation interaction and/or dynamic H-diffusion are discussed via the slow strain rate tensile tests at −196 °C. Specifically, in Regime 1, H-transportation is necessary for the triggering of IG cracking. Conversely, in Regime 2, such H-transportation plays a minor role and the initially concentrated hydrogen along GBs can sorely cause IG cracking.     

Unveiling the role of hydrogen on the creep behaviors of nanograined α-Fe via molecular dynamics simulations

Hydrogen embrittlement (HE) substantially deteriorates the mechanical properties of metals. The HE behavior of nanograined (NG) materials with a high fraction of grain boundaries (GBs) may significantly differ from those of their coarse-grained counterparts. Herein, molecular dynamics (MD) simulations were performed to investigate the HE behavior and mechanism of NG α-Fe under creep loading. The effects of temperature, sustained stress, and grain size on the creep mechanism was examined based on the Mukherjee-Bird-Dorn (MBD) equation. The deformation mechanisms were found to be highly dependent on temperature, applied stress, and grain size. Hydrogen charging was found to have an inhibitory effect on the GB-related deformation mechanism. As the grain size increased, the HE mechanism transitioned from H-induced inhibition of GB-related deformation to H-enhanced GB decohesion. The current results might provide theoretical guidance for designing NG structural materials with low HE sensitivity and better mechanical performance.     

Hydrogen embrittlement in different materials: A review

Hydrogen embrittlement (HE) is a widely known phenomenon in high strength materials. HE is responsible for subcritical crack growth in material, fracture initiation and catastrophic failure with subsequent loss in mechanical properties such as ductility, toughness and strength. This hydrogen is induced in the material during electrochemical reaction and high-pressure gaseous hydrogen environment. LIST, SSRT and TDS techniques are performed to know the effect in mechanical properties and amount of hydrogen available in the material. For microstructure examination SEM, FESEM and TEM are performed to know the effect of hydrogen in the internal crystal structure. Also, various mechanisms which are responsible for crack growth and final fracture are discussed. This paper deals with HE definition, mechanisms which causes HE, subcritical crack growth, the concentration of hydrogen measurement and prevention activities are discussed which act as a barrier for hydrogen diffusion.     

Improved hydrogen embrittlement resistance after quenching–tempering treatment for a Cr-Mo-V high strength steel

The effect of quenching-tempering (QT) treatment on the hydrogen embrittlement (HE) resistance of a reactor pressure vessel steel was studied. Decomposition of M₃C/VC carbides and precipitation of M₇C₃ carbides were confirmed by transmission electron microscopy and atom probe tomography observations. Tensile tests showed that HE sensitivity decreased to a negligible level after QT treatment. The improvement of HE resistance was mainly attributed to the decreased number of M₃C carbides which act as the reversible trapping sites for hydrogen. This was supported by the decreased concentration of reversible hydrogen as measured by thermal desorption spectroscopy. The amount of irreversible hydrogen (probably trapped at VC carbides) also decreased, which is however not considered responsible for the HE improvement.     

Atomistic simulation study of the grain-size effect on hydrogen embrittlement of nanograined Fe

Although hydrogen-induced fracture at grain boundaries has been widely studied and several mechanisms have been proposed, few studies of nanograined materials have been conducted, especially for grain sizes below the critical size for the inverse Hall-Petch relation. In this research work, molecular dynamics (MD) simulations are performed to investigate the hydrogen segregation and hydrogen embrittlement mechanism in polycrystalline Fe models. When the same concentration of H atoms is added, the H segregation ratio in the model with the smallest grain size is the highest observed herein, showing the high hydrogen trapping ability of small-grain Fe, while the H concentration at the grain boundaries (GBs) is, on the contrary, the lowest. Uniaxial tensile test simulations demonstrate that as the grain size decreases, the models show an increased resistance to hydrogen embrittlement, and for small-grain models (d < 10 nm), the GB-related deformation modes dominate the plastic deformation, where the segregated H mainly influences the toughness by inhibiting GB-related processes.     

The dependence of hydrogen embrittlement on hydrogen transport in selective laser melted 304L stainless steel

The mechanical property and hydrogen transport characteristics of selective laser melting (SLM) 304L stainless steel were investigated by tensile tests and thermal desorption spectroscopy (TDS). The heat treatment affected the hydrogen embrittlement (HE) susceptibility and the treatment at 950 °C showed the larger HE effects. Cellular structures and melt-pool boundaries were dissolved at 850 and 950 °C, respectively. TDS results indicate that the hydrogen diffusivity of the as-received SLM 304L was lower than that of wrought 304L and the hydrogen diffusion activation energy increased with the recrystallisation degree, which was related to the dislocation density. Dislocations, rather than strain-induced martensite, were the main cause of HE owing to the high austenite stability of the samples. The pre-existing dislocations in the SLM 304L sample heat-treated at 950 °C for 4 h affected the hydrogen transport behaviour during sample stretching and led to severe HE.