Improve Hydrogen Embrittlement Resistance Research Articles

Suppressing hydrogen embrittlement of a CrCoNi medium-entropy alloy by triggering co-segregation of carbon, boron, and Cr

The CrCoNi medium-entropy alloy has drawn extensive attention due to its outstanding mechanical properties, yet it suffers severe gaseous hydrogen embrittlement (HE). Here we report that co-doping of carbon and boron could greatly suppress such embrittlement, resulting in a significant reduction in ductility loss from 71 % to merely 16 %. The minor doping has no significant influence on the phase structure, grain size, and grain-boundary character. However, the co-segregation of carbon, boron, and Cr was triggered, which not only improves the grain-boundary cohesive strength but also hinders the diffusion and accumulation of hydrogen at GBs, thereby leading to improved HE resistance.

Modeling the improved hydrogen embrittlement tolerance of twin boundaries in face-centered cubic complex concentrated alloys

With increasing interest in hydrogen as an alternative fuel, there is a need to develop structural alloys with improved resistance to hydrogen embrittlement (HE) for application in the new hydrogen economy. Complex concentrated alloys (CCAs), which include high entropy alloys and their derivatives, medium entropy alloys, are a new class of structural materials, some of which have reported improved HE resistance. While some studies have suggested that the improved HE resistance in CCAs with the face-centered cubic (fcc) crystal structure may be due to the high density of nanotwins within them, a detailed mechanistic understanding is yet to be developed. Towards that end, following the approach of Zhou, Tehranchi and Curtin, we employ a density functional theory-informed Griffith–Rice model22This type of model has also been referred to in the literature as the Rice–Thomson model. In this work, we refer to it as the Griffith–Rice model to reflect that the concept of unstable stacking fault energy was introduced into the model due to Rice (1992). to predict the ductile or brittle response of a crack tip interacting with twin boundaries (TBs) in a model fcc CCA, CrCoNi, both in the absence of and presence of hydrogen. Both the model and molecular dynamics simulations predict that TBs in fcc alloys are not inherently more susceptible to HE than the bulk matrix, and could in fact improve HE resistance by retarding cracks while promoting dislocation emission along the TB. Thus, designing fcc CCAs with a high density of nanotwins or utilizing gradient nanotwinned structures could be a way forward for realizing alloys with high HE resistance.

Correlational research of microstructure characteristics and hydrogen induced cracking in hot-rolled Fe-6Mn-0.2C-3Al steels

Two-type specimens with different microstructure e.g. A70 contains finer-grained ferrite and austenite, while abundant coarse ferrite is obtained in A30. The different response on hydrogen embrittlement behavior results from the fundamental differences of microstructure. The dominant role of hydrogen in A30 lies in its local plastic enhancement influence on coarse ferrite, which is the efficient diffusion path for hydrogen, and that in A70 consists in its decohesion effect on ferrite/austenite interfaces, due to the inherited hydrogen of the fresh martensite at interfaces. This work provides a vital insight for microstructural design to improve hydrogen embrittlement resistance for medium Mn steels.

Enhancing the hydrogen embrittlement resistance of medium‑carbon high-strength steel by optimizing the tempering temperature

Hydrogen embrittlement (HE) and hydrogen diffusion in medium‑carbon high-strength steel tempered at various temperatures were investigated by in situ slow strain rate testing and electrochemical hydrogen permeation, respectively. Synergistic correlations between the various microstructural components and reversible/irreversible hydrogen traps were established. Dislocations and the size, shape, distribution, and volume fraction of cementite were found to considerably influence the HE susceptibility. High dislocation density and interconnected needle-like cementite with a high aspect ratio, oriented along grain boundaries, were produced at a low tempering temperature (560 °C); this facilitated the decohesion of atoms at the cementite–matrix interface, enabling them to act as crack propagation sites and inducing brittle intergranular fracture. However, a significantly improved HE resistance associated with a quasi-cleavage-based fracture morphology was achieved at a high tempering temperature (680 °C) owing to the low dislocation density and the presence of uniformly distributed spheroidised cementite.

The influence of L12 ordered precipitates on hydrogen embrittlement behavior in CoCrNi-based medium entropy alloys

The recently emerged multicomponent (or medium/high entropy) alloys have generated considerable excitement globally in the last 10 years because of their excellent mechanical and functional properties, particularly in terms of strength-ductility combinations that can surpass most other metallic materials. However, the achieved high strength level (above 1 GPa in many cases) fuels strong concerns about hydrogen embrittlement (HE). Detailed investigation in this field is still scarce, especially pertaining to the face-centered cubic medium entropy alloys (MEA) that are typically strengthened by ordered precipitates. Here, we unravel the effect of γ' (L12) ordered precipitates on H-induced damage behavior and the associated HE resistance in CoCrNi-based MEAs. Compared with the equi-molar CoCrNi MEA, the precipitation-hardened (CoCrNi)94Al3Ti3 MEA shows an enhanced HE resistance even at a higher strength level. Both alloys are fractured due to H-assisted intergranular cracking at the initial failure stage when loaded in the presence of H. The formation of intergranular cracks is primarily attributed to the H-induced decohesion at grain boundaries, where a high stress/strain concentration accompanied by a more intensive dislocation planar slip (or stacking fault formation) caused by H was observed. The presence of γ' precipitates serves to slow down the internal diffusion/migration of H due to the trapping effects. The precipitates with a relatively larger size (∼50 nm) also hinder dislocation planar slip thus decreasing the number of pile-up dislocations at grain boundaries. Both effects collectively reduce the tendency of H-induced intergranular cracking, leading to the improved HE resistance. The work reveals the positive role of ordered precipitates in H tolerance and thus provides some insights in further microstructure design of medium/high entropy alloys for applications in H-abundant environment.

Synergistic effect of different testing environments on the hydrogen-induced mechanical degradation to establish the role of microstructural features on the failure of X70 pipeline steel

Hydrogen-induced mechanical degradation was evaluated in different environmental conditions on two X70 pipeline steels having the same composition but different thermomechanical processing parameters, and a further correlation between microstructural features and hydrogen embrittlement (HE) susceptibility was established. HE susceptibility was determined by performing the Slow Strain Rate Tensile Test (SSRT) in air, ex-situ hydrogen-charged, in corrosive media, in-situ hydrogen-charged and Charpy impact tests before and after hydrogen charging in a wide range of temperatures.The SSRT results indicated that the synergistic action of stress and hydrogen in in-situ charged sample leads to maximum HE susceptibility in both X70 steels. Charpy results demonstrated that ductile-to-brittle transition temperature (DBTT) increases after hydrogen charging. The texture analysis revealed that X70-2 with a higher volume fraction ratio of Ƴ-fiber/{100}<011> demonstrated improved HE resistance, whereas X70-1 with higher fractions of {113}<110> and {100} || ND demonstrated a higher DBTT than before and after hydrogen charged X70-2.

The role of cryogenic pre-strain on the hydrogen embrittlement of FeNiCoCrMn high-entropy alloys

The effect of cryogenic plastic pre-strain (15 %, 30 %, and 45 % true strain) on the hydrogen embrittlement (HE) of an equimolar FeNiCoCrMn high-entropy alloy was studied through slow strain rate tension (SSRT) at 1 × 10−5 s−1. As the pre-strain increased, ductility loss under harsh electrochemical hydrogen charging showed a downward trend accompanied by the promotion of hydrogen adsorption. This work provided an effective strategy for achieving the synergy of high strength and improved HE resistance. Data availability statementThe raw/processed data required to reproduce these findings cannot be shared at this time as the data is related to an ongoing study.

Defeating hydrogen-induced grain-boundary embrittlement via triggering unusual interfacial segregation in FeCrCoNi-type high-entropy alloys

Metallic materials are mostly susceptible to hydrogen embrittlement (HE), which severely deteriorates their mechanical properties and causes catastrophic failures with poor ductility. In this study, we found that such a long-standing HE problem can be effectively eliminated in the Fex(CrCoNi)1-x face-centered-cubic (fcc) high-entropy alloys (HEAs) by triggering the localized segregation of Cr at grain boundaries (GBs). It was revealed that increasing the Fe concentration from 2.5 to 25 at. % leads to substantially improved HE resistance, i.e., the ductility loss decreases from 70% to 6%. Meanwhile, the fracture mode transformed from the intergranular to the transgranular mode. Multiscale microstructural analyses demonstrated that the Fe2.5Cr32.5Co32.5Ni32.5 and Fe25Cr25Co25Ni25 alloys show negligible differences in the phase structure, grain size, and grain-boundary (GB) character. However, interestingly, the near atomic-resolution elemental mapping revealed that an increased Fe concentration promotes the nanoscale Cr segregation at the GBs, which is primarily motivated by the strong repulsive force between Cr and Fe and the low self-binding energy of Cr. Such unusual interfacial segregation of Cr, which has not been reported before in the Fe25Cr25Co25Ni25 alloy, helps enhance the GBs’ cohesive strength and suppresses the local hydrogen segregation at GBs due to the deceased GB energy, leading to the outstanding HE resistance. These findings decipher the origins of the vastly-improved HE resistance in current FeCrCoNi-type HEAs, and meanwhile, provide new insight into the future development of novel high-performance structural alloys with extraordinary immunity to hydrogen-induced damages.

Tailoring hydrogen embrittlement resistance of pure Ni by grain boundary engineering

By using thermo-mechanical processing, 99.996 wt.% pure Ni with different grain boundary characteristics were fabricated (Sample #1, 700 ℃ × 10 h + cold rolling reduction 50%+ 650 ℃ × 2 h; Sample #2, 700 ℃ × 10 h + cold rolling reduction 50%+ 900 ℃ × 5 min). Hydrogen embrittlement sensitivity of the two samples was determined by using high-pressure hydrogen charging, low strain rate tensile test and SEM fractography. Compared with Sample #1, hydrogen embrittlement index of Sample #2 increased from 0.52 to 0.71. The effects of grain size, grain boundary type and grain boundary curviness on hydrogen embrittlement of pure Ni were discussed. The results indicate that: 1) special grain boundaries including twin grain boundaries may have a marginal effect on tailoring hydrogen embrittlement resistance in pure Ni; 2) increasing grain boundary curviness is an effective way to improve hydrogen embrittlement resistance of Ni.

Hydrogen Trapping States and Apparent Hydrogen Diffusion in Laser Additively Manufactured Ultra-High Strength AerMet100 Steel as a Function of Secondary Hardening

Microstructures, reversible hydrogen trapping states, and effective hydrogen diffusion coefficients (DH,eff) of laser additively manufactured (LAM) ultra-high-strength AerMet100 steel in tempered conditions were studied by several material characterization methods, to determine diffusible, trapped, and total hydrogen content. With secondary hardening temperatures in the range of 454°C to 566°C, increasing temperature mainly promotes M2C carbide coarsening and film-like reverted austenite thickening in the steel. Reversible hydrogen traps of tempered LAM AerMet100 steel are closely related to the precipitation behavior of highly coherent M2C carbides. The desorption activation energy of the reversible hydrogen traps in the steel is seen to increase from 17.9±0.3 kJ/mol to 21.8±1.3 kJ/mol with temperature increasing from 454°C to 566°C. This correlates with the composition and size change of M2C carbides at a higher tempering temperature. Hydrogen trapping capability of the steel has a peak value at a tempering temperature of 482°C corresponding to the combination of both high amount and medium trapping intensity of these reversible hydrogen traps. This results in the lowest diffusible and highest total hydrogen concentration for precharged H specimens of the steel. In addition, the DH,eff of LAM AerMet100 steel in the overaged condition is not only influenced by the increased thickness of film-like reverted austenite but also simultaneously affected by the altered density of M2C carbides. In comparison with the lowest DH,eff (approximately 2.4 × 10−9 cm2/s) of LAM AerMet100 steel tempered at 482°C, a slightly higher DH,eff of the steel tempered at a higher temperature is achieved by the apparent decrease of reversible hydrogen traps due to a decrease in density of the highly coherent M2C carbides. These findings are important when considering achieving improved hydrogen embrittlement resistance for LAM high Co-Ni secondary hardening ultra-high-strength steel in an over-aged condition at the strength level of interest.

Effect of vanadium content on hydrogen embrittlement of 1400 MPa grade high strength bolt steels

In this work, the hydrogen embrittlement (HE) characteristics of 1400 MPa bolt steels with three different vanadium contents of 0, 0.17 wt% and 0.34 wt% were evaluated. The characteristics of the microstructure and dislocation density of the experimental steels were analyzed, and their effects on HE were also discussed. The results showed that with increasing V content, the HE resistance of the experimental steels was improved, and the experimental steel with the highest V content possessed the best HE resistance. The V-precipitates of steels with V contents of 0.17 wt% and 0.34 wt% were reversible hydrogen traps, and the inhibitory effect of V-precipitates on hydrogen-dislocation interactions improved HE resistance. In addition, a lower dislocation density and finer martensitic structure were also beneficial for hindering hydrogen-induced cracking (HIC).

Improved Hydrogen Embrittlement Resistance of Steel by Shot Peening and Subsequent Low-temperature Annealing

The effect of shot peening and subsequent low-temperature annealing (SP treatment) on hydrogen embrittlement in tempered martensitic steel was investigated comparing the typical hydrogen charging methods, constant current controlled cathodic charging test and combined cyclic corrosion test (CCT). The hydrogen entry behavior was evaluated by hydrogen permeation technique and hydrogen visualization using secondary ion mass spectrometry. The SP treatment improved hydrogen embrittlement resistance in both hydrogen charging methods. On the other hand, the effect of SP treatment on the hydrogen entry was different depending on hydrogen charging method. The hydrogen entry was suppressed by SP treatment in the cathodic charging test because the compressive residual stress reduced hydrogen concentration in the surface layer and the potential increased by the increase of surface roughness and the formation of a surface film. In CCT, although the surface hydrogen concentration decreased due to compressive residual stress, the total hydrogen content did not decrease by SP treatment since the surface film disappeared by corrosion and an increase of surface roughness led to an increase in hydrogen entry sites. The improved hydrogen embrittlement resistance by the SP treatment in the cathodic charging test is a result of hydrogen entry suppression in addition to the effect of compressive residual stress. In CCT, the hydrogen embrittlement resistance was improved by SP treatment due to effect of compressive residual stress, i.e., the suppression of stress concentration as well as the stress-induced hydrogen diffusion in stress concentration area and the reduction of hydrogen concentration in the surface layer.

Graded Grain Structure to Improve Hydrogen-Embrittlement Resistance of TWIP Steel

The high strength of twinning-induced plasticity (TWIP) steels makes them vulnerable to the hydrogen embrittlement (HE) phenomenon, thereby limiting their potential applications. This study suggests inducing a graded grain structure (GGS) in a Fe-17Mn-0.8C TWIP steel through shot peening and subsequent heat treatment to solve the problem. The microstructures and fracture surfaces of GGS TWIP steel were compared with those of conventionally manufactured TWIP steel possessing a uniform grain structure (UGS). Compared with the conventional UGS TWIP steel, GGS steel showed similar tensile properties with a yield strength of 310 MPa, tensile strength of 1060 MPa, and elongation-to-failure of 135%. It also exhibited moderately enhanced low-cycle fatigue (LCF) resistance in terms of fatigue life (8196 cycles to failure) compared with the UGS steel (7201 cycles). Furthermore, GGS TWIP steel exhibited a marked improvement in HE resistance, both in the monotonic (by a slow-strain-rate test) and cyclic deformation modes (by the LCF test) in a hydrogen environment. A relatively fine-grained (d = 15.6 μm) surficial area enhanced the HE resistance by inhibiting hydrogen penetration and decreasing twin density, while the coarse-grained (d = 74.6 μm) interior promoted the LCF resistance by suppressing crack growth.

The effect of precipitations (NbC and carbide) in Fe–C–Mn-xNb steels on hydrogen embrittlement characteristics

Hydrogen embrittlement (HE) characteristics in Fe–C–Mn-xNb steels were examined via various analyses, including electron backscatter diffraction analysis, scanning transmission electron microscopy and three-dimensional atom-probe tomography. For the investigation, the steel samples were prepared with varying Nb contents and heat treatment processes. The material properties of steel samples that were subjected to: (i) water quenching and (ii) quenching and tempering at 170 °C for 20 min, were determined to be nearly similar, although different degrees of HE were detected. After the tempering process, ε-carbide precipitated clearly in the matrix, which could act as a trapping site for hydrogen atoms and lead to improved HE resistance. Moreover, with addition of Nb, niobium base precipitates (e.g., NbC) with a diameter of a few nanometers were obtained in the martensite matrix, which could also function as hydrogen trapping sites. There was slight improvement in the HE resistance with NbC. Hydrogen-assisted failure mechanisms under both static and cyclic loading were observed with intergranular brittle cracking for the water quenched sample, even though the brittle and ductile mix failure mode was detected for the sample after the quenching and tempering process.

The Effect of Second Tempering on Hydrogen Embrittlement of Ultra-High-Strength Steel

The objective of the present study is to enhance the resistance of hydrogen embrittlement (HE) via second tempering at 250 °C for 30, 60 and 120 minutes. Although second tempering results in a higher saturated hydrogen content for the second tempering specimens during a slow strain rate test (SSRT), it effectively reduces HE susceptibility. As the second tempering time increases, dislocation density decreases. In contrast, the size of the cementite and MoyCx precipitates increase slightly. The density of MoyCx precipitates increases, whereas the density of cementite remains approximately the same as the second tempering time increases. Regarding second tempering specimens, the volume fraction’s increase in MoyCx precipitates, which acts as a hydrogen trap with high binding energy, plays an important role in reducing the HE susceptibility, and the decrease in dislocation density can also improve HE resistance. In addition, the growth of the interface of the cementite and matrix disperses more hydrogen, which could enhance HE resistance. The result also reveals that the cementite and matrix interface is a type of low-binding-energy hydrogen trap without plastic deformation, whereas the strain interface with interfacial dislocations is a type of high binding energy hydrogen trap under plastic deformation.

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.

Enhancement of hydrogen embrittlement resistance of Fe-Mn-C twinning-induced plasticity steel by partial recrystallization technique

We suggest that the partial recrystallization is a novel pathway to increase the hydrogen embrittlement resistance of Fe-Mn-C twinning-induced plasticity steels. The Fe-18Mn-0.6C (wt%) cold-rolled steel was annealed for from 3 min to 30 min at 650 °C. When the annealing time was shorter than 5 min, the specimen was partially recrystallized, while when the annealing time exceeded 10 min, the matrix was almost fully recrystallized. The partially recrystallized specimens exhibited the improved hydrogen embrittlement resistance because the inherited dislocation and mechanical twins in unrecrytallized grains acted as a trap site for diffusible hydrogen.

Grain boundary engineering approach to improve hydrogen embrittlement resistance in Fe[sbnd]Mn[sbnd]C TWIP steel

This investigation aimed to improve hydrogen embrittlement (HE) resistance of Fe17Mn−0.8C (wt %) TWIP steels by use of grain boundary engineering approach. The degree of enhancement in HE resistance by grain boundary engineering was compared with those of Al-added TWIP steels (Fe17Mn−0.8C1Al and Fe17Mn−0.8C2Al). By applying grain boundary engineering, the special boundaries that shared a high fraction of lattice points between neighboring grains were increased from 48% to 59%, and the morphology of grain boundaries was changed to ragged type. After hydrogen charging, fracture strength and reduction of area were clearly degraded in all four samples (Base, GBE, 1Al, and 2Al), revealing a transition of fracture mode from dimple fracture to intergranular fracture. However, the alloy with higher fraction of special boundaries had higher resistance to hydrogen-induced intergranular embrittlement. The HE resistance of GBE was obviously improved to a level similar to that of Al-added TWIP steels. Important implication of this work is that HE resistance can be greatly enhanced by controlling the distributions of grain boundary characters (GBCs) without adding Al. This was attributed to not only the higher fraction of special boundaries, but also the loss of continuity of random boundaries.

Mechanism of improved hydrogen embrittlement resistance of low-temperature plasma carburised stainless steel

ABSTRACTThe present paper focuses on the underlying principles of the effect of S-phase on reducing hydrogen embrittlement susceptibility. Hydrogen permeation test indicates that S-phase formed by low-temperature carburising has minor effect on hydrogen diffusion. Moreover, hydrogen-induced phase transformation caused by hydrogen charging is suppressed in the presence of S-phase. The kernel average misorientation result indicates that lattice distortion introduced by hydrogen charging is suppressed in the presence of S-phase. Cross-sectional EBSD investigations on the surface layer of fractured samples show that S-phase remains extremely stable during tensile deformation after hydrogen charging.

Effects of vanadium carbides on hydrogen embrittlement of tempered martensitic steel

Precipitation of V carbides is known to suppress the hydrogen-embrittlement (HE) phenomenon as well as to increase material strength. Despite increasing attention to V carbides, there have been few systematic and quantitative investigations on their effects on HE resistance. This study reveals the role of V carbides on the HE behavior of tempered martensitic steel while eliminating other factors, such as chemical composition of other elements, mechanical strength, and dislocation density. The amount of trapped hydrogen increased with increasing V content, whereas the best HE resistance was attained at 0.2 wt% V and it decreased with further V addition. V carbide was considered as a non-diffusible hydrogen-trapping site in this study. However, excessive V content led to the formation of large undissolved carbides that gave rise to brittle fracture and decreased HE resistance. This study suggests that improved HE resistance can be achieved by minimizing the size and amount of undissolved V carbides.