Conformity between macro-mechanics and microscopic functions of hydrogen in failure is reviewed in reference to some models of hydrogen embrittlement (HE), focusing on the role of plasticity. Plastic strain localization, a characteristic feature of HE, is consistent with the hydrogen-enhanced creation of vacancies during plastic deformation. Constitutive relations that take into account the presence of voids describe well the ductile fracture process in HE. The effect of hydrogen on increasing the density of strain-induced vacancies likely promotes plastic instability and decreases ductile crack growth resistance, thus leading to enhanced shear localization and a premature failure.

View full abstract

Round-robin tests were conducted for designing standard procedures for thermal desorption analysis (TDA) of hydrogen. Scatters of diffusive and non-diffusive hydrogen contents measured at various institutions using common materials and experimental procedures were examined. Some factors causing the scatter and their respective contributions were examined in optional tests.

View full abstract

Numerical modeling of thermal desorption spectra of hydrogen, which is used for identifying the hydrogen state in metals, is reviewed. The previously proposed models are described in the historical perspective by categorizing them according to the rate-determining processes of hydrogen detrapping and diffusion in the thermal desorption spectra. The range of validity of each model is also described.

View full abstract

In order to study hydrogen-defect interactions, the effects of ion irradiation and annealing on the lattice location of hydrogen dissolved in Nb, NbH_0.023, have been investigated at room temperature by the channelling method utilizing a nuclear reaction ¹H(¹¹B,α)αα with a 2 MeV ¹¹B⁺ beam. By irradiation at room temperature with a dose of about 1.4 × 10¹⁶/cm² of about 2 MeV ¹¹B ions, the lattice location changes from the original tetrahedral (T) site (T-1 state) to the T_tr site, which is displaced from a T site by 0.45–0.55 Å towards its nearest neighbour lattice point. The T_tr-site occupancy remains the same even for an approximately three times higher irradiation dose. On subsequent annealing at 523 K for 1 h, the site occupancy changes to the occupancy of (55–70)% of H atoms at T sites (T-2 state) and (30–45)% of them at random (R) sites. By additional irradiation with a dose of about 1.4 × 10¹⁶/cm² at room temperature subsequent to the annealing, the site occupancy changes to the occupancy of (35–50)% at T + (50–65)% at R or (30–40)% at T + (10–20)% at T_tr + (50–60)% at R. It is concluded that the irradiation-induced site change to the T_tr site is a result of the trapping of hydrogen by monovacancies, i.e., the formation of H-vac. complex-1. Most of the H atoms in the T-2 state are not free hydrogen, but associated with more vacancies, for which hydrogen located at a T site in a tetrahedron consisting of four vacancies (tetravacancy), i.e., H-4vac. complex-2, is proposed. The R-site occupancy corresponds to hydrogen in a H-associated larger vacancy cluster complex-3. It is demonstrated that the complex-1 does not act as a nucleus, whereas the complex-2 acts as a nucleus for the growth to the H-associated larger vacancy cluster complex-3 by trapping more irradiation-introduced vacancies at room temperature.

View full abstract

Hydrogen behavior and hydrogen-enhanced lattice defect formation under elastic stress of tempered martensitic steel were clarified with respect to dislocations and vacancies by thermal desorption analysis (TDA) using hydrogen as a probe of defects and a positron probe microanalyzer (PPMA). The relationship between hydrogen embrittlement and lattice defects associated with hydrogen was also investigated. The amount of lattice defects increased gradually with increasing time of applied stress during hydrogen charging. The specimen fractured under elastic stress in the presence of hydrogen macroscopically showed brittle fracture without necking. Whereas fracture surface was attributed to localized plastic deformation, since the morphology of the microscopic fracture surface was mostly quasi-cleavage fracture. The increased lattice defects in the near-fracture area were subsequently removed by annealing at 200°C. The mean positron annihilation lifetime measured with the PPMA for a fractured specimen was longer in the near-fracture area than in other areas. Thus, the most probable reason for the increase in the amount of lattice defects can be ascribed to an increase in the amount of vacancies or vacancy clusters. Regarding hydrogen embrittlement involving microscopic plastic deformation, the localized enhanced vacancies due to interactions between dislocations and hydrogen under elastic stress directly caused ductility loss, because ductility loss occurred even though hydrogen was completely removed by degassing before the tensile test. Besides hydrogen content and applied stress, the time of formation and accumulation of vacancies are also concluded to be important factors causing hydrogen embrittlement.

View full abstract

This study investigated the characteristics of hydrogen-related crack propagation in low carbon lath martensite steel through orientation analysis using electron backscattering diffraction. The orientation analysis revealed that the hydrogen-related fracture surface consisted of the {011}_M facets, which were also parallel to the block boundaries or lath boundaries in the lath martensite structure. In addition, micro-cracks were observed on or in the vicinity of the prior austenite grain boundaries. On the basis of the experimental results, we proposed that hydrogen enhanced local plastic deformation occurred in the vicinity of prior austenite grain boundaries. The mechanism of hydrogen-related fracture is characterized by the formation of micro-cracks around prior austenite grain boundaries and subsequent crack propagation along block boundaries or lath boundaries.

View full abstract

The effect of uniform distribution of fine cementite on resistance of ultra-high strength steels to hydrogen embrittlement was studied. The materials used were directly-quenched and tempered 1000–1300 MPa class low carbon steel plates for welded structures with lath martensite structure. Cementite morphology was different at different heating rates to tempering temperatures. Finer cementite was distributed in rapidly-heated steels (20°C/s) than in slowly-heated steels (0.3°C/s). The rapidly-heated steels showed higher resistance to hydrogen embrittlement than the slowly-heated steels for a slow strain rate test (SSRT), whereas they showed almost the same resistance to hydrogen embrittlement for a constant load test (CLT). The specimens fractured in a plastic region for the SSRT, on the other hand, the CLT was conducted in an elastic region. The difference in hydrogen embrittlement resistance between plastic and elastic loading methods was concluded to result from a change in the hydrogen trap state at cementite in association with plasticity. Hydrogen is more strongly trapped at and/or around the strained interfaces between the matrix and cementite after plastic deformation. A close observation of fracture surfaces, hydrogen thermal desorption analysis and hydrogen microprint technique revealed that the high resistance of the rapidly-heated and tempered steels to hydrogen embrittlement for the SSRT is due to a shift of the fracture mode from quasi-cleavage fracture to ductile fracture. This shift was caused by the suppression of the quasi-cleavage fracture due to less hydrogen at lath boundaries accompanied by the uniform distribution of fine cementite.

View full abstract

Hydrogen deteriorates the strength of steel, and it is well known as hydrogen embrittlement. The susceptibility to this embrittlement is generally increased more with higher strength steels under the usual static loading. Meanwhile fatigue strength is more important in structural use; it is desired to clarify the effect of hydrogen on the properties under cyclic loading. Present paper deals with the effects of hydrogen entry on the strain controlled low cycle fatigue properties of low carbon steel (JIS S10C), which is the most basic kind of steels. With the information about hydrogen in steels, the extent of hydrogen damage in fatigue life and its dependence to cementite morphology were discussed. The total hydrogen content of 0.5–1.5 mass-ppm severely degrades the fatigue life of normalized S10C, which is not so strong and almost insusceptible to hydrogen embrittlement under static loading. The fracture surface is accompanied with fish-eye fracture surface. This is originated at inclusion and propagating with quasi-cleavage and vague striation. The hydrogen gas atmosphere formed around inclusion during cyclic loading is claimed to be the major reason for the degradation. Spheroidizing heat treatment makes the steel free from the degradation. The hydrogen claimed for the degradation of fatigue life is what has been trapped by pearlite or cementite/ferrite interface, because this interface is reduced by the spheroidizing treatment. It is not the hydrogen that was trapped by non-metallic inclusion before fatigue test.

View full abstract

This paper offers a detailed analysis of the hydrogen-assisted micro-damage (HAMD) region in axisymmetric round-notched samples of high-strength eutectoid steel under hydrogen embrittlement environmental conditions. Emphasis is placed on the microscopic appearance and the evolution of such a microscopic topography from the initiation (sub-critical) to the fracture (critical) point. The use of very different notched geometries —with the subsequent various triaxial stress distributions in the vicinity of the notch tip— allows an analysis of the influence of stress state on hydrogen diffusion and micro-cracking. In all cases, the microscopic appearance of the hydrogen-affected zone resembles micro-damage, micro-cracking or micro-tearing due to hydrogen degradation.

View full abstract

Recent research on Hydrogen Environment Embrittlement (HEE) susceptibility of stainless and low alloy steels in highly pressurized gaseous hydrogen environments was reviewed from the viewpoint of tensile properties, hydrogen absorption and fatigue properties.
HEE susceptibility evaluated by Slow Strain Rate Test (SSRT) in high pressure hydrogen environments strongly depended on steel chemical compositions. Austenitic stainless steels such as type 316L or iron-based superalloy as A286 showed sufficient resistance to HEE, while stainless steels with low levels of alloying elements such as type 304L showed a remarkable ductility loss in high pressure gaseous hydrogen due to martensitic transformation. Martensitic stainless or low alloy steels also showed a remarkable ductility loss in gaseous hydrogen.
Relationship between HEE susceptibility and an amount of hydrogen absorption was investigated. HEE susceptibility and hydrogen embrittlement under cathodic charging in aqueous solution showed the same dependence on the amount of hydrogen absorption, which implies HEE occurs by hydrogen absorption from external gaseous hydrogen environments.
Fatigue properties in high pressure gaseous hydrogen environments were evaluated by means of internal or external pressurization tests. Austenitic stainless steels such as type 316L showed little decrease in fatigue life by hydrogen, while metastable stainless steel as type 304 or precipitation hardened superalloy as A286 showed degradation in fatigue life by hydrogen gas. Low alloy steel also showed a decrease in fatigue life in hydrogen, while high strength low alloy steel with much Mo and V showed longer fatigue life than conventional steel.

View full abstract

The internal reversible hydrogen embrittlement (IRHE) of austenitic Fe(10–20)Ni17Cr2Mo alloys based on type 316 stainless steels hydrogen-charged to around 40 mass ppm was investigated by performing tensile tests using the slow strain rate technique at temperatures from 80 to 300 K. The susceptibility to IRHE depended on the Ni content. IRHE occurred below a Ni content of 15% (Ni equivalent of 29%), increased with decreasing temperature, reached a maximum at 200 K and decreased with further decreasing temperature. Hydrogen-induced fracture due to IRHE occurred in brittle transgranular mode associated with the strain-induced α' martensite structure at temperatures from 200 to 300 K and occurred simultaneously with fracture along the prior annealed-twin boundary at 200 and 250 K, then changed to dimple rupture mode due to hydrogen localization at 150 K. IRHE was controlled by the amount of strain-induced α' martensite above 200 K, whereas it was controlled by hydrogen diffusion below 200 K.

View full abstract

Fatigue crack growth tests were carried out in high-pressure gaseous hydrogen at 90 MPa at room temperature for SUS316-based stainless steels containing different amounts of Ni and Cr, and the effects of these alloying elements on fatigue crack propagation were investigated. The fatigue crack growth rate of the SUS316-based steels with a Ni content lower than 12 mass% was accelerated in hydrogen gas with decreasing Ni content. The Cr content had little effect on the fatigue crack growth rate in hydrogen gas. The combined effect of the Ni and Cr contents on the fatigue crack growth rate was closely related to Md30 and a modified Ni equivalent, [Ni]+0.37[Cr], where [Ni] and [Cr] are the Ni and Cr contents, respectively. The fatigue crack growth rate in the second region in the da/dN-ΔK relationship for SUS316-based stainless steels can be estimated with the Paris equation, da/dN = C (ΔK)^m, and C = 8·10^–11·exp (0.0235·Md30) in steels with Md30 > –95°C. No clear degradation ascribable to high pressure hydrogen gas was observed in steels with Md30 ≤ –95°C or [Ni]+0.37[Cr] ≥ 17.5 mass%. Fatigue cracks propagated mainly in the γ phase, and the α′ deformation induced martensite phase sometimes became the crack propagation path in tested steels with low γ phase stability. Faceted fracture surfaces consisting of the {111}γ plane and α′ martensite were observed at the fatigue fracture surfaces and considered to be formed as a result of hydrogen gas embrittlement at or near the interphase boundaries between the two phases, where the hydrogen concentration was considered to be high.

View full abstract

The present study has investigated whether the hydrogen embrittlement behavior of Ni–Ti superelastic alloy can be changed by modifying the hydrogen absorption conditions. Upon immersion in H₂SO₄ or H₃PO₄ solution, the stress plateau due to the stress-induced martensite transformation becomes unclear and forms a gentle slope. In addition, the superelastic strain decreases with increasing immersion time. The peripheral part of the fracture surface of the immersed specimens is flat as usual, whereas the center part of the fracture surface is rough compared with that under other hydrogen absorption conditions reported previously. For the specimens immersed in H₃PO₄ solution, hydrogen thermal desorption tends to be observed at higher temperatures compared with the specimens immersed in H₂SO₄ solution. Moreover, for a longer immersion time, a second peak of hydrogen desorption is observed at a high temperature, indicating that the hydrogen states change with the hydrogen absorption conditions. The results of this study suggest that changing the hydrogen embrittlement behavior by modifying the hydrogen absorption conditions may enable the determination of the embrittlement mechanism of the alloy.

View full abstract

It has been reported that cracks grow along {100} independent of the tensile direction in hydrogen embitterment in nickel single crystal. Behavior of crack initiation by cathodic hydrogen charging was investigated in the present work to clarify the mechanism of hydrogen embrittlement. Hydrogen was charged on (100) or (111) surface of specimens during 180 hours at 100 A/m² cathodic current density. Nickel α phase was transformed into the hydride by hydrogen. A lot of cracks were observed in the nickel-hydride layer without external stress. They were along {100} independent of the plane charged hydrogen, though the distribution of the cracks on the surface depends on it. Therefore, {100} cracking is preferred in nickel hydride. It corresponds to the morphology of the fracture surface in hydrogen embrittlement in nickel single crystal.

View full abstract

Research on hydrogen embrittlement (delayed fracture) susceptibility of high strength steel bolts was reviewed from the viewpoint of metallurgical factors enhancing the resistance to delayed fracture, assessment of delayed fracture susceptibility based on the hydrogen absorption from atmospheric environments, and the mechanism of hydrogen entry into steel under atmospheric exposure.
Resistance to delayed fracture was quantified based on the threshold hydrogen concentration C_th by means of laboratory constant load tests under cathodic hydrogen charging. Vanadium containing anti-delayed fracture steel bolts had higher C_th values than conventional steel bolts, and showed good resistance to delayed fracture at tensile strength levels of 1400 MPa under actual atmospheric exposure.
Susceptibility to delayed fracture was evaluated by comparing C_th to absorbed hydrogen concentration C₀ into steel bolts under atmospheric exposure. The good resistance to delayed fracture of Vanadium steel high strength bolts was attributable to sufficiently higher C_th than C₀. Hydrogen permeation experiments under atmospheric exposure enabled more exact measurements of sub-surface hydrogen concentration into steel. Hydrogen permeability under atmospheric exposure showed a strong dependence on the time of the day, seasons and exposure locations.
Mechanism of hydrogen entry was investigated by hydrogen permeation measurements under laboratory wet-dry cyclic conditions. Dominant factors that control hydrogen entry were temperature, relative humidity, and the amount of sea salt on the steel surface. Dependence of hydrogen permeability under actual atmospheric environments on daily temperature-humidity cycle, season and locations were explainable based on the effect of temperature, humidity and sea salts.

View full abstract

To ensure safety against delayed fracture caused by hydrogen embrittlement under service environments of steel sheets for auto body parts used after forming and paint baking, it is quite important to understand hydrogen entry into steels under the service environments, which could be influenced by part processing parameters such as forming and the following paint baking conditions. In this study, the influence of temperature and holding time in low-temperature heat treatment after deformation on hydrogen entry into steel sheets was investigated in detail, and the mechanism of the decrement of hydrogen entry due to the heat treatment was discussed using 0.2%C steel, 0.002%C steel and IF steel. The steels were 10–20% cold rolled to introduce plastic strain, then heat-treated at 50–170°C for 0.2–20160 min, followed by immersion in pH 1-HCl for at most 72 h to introduce hydrogen into the steels. Hydrogen entry into the steels during immersion in pH 1-HCl increased with the cold rolling and decreased with the following low-temperature heat treatment, and as the heating temperature and holding time increased, hydrogen entry during immersion in HCl decreased. This indicates hydrogen trapping sites, which were introduced by the cold rolling, decreased due to the low-temperature heat treatment. From a comparison of steels with and without solute carbon atoms, the decrement of hydrogen trapping sites due to the low-temperature heat treatment was attributed to both the decrement of vacancies and occupation of hydrogen trapping sites around dislocations by interstitial solute atoms such as carbon.

View full abstract

It is needless to say by now that delayed fracture is an intrinsic issue in high strength bolt steels. Since hydrogen absorption of bolt steels in galvanizing must be taken into account besides the absorption from the environment of bolt usage, baking is integrated within the manufacturing process of galvanized bolts in order to decrease the absorbed hydrogen. But there are processes in bolt manufacturing other than galvanizing in which the hydrogen absorption is assumed to take place. The authors investigated the effect of each manufacturing process on the amount of the absorbed hydrogen in bolt steels.

View full abstract

The time dependence of hydrogen concentrations in steel was investigated using immersion, galvanostatic-charging, and potentiostatic-charging methods in various environments in order to establish a method for stable hydrogen entry into steel. Stable hydrogen charging conditions can be obtained under galvanostatic-charging in a deaerated buffer solution of a pH more than 5.0. Hydrogen can also stably enter into steel in immersion tests such as those in the environment of a pH more than 5.0 with poisonous substances such as H₂S and ammonium thiocyanate. A different hydrogen concentration can be accurately obtained by an immersion test with different concentrations of poisonous substances such as H₂S or ammonium thiocyanate or with a different pH, and it is also obtained by applying the different low current densities under galvanostatic-charging.

View full abstract

Delayed fracture is affected by two parameters: stress and hydrogen. Hydrogen enters the material from the environment as a result of corrosion. Generally, it takes a relatively long time, so that fracture occurs with a time lag after loading. The delayed fracture occurs when the driving force (S-H)_E becomes greater than the material resistance (S-H)_C. Extensive research has been carried out to evaluate the material resistance against delayed fracture using the constant load test (CLT) and slow strain rate test (SSRT). Recently, the author has developed a test method called conventional strain rate test (CSRT). In CSRT, hydrogen, at a content corresponding to the accumulated hydrogen concentration in CLT and SSRT, is introduced uniformly into the specimen and loading is applied at a conventional strain rate with negligible hydrogen diffusion. The advantage of CSRT is that it requires no special testing equipment and the testing time is very much shorter than those of the previous test methods. The principle, test procedure and some experimental results of CSRT are discussed in this paper.

View full abstract

The delayed fracture characteristics of V-bearing steel were evaluated using conventional strain rate test (CSRT) and the hydrogen absorption and desorption behaviors were studied with the specimens hydrogen-charged and then exposed to air of 30°C for up to 2.5 months. CSRT was carried out at two test sites, and nearly the same delayed fracture resistance was obtained for the V-bearing steel. The fracture appearance changed from quasicleavage to intergranular with increasing hydrogen content. The hydrogen content of the boundary between fracture appearances was approximately 4 mass ppm. The hydrogen introduced into the V-bearing steel was composed of a diffusible one which decreased in concentration in 24 h when exposed to air of 30°C, and two types (weakly and strongly) of trapped ones. The strongly trapped hydrogen remained in the specimen after 2.5 months of exposure in air. By analyzing the thermal desorption profiles with Gaussian function, the peak temperatures of these hydrogen types were 100°C, 167°C and 198°C, corresponding to diffusible, weakly and strongly trapped hydrogen, respectively. The hydrogen-charged specimens of more than 4 mass ppm were fractured in the intergranular mode. After exposure in air and the hydrogen content became less than 4 mass ppm, the fracture mode changed to quasicleavage. After recharging the hydrogen to more than 4 mass ppm, the fracture mode became intergranular again.

View full abstract

Experimental studies attempting to evaluate the delayed fracture property of high strength bolt steels by using slow strain rate tests (SSRT) of circumferentially notched round bar specimens are reviewed in this paper. The relationship between the notch tensile strength (NTS) of the hydrogen-precharged specimens and the hydrogen content was approximated by power law relationship. It has been found that the local stress and the local diffusible hydrogen concentration control the occurrence of delayed fracture. To take into account the effect of hydrogen uptake from the environment, the notched bar specimens were subjected to cyclic corrosion test (CCT) and outdoor exposure, and their NTSs were measured using SSRT. The susceptibility of high strength steels to delayed fracture estimated from the decrease of NTS was correspondent to the fracture ratio of high strength bolts of the steels obtained by outdoor exposure tests, and was successfully evaluated with consideration of hydrogen uptake. Hydrogen entry behavior under CCT was monitored by electrochemical hydrogen permeation test, and continuous increase in hydrogen entry with growth of rust layer was observed. It is suggested that the “delay” of delayed fracture is the time required for the enhancement of the hydrogen entry efficiency.

View full abstract

When ultra high strength steel (UHSS) sheets with tensile strength over 980 MPa are applied in automobiles, there is a risk that a type of hydrogen embrittlement fracture called delayed fracture may occur while a vehicle is in use. This paper summarizes the effects of stress, strain, diffusible hydrogen content and the forming mode on the hydrogen embrittlement resistance of UHSS sheets for automotive applications. In this study, 1180 MPa grade ferrite-martensite dual phase steel was used. This material was evaluated by the U-bending and drawn cup methods. It was concluded that high strain, high stress and high diffusible hydrogen content reduced hydrogen embrittlement resistance.
In addition, an improved immersion-type hydrogen charging method using an ammonium thiocyanate (NH₄SCN) aqueous solution was introduced in this paper. The NH₄SCN solution enables control of the diffusible hydrogen content from low to high concentrations using the NH₄SCN concentration, and dissolution of the specimens during immersion in NH₄SCN was minimal, making it possible to maintain substantially the same surface condition as before immersion.

View full abstract

This study proposes a new evaluation model of hydrogen-induced unstable fracture of high strength steel and/or its welds. In this model, a new driving force for hydrogen-induced fracture, Hydrogen-Weibull stress, is proposed on the basis of the original Beremin model, where cohesive energy reduction due to diffusible hydrogen is implemented into the local unstable fracture model. The Hydrogen-Weibull stress, which is enlarged not only by the local stress but also by diffusible hydrogen content, can be independent of geometrical parameters of components as well as diffusible hydrogen content. The validity of this Hydrogen-Weibull stress is demonstrated by constant load test conducted with different load level and with various initial hydrogen contents.

View full abstract