Takai Kenichi

Factors causing hydrogen embrittlement of cold-drawn pearlitic steel fractured in plastic/elastic region have been investigated from the viewpoint of lattice defects. Tensile tests were conducted for hydrogen-charged specimens containing 1.5 and 4.0 ppm hydrogen under various crosshead speeds. The amount of tracer hydrogen which corresponds to the amount of lattice defects in the specimens subjected to plastic strain or elastic stress was measured using a thermal desorption analysis. As a result, specimens containing 1.5 ppm hydrogen fractured in the plastic region under all experimental conditions in the present study. In contrast, specimens containing 4.0 ppm hydrogen fractured in the elastic region at crosshead speed of 0.01 mm·min-1 or less and fractured in the plastic region at 0.1 mm·min-1 or more. Subjecting plastic strain in the presence of hydrogen increased the amount of lattice defects corresponding to vacancies. In contrast, the presence of hydrogen had no effects on the formation of lattice defects under subjecting elastic stress. The amount of lattice defects in the specimens fractured in plastic region with hydrogen was equivalent to that of lattice defects in the specimens fractured under same conditions without hydrogen. The amount of lattice defects in the specimens fractured in elastic region with hydrogen was less than that of lattice defects in the specimens fractured under same conditions without hydrogen. These results indicated that lattice defects enhanced by hydrogen affected the fracture in plastic region with hydrogen. However, the effects of lattice defects on the fracture in elastic region with hydrogen were small.

States of hydrogen present in high-strength steels for use as bearing steel SUJ2 and hydrogen embrittlement susceptibility were examined using thermal desorption analysis (TDA) and tensile tests. SUJ2 specimens containing the retained austenite phase (R) in the martensitic phase exhibited three hydrogen desorption peaks in the TDA profile. Two of the peaks desorbed at higher temperatures decreased with a decreasing amount of R, indicating they corresponded to desorption associated with R. Fracture strength in the presence of hydrogen increased with a decreasing amount of R and with an increasing strain rate. For the specimens containing R and hydrogen, a flat facet at the crack initiation site and a quasi-cleavage (QC) fracture in the initial crack propagation area were observed on the fracture surface. Local characterization using electron back-scattered diffraction (EBSD) revealed that the flat facet on the fracture surface corresponded not to R but to strain-induced martensite. In addition, the facet was on the {112} plane of martensite, which is the slip plane or deformation twin plane of body-centered-cubic metals. The reason for high hydrogen embrittlement susceptibility of the specimens containing R was attributed to the straininduced phase transformation at the crack initiation site of the flat facet and in the initial crack propagation area of the QC fracture. Furthermore, the strain rate dependency of hydrogen embrittlement susceptibility is presumably ascribable to local plastic deformation, i.e., the interaction between dislocations and hydrogen.

The hydrogen embrittlement behavior of Fe-19Cr-8Ni-0.05C and Fe-19Cr-8Ni-0.14C metastable austenitic steels was investigated using tensile tests under hydrogen-charging, cryogenic thermal desorption spectroscopy, and in situ deformation experiments. Coupled with post-mortem microstructure characterization, the cracking paths were clarified to be transgranular along {110}(alpha) and {100}(alpha) in the Fe-19Cr-8Ni-0.05C steel and (100)(alpha) in the Fe-19Cr-8Ni-0.14C steel. Intergranular cracking also occurred in the Fe-19Cr-8Ni-0.05C steel when alpha '-martensite thoroughly covered the grain boundaries. Occurrence of the transgranular and intergranular hydrogen-assisted cracking in the steels is assisted by (1) an increase in the hydrogen-affected zone associated with presence of thermally induced alpha '-martensite, and (2) an increase in the local mobility of hydrogen that occurs with the deformation-induced alpha '-martensitic transformation. Additionally, (3) the trans granular hydrogen-assisted cracking is assisted by the intersection of deformation bands driven by the maximum Schmid factor and the stress concentration at the crack tip. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Hydrogen embrittlement is a serious problem in high-strength steels. Drawn pearlitic steel shows excellent resistance to hydrogen embrittlement despite its high strength, and aging treatment at a low temperature can simultaneously improve its strength and hydrogen-embrittlement resistance. To clarify the mechanism for this we have used thermal desorption analysis (TDA) and the newly developed precession electron diffraction analysis method in the transmission electron microscope. After aging at 100 °C for 10 min, the amount of hydrogen seen amount on the TDA curve reduced at around 100 °C. In contrast, when aging was performed at 300 °C, the hydrogen amount further reduced at around 100 °C and the unevenly deformed lamellar ferrite zone was locally recovered. For the samples that were aged at the low temperature, we confirmed that their yield strength and relaxation stress ratios increased simultaneously with improvement in the hydrogen-embrittlement property. We infer that segregation of carbon or formation of very fine carbide in dislocations during aging is the cause of these behaviors.

Studies to date have not completely determined the factors influencing hydrogen embrittlement of ferrite/bainite X80 pipeline steel. Hydrogen embrittlement susceptibility was evaluated based on fracture strain in tensile testing. We conducted a thermal desorption analysis to measure the amount of tracer hydrogen corresponding to that of lattice defects. Hydrogen embrittlement susceptibility and the amount of tracer hydrogen significantly increased with decreasing crosshead speed. Additionally, a significant increase in the formation of hydrogen-enhanced strain-induced lattice defects was observed immediately before the final fracture. In contrast to hydrogen-free specimens, the fracture surface of the hydrogen-charged specimens exhibited shallower dimples without nuclei, such as secondary phase particles. These findings indicate that the presence of hydrogen enhanced the formation of lattice defects, particularly just prior to the occurrence of final fracture. This in turn enhanced the formation of shallower dimples, thereby potentially causing premature fracture of X80 pipeline steel at lower crosshead speeds.

The difference in the hydrogen charging methods, immersion in a NH4SCN aqueous solution, and cathodic electrolysis in a NaOH aqueous solution, did not affect the hydrogen state present in the steel, but it did affect the surface state of the specimens through corrosion, causing fracture strength to fluctuate in tensile testes. As for stress application method, the fracture strength at lower crosshead speeds in tensile tests was consistent with that found for hydrogen precharging prior to stress application in CLTs as long as hydrogen charging was conducted by cathodic electrolysis. However, the fracture strength obtained with concurrent hydrogen charging without precharging prior to stress application in CLTs was higher than that with hydrogen precharging prior to stress application in CLTs regardless of the same hydrogen content. In other words, delayed fracture susceptibility was affected by the order of hydrogen charging and stress application for quasi-cleavage fracture associated with local plastic deformation, i.e., dislocation motion. Therefore, by taking into account the cathodic electrolysis in the NaOH solution, the low crosshead speed and the order of hydrogen charging and stress application, the fracture strength in CLTs, and tensile tests coincided with respect to quasi-cleavage fracture even though the stress application methods were different.

Clarifying the states of hydrogen present in iron and steel is important in order to understand hydrogen embrittlement mechanisms and develop materials with high resistance to hydrogen embrittlement. Although it is widely recognized that the fracture strain of iron and steel decreases with increasing amounts of absorbed hydrogen, it is not yet well understood whether hydrogen directly decreases the fracture strain. Therefore, the objective of this study is to clarify the atomic-scale changes in strained alpha-iron specimens containing hydrogen. Low temperature thermal desorption spectroscopy (L-TDS), which can heat samples from lower temperatures than conventional TDS, was used to identify the peak temperatures and hydrogen states corresponding to various lattice defects in alpha-iron. The results indicate that new hydrogen trap sites in strained alpha-iron specimens containing hydrogen are enhanced compared to those without hydrogen. These sites are not dislocations, but hydrogen-enhanced strain-induced vacancies, because they are removed during aging at 30 degrees C.

Factors causing hydrogen embrittlement of cold-drawn pearlitic steel fractured under plastic/elastic region have been investigated from the perspective of lattice defects. Tensile tests were conducted for hydrogen-charged specimens containing 1.5 and 4.0 mass ppm hydrogen to evaluate mechanical properties. The amount of tracer hydrogen, i.e., lattice defects in the specimens unloaded just before tensile fracture strength, was measured using a thermal desorption analysis. Specimens containing 1.5 and 4.0 mass ppm hydrogen fractured under plastic and elastic region, respectively. The specimen fractured under plastic region showed enhanced formation of lattice defects corresponding to vacancies, which directly caused embrittlement. In contrast, the specimen fractured under elastic region showed no enhancement to the formation of lattice defects. These results reveal that one of the factors causing hydrogen embrittlement under plastic region is due to hydrogen-enhanced strain-induced vacancies, whereas the factors causing hydrogen embrittlement under elastic region are due to others.

States of hydrogen present in high-strength steels for use as bearing steel SUJ2 and hydrogen embrittlement susceptibility were examined using thermal desorption analysis (TDA) and tensile tests. SUJ2 specimens containing retained austenite phase (gamma(R)) in the martensite phase exhibited three hydrogen desorption peaks in the TDA profile. Two of the peaks desorbed at higher temperatures decreased with a decreasing amount of gamma(R), indicating they corresponded to desorption associated with gamma(R). Fracture strength in the presence of hydrogen increased with a decreasing amount of gamma(R) and with an increasing strain rate. When the specimens contained gamma(R) and hydrogen, a flat facet at the crack initiation site and a quasi-cleavage (QC) fracture in the initial crack propagation area were observed on the fracture surface. Local characterization using electron back-scattered diffraction (EBSD) revealed that the flat facet on the fracture surface corresponded not to gamma(R) but to stress-induced martensite. In addition, the facet was {112} plane of martensite, which is the slip plane or deformation twin plane of body-centered-cubic metals. The reason for high hydrogen embrittlement susceptibility of the specimens containing gamma(R) was attributed to the stress-induced phase transformation at the crack initiation site of the flat facet and in the initial crack propagation area of the QC fracture. Furthermore, the strain rate dependency of hydrogen embrittlement susceptibility is presumably ascribable to local plastic deformation, i.e., the interaction between dislocations and hydrogen.

An ammonium thiocyanate (NH₄SCN) solution is widely used in hydrogen embrittlement evaluations of high-strength steel materials. It is known that an increase in the specific solution volume to the specimen surface area results in a severe evaluation in hydrogen embrittlement testing. The reason for that is explained in this paper based on the change in the solution pH induced by a cathodic reaction, which accompanies thiocyanate ion decomposition and governs hydrogen absorption. In addition, the pH dependence of the cathodic reaction governing hydrogen absorption is made clear by using a sodium thiocyanate solution containing a buffer solution to control the solution pH. It is shown that immersing steel specimens in the pH-controlled sodium thiocyanate solution achieves a higher hydrogen content compared with the level attained with the NH₄SCN solution.

Ammonium thiocyanate is widely used as a reagent for promoting hydrogen absorption by high-strength steels. The effects of the solution concentration, temperature and dissolved oxygen on corrosion film formation, corrosion reactions and hydrogen absorption were investigated in a hydrogen embrittlement test environment using an ammonium thiocyanate aqueous solution. A still bath of a 20 mass% ammonium thiocyanate solution displayed a supply limitation, whereas a solution with flow influenced the corrosion and hydrogen absorption by the tested steel. The concentration of dissolved oxygen that forms an iron oxide film was found to have a small effect on hydrogen absorption. The change in the hydrogen content of the tested steel with elapsed time is explained in terms of changes in the equilibrium concentration of hydrogen on the steel surface and in the corrosion rate. The change in the solution pH during potentiostatic electrolysis tests in an ammonium thiocyanate solution is also discussed.

Hydrogen embrittlement has become a crucial issue with the promotion of high-strength steel. As-drawn pearlitic steel wire is well known to have superior resistance to hydrogen embrittlement. The resistance to hydrogen embrittlement is clarified as being further improved by aging treatment at 100 degrees C and 300 degrees C for 10-min. of as-drawn 0.8 mass% C pearlitic steel wire with phi 5.0 mm (e=1.9). The higher the aging temperature is, the better the resistance to hydrogen embrittlement becomes. Simultaneously, the strength even increased slightly by aging treatment. The mechanism is investigated by exploiting thermal desorption analysis (TDA) and the newly developed TEM precession analysis. Aging at 100 degrees C led to a decrease in the hydrogen content at peak I around 100 degrees C in the TDA curve, which is inferred to be caused by C segregation to dislocations resulting in improvement of hydrogen embrittlement. Aging at 300 degrees C further improved the resistance to hydrogen embrittlement, which is presumably brought about by the local recovery of the heterogeneously deformed lamellar ferrite area together with the C segregation to dislocations. Here, the strength increased slightly by aging due to the softening factor of recovery and the hardening factor of strain aging.

It has been proposed that several corrosion reactions occur in the FIP (Federation International de la Precontrainte) test when using a 20 mass% ammonium thiocyanate solution. The results of the present study show that a cathodic reaction accompanying the decomposition of SCN- ions contributes significantly to hydrogen absorption in the FIP test environment. The cathodic reaction continues even during the period when a gradual decline is seen in the hydrogen content of PC steel specimens in the FIP test. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Hydrogen embrittlement has become a crucial issue with the promotion of high-strength steel. As-drawn pearlitic steel wire is well known to have superior resistance to hydrogen embrittlement. The resistance to hydrogen embrittlement is clarified as being further improved by aging treatment at 100-degrees C and 300-degrees C for 10-min. of as-drawn 0.8 mass% C pearlitic steel wire with phi 5.0 mm (epsilon=1.9). The higher the aging temperature is, the better the resistance to hydrogen embrittlement becomes. Simultaneously, the strength even increased slightly by aging treatment. The mechanism is investigated by exploiting thermal desorption analysis (TDA) and the newly developed TEM precession analysis. Aging at 100-degrees C led to a decrease in the hydrogen content at peak I around 100-degrees C in the TDA curve, which is inferred to be caused by C segregation to dislocations resulting in improvement of hydrogen embrittlement. Aging at 300-degrees C further improved the resistance to hydrogen embrittlement, which is presumably brought about by the local recovery of the heterogeneously deformed lamellar ferrite area together with the C segregation to dislocations. Here, the strength increased slightly by aging due to the softening factor of recovery and the hardening factor of strain aging.

The role of solute hydrogen and hydrides in the degradation of the mechanical properties of pure titanium was examined in this study. Commercially pure (99.5.) titanium was electrolytically charged with hydrogen from 50 to 2500 mass ppm and then heated in Ar at 723 K for 3 h to obtain a homogeneous solution of hydrogen. Three types of cooling process after heating were employed to obtain hydrides with different morphologies and solute hydrogen: furnace cooling and water quenching, both to room temperature, and air cooling to 473 K. The microstructures obtained with each process were coarsely precipitated hydrides or finely dispersed hydrides with dissolved hydrogen, and solute hydrogen, respectively. The ratio of the fracture strain of the hydrogen. charged specimens to that of the as-received specimen was used as an index of hydrogen degradation susceptibility (DS). The specimens having coarse hydrides showed a steep increase in DS above 600 mass ppm of absorbed hydrogen due to hydride precipitation and fracture, while the specimens with solute hydrogen showed a milder increase in DS, suggesting that solute hydrogen had a different effect on degradation. The change in DS with the strain rate of the tensile test varied for each type of specimen. The specimens with coarse hydrides showed an increase in DS in the high strain rate region due to preferred fracture of hydrides. On the other hand, DS of the specimens with solute hydrogen increased with a lower strain rate, suggesting interaction between solute hydrogen and mobile dislocations, which is also found in other metallic materials.

The effects of the hydrogen state, temperature, strain rate and hydrogen content on hydrogen embrittlement susceptibility and hydrogen-induced lattice defects were evaluated for cold-drawn pearlitic steel that absorbed hydrogen in two trapping states. Firstly, tensile tests were carried out under various conditions to evaluate hydrogen embrittlement susceptibility. The results showed that peak 2 hydrogen, desorbed at temperatures above 200 degrees C as determined by thermal desorption analysis (TDA), had no significant effect on hydrogen embrittlement susceptibility. In contrast, hydrogen embrittlement susceptibility increased in the presence of peak 1 hydrogen, desorbed from room temperature to 200 degrees C as determined by TDA, at temperatures higher than 30 degrees C, at lower strain rates and with higher hydrogen content. Next, the same effects on hydrogen-induced lattice defects were also evaluated by TDA using hydrogen as a probe. Peak 2 hydrogen showed no significant effect on either hydrogen-induced lattice defects or hydrogen embrittlement susceptibility. It was found that hydrogen-induced lattice defects formed under the conditions where hydrogen embrittlement susceptibility increased. This relationship indicates that hydrogen embrittlement susceptibility was higher under the conditions where the formation of hydrogen-induced lattice defects tended to be enhanced. Since hydrogen-induced lattice defects formed by the interaction between hydrogen and strain were annihilated by annealing at a temperature of 200 degrees C, they were presumably vacancies or vacancy clusters. One of the common atomic-level changes that occur in cold-drawn pearlitic steel showing higher hydrogen embrittlement susceptibility is the formation of vacancies and vacancy clusters. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The numbers of lattice defects formed by applying cyclic prestress with/without hydrogen for various numbers of cycles and strain rates during cyclic prestress were compared for tempered martensitic steel. A tensile test was also carried out to evaluate hydrogen embrittlement susceptibility following the application of cyclic prestress. The results showed that when cyclic prestress was applied without hydrogen, the number of cycles and strain rate had no apparent effect on mechanical properties and fracture morphology at the time of the subsequent tensile test. In contrast, when cyclic prestress was applied with hydrogen, the fracture strain and fracture stress decreased with an increasing number of prestress cycles and a decreasing strain rate, and the fracture morphology exhibited brittle fracture, signifying an increase in hydrogen embrittlement susceptibility at the time of the tensile test. The number of hydrogen-enhanced lattice defects also increased with increasing number of cycles and a decreasing strain rate was found when cyclic prestress was applied with hydrogen. These results indicate a correlation between hydrogen embrittlement susceptibility and the number of hydrogen-enhanced lattice defects. The kinds of increased hydrogen-enhanced lattice defects were probably vacancies and vacancy clusters formed by the interactions between hydrogen and dislocation movement during the application of cyclic prestress. The vacancies and vacancy clusters formed during the application of cyclic prestress with hydrogen presumably caused intergranular fracture and increased hydrogen embrittlement susceptibility. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The hydrogen-releasing properties from weld metals that mainly consist of martensite and contain some amount of retained austenite were experimentally investigated in order to determine the physical properties such as the activation energies for the simulation of the hydrogen distribution in welded joints and to consider the time period to age a cold cracking test specimen before crack observation. The numerical calculations for single-pass welding show that in the case of the 2.5 % and 4.2 % retained austenite, the hydrogen contents in the martensite become sufficiently low in 2 days. In the case of 16 % retained austenite, about 40 % of the initial hydrogen content still exists in the retained austenite 7 days after the completion of the welding. But the hydrogen content in the martensite becomes less than 1 % of the initial content in 7 days. Hence, the 7-day-ageing time before crack observation is considered to be adequate. The numerical calculations for multi-pass welded joints show that the 7-day ageing is also considered to be appropriate.

Improvement of the surface layer as well as the microstructure has been needed to develop high-strength steels, since delayed fracture cracks initiate in the surface layer. In the present study, two approaches were taken to reduce the delayed fracture susceptibility of tempered martensitic steel with tensile strength of 1450 MPa. One was by increasing the Si content, which was intended to improve the microstructure. The other was by a surface-softening treatment, which was for improving the surface layer. Delayed fracture susceptibility was evaluated by conducting tensile tests and constant load tests in a NH4SCN aqueous solution. It was found that increasing the Si content from 0.2 mass% to 1.88 mass% prevented intergranular fracture and reduced delayed fracture susceptibility. One reason for this improvement is that the Fe3C particle size on prior-gamma grain boundaries and in the matrix decreases with increasing Si content, which implies that Si stabilizes dislocation structures. When the surface strength of surface-softened steel specimens was lowered to 1150 MPa, delayed fracture susceptibility was reduced further. This is attributed to not only a reduction of the Vickers hardness of the surface layer but also a reduced hydrogen concentration at the surface layer. The rearrangement and annihilation of dislocations and also the spheroidizing and coarsening of Fe3C particles at the surface layer subjected to a high tempering temperature lead to a reduction of the hydrogen concentration at the surface layer.

Improvement of the surface layer as well as the microstructure has been needed to develop high-strength steels, since delayed fracture cracks initiate in the surface layer. In the present study, two approaches were taken to reduce the delayed fracture susceptibility of tempered martensitic steel with tensile strength of 1 450 MPa. One was by increasing the Si content, which was intended to improve the microstructure. The other was by a surface-softening treatment, which was for improving the surface layer. Delayed fracture susceptibility was evaluated by conducting constant strain rate tensile tests (tensile tests) and constant load tests in a NH4SCN aqueous solution. It was found that increasing the Si content from 0.2 mass% to 1.88 mass% prevented intergranular fracture and reduced delayed fracture susceptibility. One reason for this improvement is that the Fe3C particle size on prior-grain boundaries and in the matrix decreases with increasing Si content, which implies that Si stabilizes dislocation structures. When the surface strength of surface-softened steel specimens was lowered to 1 150 MPa, delayed fracture. susceptibility was reduced further. This is attributed to not only a reduction of the Vickers hardness of the surface layer but also a reduced hydrogen concentration at the surface layer. The rearrangement and annihilation of dislocations and also the spheroidizing and coarsening of Fe3C particles at the surface layer subjected to a high tempering temperature lead to a reduction of the hydrogen concentration at the surface layer.

States of hydrogen present in cold-rolled pure iron specimens charged by hydrogen gas and cathodic electrolysis have been compared using thermal desorption analysis to obtain the fundamental properties for evaluating the hydrogen embrittlement susceptibility of hydrogen gas pipeline steels. Hydrogen was charged into the cold-rolled pure iron specimens in gaseous hydrogen at pressures of 4, 7, and 10 MPa at temperatures of 30, 60, 90, 120 degrees C, and in an aqueous solution of NaOH with pH of 13, to which 5 g. L-1 of NH4SCN was added, at current densities in the range of 0 to 50 A.m(-2) and at temperatures of 30, 60 and 90 degrees C. For charging at 30 degrees C under various hydrorgen pressures, hydrogen peak temperatures are approximately 60 degrees C and the peak profiles are identical. In contrast, when the charging temperature is increased to 90 degrees C under various hydrorgen pressures, hydrogen desorption newly occurred above approximately 100 degrees C. The hydrogen peak at 60 degrees C corresponds to hydrogen trapped at dislocations and vacancies, whereas the hydrogen desorption above approximately 100 degrees C corresponds to hydrogen trapped at vacancy clusters formed during hydrogen charging at higher temperatures. In addition, equilibrium hydrogen content in solid solution and at trapping sites decreases with increasing charging temperature, since enthalpy of hydrogen solution is -27.3 kJ.mol(-1). These results indicate that higher. temperature charging causes the states of hydrogen present in cold. rolled pure iron to change due to the formation of vacancy clusters and reduces the equilibrium hydrogen content including that in solid solution and at trapping sites.

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 micro-analyzer (PPMA). The relationship between hydrogen embrittlement and lattice defects associated with hydrogen was also investigated. The amount of lattice defects increased gradually with increasing the 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 degrees 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.

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.

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 degrees 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 degrees 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 degrees C, 167 C and 198 degrees 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.

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.

A solution of ammonium thiocyanate is used in the FIP (Fédé ration International de la Précontrainte) test as a hydrogen charging method. Though this method is comparatively simple, fracture time in the FIP test and hydrogen content often differ among various testing institutes. However, the detailed hydrogen absorption behavior in the solution is still not clear. In this context, the effects of existing states of hydrogen, oxide film on the specimen surface, specific solution volume to specimen surface area, immersion time and solution temperature on the hydrogen absorption behavior of a steel bar for reinforcing prestressed concrete were investigated by immersing it in the solution. The amount of absorbed hydrogen increased with immersion time, reached its maximum, and then decreased with increasing immersion time. A main factor of the decrease in the amount of absorbed hydrogen was corrosion products, including Fe, O and S, formed on the specimen surface, since the amount of absorbed hydrogen increased again as a result of merely polishing the surface. This indicates that corrosion products formed on immersing specimens in a solution of ammonium thiocyanate strongly affect hydrogen absorption behavior. Whereas, variation of the solution, such as increase in pH, during immersion also affects slightly hydrogen absorption behavior.

Plastic strain takes place at the bottom of the thread, when fastening force is applied to the high strength bolts. The effect of plastic strain on the delayed fracture characteristics was studied using conventional strain rate test (CSRT). For this purpose two kinds of methods were used in the experiments: one is applying uniform plastic strain to the specimen before machining notch and the other is preloading of notched round bar (NRB) specimen. The hydrogen content increased as increasing plastic strain and in contrast, the relationship between fracture stress and hydrogen content using CSRT was independent of plastic strain, which indicates that plastic strain increases hydrogen content and results in decreasing fracture stress based on the above-mentioned relation. On the contrary, the preloading affected the fracture nominal stress obtained by CSRT of hydrogen prechargeci NRB specimen. Using finite element stress analysis, the maximum stress ahead of the notch tip for preloaded NRB specimen was obtained. The relationship between the fracture maximum stress and hydrogen concentration becomes unique irrespective preloading. It is concluded that the relation of local maximum stress the hydrogen concentration at the delayed fracture initiation site is the material constant and control delayed fracture.

Hydrogen was introduced in commercial-purity (99%) aluminum by electrochemical charging to study the existing state of hydrogen and its effects on the mechanical properties of aluminum. Electrochemical charging was conducted in an aqueous H2SO4 solution with 0.1% NH4SCN as a hydrogen recombination poison. The potential and during the charging were determined from the immune. passive, and corrosive regions in the Pourbaix diagram to determine the optimum conditions or the charging. The maximum amount of hydrogen absorbed was obtained in the immune region. The amount of hydrogen and its existing state were examined using hydrogen desorption curves, which were obtained by thermal desorption spectroscopy. The curves showed distinctive peaks corresponding to trapping sites of hydrogen in the material. One of the peaks was observed at approximately 100 degrees C, and it corresponds to vacancies and dislocations in the material; another peak was observed at approximately 400 degrees C and it corresponds to molecular hydrogen in blisters. It was presumed that charged hydrogen diffuses into the bulk of the material to form hydrogen-vacancy pairs, and then these pairs cluster to form blisters. The fracture strain of charged aluminum in the immune region decreased with decreasing strain rate, showing an inverse dependence on the fracture strain of the uncharged material. This phenomenon was considered to be caused by hydrogen transport by dislocations through the interaction between hydrogen and dislocations. The phenomenon was further confirmed by the observation of hydrogen release during tensile deformation, where the amount of hydrogen was high in the strain rate range where the interaction between dislocations and hydrogen was prominent. [doi: 10.2320/matertrans.M2011035]

The electrochemical properties of Mg + 2LiH and Al + 3LiH are investigated by applying a Li-ion insertion and extraction system to form magnesium and aluminum hydrides. For MgH(2) formation, the voltage-composition (VC) curve for Mg + 2LiH during charging exhibits a plateau voltage at 0.58 V, then the final composition is obtained with 1.05 mol Li extraction at 3.0 V. After the charging, the MgH(2) phase is observed by XRD measurement. Therefore, MgH(2) is produced from Mg and LiH by electrochemical charging. With respect to AlH(3) formation, Al + 3LiH is charged at a plateau voltage of 0.81 V, which corresponds to the reaction of Al with hydrogen in LiH to form AlH(3). And the final composition at 3.0 V is 0.6 mol Li. In the XRD profile after charging, the AlH(3) phase is not detected. (C) 2010 Elsevier B.V. All rights reserved.

In this study, we analyzed the effects of deformation on hydrogen absorption and desorption properties of titanium to improve such properties. Hydrogen was introduced into commercially pure (99.5%) titanium by the electrochemical method. The amount and existing state of hydrogen were examined using hydrogen desorption curves obtained by thermal desorption spectroscopy. Hydrogen absorption was promoted by applying tensile deformation prior to charging, which leads to hydride formation within a short charging time. The amount of hydrogen absorbed decreased when the volume fraction of deformation twins exceeded about 0.2. It was considered that hydrogen was mainly trapped by dislocations forming hydride while a large fraction of deformation twins hindered dislocation motion, thus reducing dislocation density leading to a decrease in the amount of absorbed hydrogen. Almost half the charged hydrogen was released when in-plane compressive stress was applied to a charged plate specimen at room temperature due to hydride decomposition under compressive stress. (C) 2010 Elsevier B.V. All rights reserved.

The particulate composite model derived to explain the high contact angle of water on the water repellent particulate composite materials was applied for the contact angle of other types of liquid such as methylene iodide and α-bromonaphthalene on the particulate composite materials. By substituting the experimental data of contact angle of water, methyleneiodide and α-bromonaphtalene to the formula of this model, following results were obtained. For water case, the more the PTFE volume fraction, the more area of PTFE surface is covered by air and that 100% of binder surface can be covered by water. For methylene iodide case, both PTFE and binder are covered thoroughly by methylene iodide for all the PTFE volume fraction. For α- bromonaphtalene case, the more the PTFE volume fraction, the more area of PTFE surface is covered by air and that 100% of binder surface can be covered by α-bromonaphtalene. By assuming that binder is covered by each of liquid, the contact angle of three different types of liquid on the PTFE particulate composites material can be skeptically expressed using the parameters of PTFE volume fraction and liquid coverage of PTFE. © 2011 The Society of Materials Science, Japan.

The delayed fracture characteristics of V-bearing steel were evaluated using the CSRT method, and the hydrogen trapping and de-trapping behavior was studied with the specimens hydrogen-charged and then held in air for up to 2.5 months. The CSRT tests were carried out at two test sites and nearly the same delayed fracture characteristics were obtained using the V-bearing steel. The fracture appearance changed from quasi-cleavage to inter-granular fracture with an increase of hydrogen content. The boundary hydrogen content of the fracture appearance change is around 4 mass ppm. The hydrogen charged in V-bearing steel was composed of diffusible one, which comes out in 24 h when held in air of 30 degrees C and two kinds of (weakly and strongly) trapped ales. The strongly trapped hydrogen remained in specimen after 2.5 months held in air. By analyzing the thermal desorption profiles with Gaussian function, the peak temperatures of these hydrogen were 100 degrees C, 167 degrees C and 198 degrees C, which corresponds to diffusible, weakly and strongly trapped hydrogen. The specimens hydrogen-charged more than 4 mass ppm were fractured in inter-granular mode. After held in air and the hydrogen comely: became less than 4 mass ppm the fracture mode changed to quasi-cleavage. Re-charging the hydrogen more than 4 mass ppm, the fracture mode became inter-granular again.

Hydrogen is introduced in commercial (99% pure) aluminum by electrochemical charging to study the existing state of hydrogen and its effect on the mechanical properties of aluminum. Electrochemical charging is conducted in an aqueous solution of H2SO4 with 0.1 mass% NIH4SCN as a hydrogen recombination poison. The potential and pH during the charging are chosen from the immune, passive, and corrosive regions on Pourbaix diagram to determine the optimum conditions for the charging. The maximum amount of hydrogen absorbed is obtained in the immune region. The amount of hydrogen and its existing state are examined using hydrogen desorption curves, which are obtained by thermal desorption spectroscopy. The curves show distinctive peaks that correspond to trapping sites of hydrogen in the material. One of the peaks is observed at approximately 100 degrees C and it corresponds to vacancies and dislocations in the material; another peak is observed at approximately 400 degrees C and it corresponds to molecular hydrogen in blisters. It is presumed that charged hydrogen diffuses into the bulk of the material to form hydrogen vacancy pairs, and then these pairs cluster to form blisters. The fracture strain of charged aluminum in the immune region decreased with a slower strain rate, showing an inverse dependence on the fracture strain of the uncharged material. This phenomenon is considered to be caused by the transport of hydrogen by dislocations through the interaction between hydrogen and the dislocations. The phenomenon is further confirmed by the observation of hydrogen release during tensile deformation, where the amount of hydrogen is higher in the strain rate region where the interaction between the dislocations and hydrogen is more prominent.

Dynamic behavior of hydrogen desorption from pure iron with a body-centered-cubic lattice and Inconel 625 with a face-centered-cubic lattice was examined during tensile deformation using a quadrupole mass spectrometer in a vacuum chamber integrated with a tensile testing machine. Hydrogen desorption from hydrogen-charged specimens was detected under various strain rates and cyclic stresses. Hydrogen desorption rarely increased under elastic deformation. In contrast, it increased rapidly at the proof stress when plastic deformation began, reached its maximum, and then decreased gradually with increasing applied strain for both pure iron and Inconel 625. This desorption behavior is closely related to hydrogen dragging by moving dislocations. The thermal desorption analysis results showed that the amount of desorbed hydrogen differed at each strain rate. This difference in the amount of desorbed hydrogen transported by dislocations depends on the balance between the hydrogen diffusion rate and mobile dislocation velocity.

For a formation of metal hydride of MgH2 or AlH3 under room temperature and ambient pressure, the cathode electrodes of metal and lithium hydride are electrochemically charged with Li anode electrodes in the system of Li-ion extraction. For MgH2 formation, the VC (Voltage-Composition) curve of Mg + 2LiH during charge shows a plateau voltage at 0.6 V until the final composition of 1.05 Li extraction. After charging MgH2 phase is observed by the XRD measurement. Therefore MgH 2 is produced by the electrochemical charge from Mg and LiH. For AlH3 formation, Al + 3LiH is charged until the final composition of 0.6 Li at a plateau voltage of 0.8 V which corresponds to the reaction between Al and LiH for the formation of AlH3. In the XRD profile after charging AlH3 phase is not detected although the intensities of Al and LiH decrease compared with these before charging, which suggests the reaction leading to the formation of AlH3. © 2010 Materials Research Society.

The hydrogen desorption behavior of pure iron with a body-centered-cubic (BCC) lattice and Inconel 625 with a face-centered-cubic (FCC) lattice was examined during tensile deformation using a quadrupole mass spectrometer in a vacuum chamber integrated with a tensile testing machine. Hydrogen and water desorption was continuously detected simultaneously under the application of a tensile load and strain to the specimens. Hydrogen desorption promoted by tensile deformation can be found by deducting both fragment hydrogen dissociated from H(2)O and H(2) desorbed under unloading from the total amount of hydrogen desorbed from hydrogen-charged specimens during tensile deformation. Hydrogen desorption from hydrogen-charged specimens was detected under various strain rates of 4.2 x 10(-5)/s, 4.2 x 10(-4)/s and 4.2 x 10(-3)/s. Hydrogen desorption rarely increased under elastic deformation. In contrast, it increased rapidly at the proof stress when plastic deformation began, reached its maximum, and then decreased gradually with increasing applied strain for both pure iron and Inconel 625. This desorption behavior is closely related to hydrogen dragging by moving dislocations. The amount of desorbed hydrogen promoted by tensile deformation was measured by thermal desorption analysis (TDA). The TDA results showed that the amount of described hydrogen differed at each strain rate. The largest amount of desorbed hydrogen promoted by tensile deformation was 16% of the initial hydrogen content in pure iron with a high hydrogen diffusion rate when the specimen was deformed at a strain rate of 4.2 x 10(-4)/s. In contrast, that of Inconel 625 with a low hydrogen diffusion rate was 9% of the initial hydrogen content when the alloy was deformed at a strain rate of 4.2 x 10(-6)/s. This difference in the amount of desorbed hydrogen transported by dislocations depends on the balance between the hydrogen diffusion rate and mobile dislocation velocity.

Hydrogen gas is generated by the electrolysis of liquid ammonia which has high hydrogen capacity of 17.8 mass%. The metal amides are used as supporting electrolytes to dissolve the amide ion in liquid ammonia. The results presented here indicate that liquid ammonia is promising as an energy medium for hydrogen storage and generation.

The delayed fracture Characteristics of steels are expressed by the relationship between the maximum fracture stress and the hydrogen content at the notch lip of circumferentially notched round bar specimen. where delayed fracture initiates. This material constant is easily obtained by CSRT (Conventional Strain Rate Technique) method. The CSRT tests with the notch tip radius of 0.1, 0.25 or 0.8 mm were carried out on the high strength steel having 1300 MPa in tensile strength. Based on the probabilistic and statistical evaluation of the CSRT test results, the specimen with the notch root radius of 0.25 min gives stable and average results of all experiments. Moreover, this notch geometry has the comparable stress concentration factor to the bottom of the actual bolt screw and easily machined with high accuracy. From these points of view the 0.25 fruit radius notch is Considered to become the standard specimen geometry. The scattering of delayed fracture characteristics was evaluated by applying the P-S-N method Of fatigue test and the P-S-H method was demonstrated.

Hydrogen desorption behaviors of pure iron with a body-centered-cubic (bcc) lattice and Inconel 625 with a face-centered-cubic (fcc) lattice were examined during tensile deformation using a quadrupole mass spectrometer in a vacuum chamber integrated with a tensile testing machine. Hydrogen and water desorption was continuously detected simultaneously under the application of a tensile load and strain to the specimens. Hydrogen desorption promoted by tensile deformation can be found by deducting both fragment hydrogen dissociated from H₂O and H₂ desorbed under unloading from the total hydrogen desorption out of hydrogen-charged specimens during tensile deformation. Hydrogen desorption from hydrogen-charged specimens was detected under various strain rates of 4.2×10⁻⁵/s, 4.2×10⁻⁴/s and 4.2×10⁻³/s.<br>Hydrogen desorption did not increase under elastic deformation. In contrast, it increased rapidly at the proof stress when plastic deformation began, and reached its maximum, then decreased gradually for both pure iron and Inconel 625. This desorption behavior is considerable related to hydrogen dragging by dislocation mobility. The desorbed hydrogen contents promoted by tensile deformation were measured using thermal desorption analysis (TDA). The TDA results showed that the desorbed hydrogen content differed at each strain rate. The largest desorbed hydrogen content promoted by tensile deformation was 16% of the initial hydrogen content in pure iron with high hydrogen diffusion rate when it was deformed at a strain rate of 4.2×10⁻⁴/s. In contrast, that of Inconel 625 with low hydrogen diffusion rate was 9% of the initial hydrogen content when it was deformed at a strain rate of 4.2×10⁻⁶/s. This deference of the desorbed hydrogen content transported by dislocations depended on the balance between the hydrogen diffusion rate and moving dislocation velocity.

We present a systematic benchmark study on different numerical models for analyzing hydrogen thermal desorption spectra, by focusing on the adoption of the local equilibrium hypothesis in these models. We find that the direct numerical method of the full set of the extended mass conservation equations is only able to predict the experimental behavior of thermal desorption spectra for pure iron in the thin specimen limit, while other models incorporating the local equilibrium hypothesis fail to predict this behavior.

We have developed a numerical model to simulate the hydrogen desorption profiles for pure iron and eutectoid steel, which is obtained in thermal desorption analysis (TDA). Our model incorporates the equation of McNabb and Foster without the hydrogen diffusion term combined with the Oriani&apos;s local equilibrium theory. It is found that the present numerical model successfully simulates the hydrogen desorption profile using the concentration of hydrogen trapping sites which is inferred from experiments both for Pure iron and for eutectoid steel. We further verify the model by discussing the trapping site concentration and the effect of hydrogen diffusion.

The factor that plays the essential role in hydrogen-related failure has been examined for Inconel 625 and iron by means of tensile testing with interposed unloading and reloading with/without hydrogen charging. Aging at 30 degrees C or annealing at 200 degrees C was conducted during the unloaded stage in order to diffuse out hydrogen or to anneal out strain-induced defects. Hydrogen thermal desorption analysis was used to evaluate strain-induced defects that act as trapping sites of hydrogen. Fracture strain decreased in the initially hydrogencharged specimens even though hydrogen was absent at the late stage of straining. Annealing at 200 degrees C at the unloaded stage almost completely recovered the decrease in fracture strain. Enhancement of strain-induced defects by hydrogen and their involvement in degradation were revealed by means of hydrogen thermal desorption analysis. The results provide direct evidence of the primary role of vacancies rather than hydrogen itself in hydrogen degradation, and agree well with the hydrogen-enhanced strain-induced vacancy model with respect to the mechanism of hydrogen-related failure. (C) 2008 Published by Elsevier Ltd on behalf of Acta Materialia Inc.