Takai Kenichi

Abstract The effect of hydrogen-enhanced strain-induced lattice defects formed during the incubation period on hydrogen embrittlement fracture of iron was clarified by quantitative evaluation using low-temperature thermal desorption spectroscopy (L-TDS) from −200 ºC. Tensile testing of iron specimens was conducted in solution with various concentrations of ammonium thiocyanate as a catalyst poison. Cathodic electrolysis was employed to establish conditions of low and high hydrogen content to examine the fracture characteristics of the iron specimens. The fracture elongation of the hydrogen-charged iron specimens was lower than that of the hydrogen-free specimens, although the elongation was the same regardless of the hydrogen content. In contrast, the flow stress during the deformation process increased with increasing hydrogen content. Specimens were prepared under the same hydrogen charging conditions and unloaded within a uniform elongation range. L-TDS was used to detect lattice defects with hydrogen re-charged as a probe under equilibrium conditions with dislocation cores and strain fields around the cores and vacancies in the specimens. The formation of vacancy-type defects was promoted in the presence of hydrogen during plastic deformation, and the extent of promotion was similar regardless of the hydrogen content. The concentration of hydrogen-enhanced strain-induced vacancies may thus affect the decrease in ductility due to the presence of hydrogen, and the hydrogen coordination number to its vacancies is responsible for the increase in flow stress.

Abstract The effects of aging treatments on the annihilation and accumulation behavior of hydrogen-enhanced strain-induced vacancies (HESIVs) formed in tempered martensitic steel were investigated. The vacancies were formed by applying plastic deformation in the presence of hydrogen. The tracer hydrogen content and peak temperature under various aging treatment conditions were measured using low-temperature thermal desorption spectroscopy (L-TDS). Aging treatments were performed in the absence and presence of hydrogen at 30 °C for 0, 2, 5, and 9 d. The spectra measured by L-TDS were divided into two peaks by using Gaussian curves: Peak 1H corresponding to hydrogen desorption from dislocations and Peak 2H corresponding to hydrogen desorption from vacancies. The amount of Peak 2H, i.e., the amount of vacancies decreased and the peak temperature of Peak 2H, i.e., clustered vacancy size increased with increasing aging time. The change in the amount and the peak temperature of Peak 2H was smaller than that in previous studies for pure iron. Furthermore, the change was greater for aging in the absence of hydrogen than for aging in the presence of hydrogen. Therefore, the impurities in the steel such as solid solute carbon and hydrogen probably stabilize vacancies, decrease the diffusion coefficient of vacancies, and then partially suppress the annihilation and accumulation of vacancies.

Abstract The effects of grain boundary characteristics and grain sizes on crack propagation of hydrogen embrittlement were investigated for pure iron. Specimens with different grain boundary characters such as the low- and high-angle grain boundaries and different grain sizes were charged with hydrogen and subjected to tensile testing at a tensile rate of 0.01 mm·min⁻¹. The fracture modes were observed using a field emission scanning electron microscope (FE-SEM). As a result, intergranular (IG) fracture was observed in the fine grain area and cleavage fracture was observed in the coarse grain area. In order to characterize the crystallography of hydrogen embrittlement cracks, the grain boundary character was analyzed using electron backscatter diffraction patterns (EBSD). The results revealed that the secondary cracks originated and propagated on the high-angle grain boundaries in the fine grain area. Whereas, the fracture surface was parallel to {001} cleavage plane in the coarse grain area. In addition, the specimens were strained plastically to determine the effect of dislocations on hydrogen embrittlement cracks. The IG fracture appeared in the fine grain area, while the QC fracture parallel to {011} slip plane appeared in the coarse grain area. The location of the secondary crack and the effect of dislocations suggest that the binding energies between hydrogen and lattice defects are related to crack propagation passes.

Abstract The tensile speed dependence of crack initiation and propagation behavior in hydrogen embrittlement fracture was investigated for a tempered martensitic steel with plate-like carbides on prior austenite (γ) grain boundaries. Notched specimens with a stress concentration factor of 2.8 charged with hydrogen of 0.2 mass ppm were tensile tested at tensile speeds of 0.001 and 0.1 mm/min. Fractured surfaces and specimen sides in which the internal crack propagation was arrested by unloaded immediately upon reaching the maximum stress in tensile tests were observed using a scanning electron microscope. At 0.001 mm/min, intergranular (IG) fracture was dominant at the notch tip on the fracture surface. A crack initiation and propagation were observed in and on the prior γ grains at the notch tip, respectively. At a site away from the notch tip, discontinuous crack initiation and propagation on grain boundaries were observed. In contrast, at 0.1 mm/min, quasi-cleavage (QC) fracture was dominant at the notch tip on the fracture surface. A crack initiated and propagated only in the prior γ grains at the notch tip. These findings indicate that even with the same hydrogen content, at lower tensile speeds, there is an increase in the hydrogen concentration on the prior γ grain boundaries, leading to decohesion of prior γ grain boundaries. In contrast, at higher tensile speeds, the involvement of plastic deformation may be significant. Therefore, the mechanism of crack initiation and propagation in hydrogen embrittlement fracture probably depends on tensile speeds.

Abstract Hydrogen embrittlement susceptibility (HES) and morphologies of hydrogen-related fracture for ferrite-martensitic dual-phase (DP) steel sheets and transformation-induced plasticity (TRIP) steel sheets with the tensile strength of 1180 MPa class were investigated. HES was evaluated with fracture displacement obtained by three-point bending tests at a constant displacement speed. Fracture morphologies were observed using a field-emission scanning electron microscope. Specimens were electrochemically precharged with three different hydrogen charging conditions, and then three-point bending tests were carried out simultaneously with hydrogen charging under the same conditions as the precharging. The results indicated that HES for both kinds of steel sheets markedly increased with increasing the hydrogen content. The fracture displacements were different between the DP and TRIP steels, indicating that the HES varied based on the microstructure. The quasi-cleavage (QC) fracture was observed on the compression side and tension side for the DP steel. The area of QC fracture surface on the compression-side for the DP steel increased with increasing hydrogen content. Whereas, for the TRIP steel, QC fracture was observed in the whole area of the fracture surface regardless of hydrogen content, unlike the case of DP steel. These findings indicate that differences in the crack initiation sites and propagation paths probably cause the differences in HES and fracture morphologies.

Hydrogen embrittlement (HE) is increasingly becoming a critical issue for using high-strength steels in the automotive and infrastructure industries. To overcome the risk posed by HE of structural components under a hydrogen uptake environment in long-term service, it is necessary to clarify the mechanism of HE. In the present study, the presence of hydrogen-enhanced strain-induced vacancies (HESIVs)—one type of defect associated with proposed HE mechanisms—was validated by low-strain-rate tensile tests with in-situ electrochemical hydrogen charging for tempered martensitic steel showing quasi-cleavage fracture with a tensile strength. The effect HESIVs on the mechanical properties of tempered martensitic steel was also studied. The combined use of low-temperature thermal desorption spectroscopy and tensile tests led to the following observations: (i) hydrogen enhanced the accumulation of vacancy-type defects under plastic strain, (ii) accumulated vacancy-type defects adversely affected the ductility of the tempered martensitic steel after hydrogen release, and (iii) aging at 150 °C after applying a given plastic strain with hydrogen charging decreased the amount of newly formed vacancy-type defects and resulted in recovery of ductility.

<title>Abstract</title>We obtained thermal desorption spectra of hydrogen for a small-size iron specimen to which strain was applied during charging with hydrogen atoms. In the spectra, a shoulder-shaped peak in the high-temperature side was enhanced compared with the spectra of the specimen to which only strain was applied. We also observed that the peak almost disappeared by aging processes at ≥ 373 K. Then, assuming that the shoulder-shaped peak results from hydrogen atoms released by vacancies, we simulated the thermal desorption spectra using a model incorporating the behavior of vacancies and vacancy clusters. The model considered up to vacancy cluster <inline-formula><alternatives><tex-math>$${ { V_9 } }$$</tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>V</mml:mi> <mml:mn>9</mml:mn> </mml:msub> </mml:math></alternatives></inline-formula>, which is composed of nine vacancies, and employed the parameters based on atomistic calculations, including the H trapping energy of vacancies and vacancy clusters that we estimated using the molecular static calculation. As a result, we revealed that the model could, on the whole, reproduce the experimental spectra, except two characteristic differences, and also the dependence of the spectra on the aging temperature. By examining the cause of the differences, the possibilities that the diffusion of clusters of <inline-formula><alternatives><tex-math>$${V_2}$$</tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>V</mml:mi> <mml:mn>2</mml:mn> </mml:msub> </mml:math></alternatives></inline-formula> and <inline-formula><alternatives><tex-math>$${V_3}$$</tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>V</mml:mi> <mml:mn>3</mml:mn> </mml:msub> </mml:math></alternatives></inline-formula> is slower than the model and that vacancy clusters are generated by applying strain and H charging concurrently were indicated.