高井 健一

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.

The local plastic strain evolution associated with crack growth in hydrogen-assisted quasi-cleavage fracture was investigated using tempered lath martensitic steels. The quasi-cleavage crack grew via the following process. After crack initiation, the crack grew sharply to a certain length by repeated episodes of nano-void nucleation and coalescence. When the sharp crack tip intersected microstructural boundaries such as a lath or block, crack deflection/branching occurred. This was followed by crack tip blunting, which temporarily stopped crack growth. Further crack growth was possible via one of the following two routes: (1) sharp crack initiation/growth from the blunt crack tip, and (2) new crack initiation near the blunt crack tip. The newly formed cracks subsequently coalesced. Repetition of this multi-scale crack growth mechanism finally caused a quasi-cleavage fracture. Correspondingly, hierarchical crack morphologies were observed, which coincided with the lath martensitic microstructures and fractographic features. Furthermore, specific correlations were found between hydrogen-assisted cracking behavior and local plastic strain evolution at different spatial scales. Specifically, the largest plastic strain evolution occurred in the region where crack coalescence was observed. The second largest plastic strain evolution occurred when crack tip blunting occurred. Nanoscale local plasticity evolution around a sharp crack was also observed as an appearance of intense slip bands, indicating that the local plasticity played a key role in the hydrogen-related sharp crack growth.

<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.

© 2019 Hydrogen Energy Publications LLC The effects of martensitic transformation on microstructural hydrogen distribution in a multiphase transformation-induced plasticity (TRIP) steel were investigated using silver decoration before and after cooling and cryogenic thermal desorption spectroscopy (C-TDS) during cooling. Transformation-induced hydrogen effusion occurred; however, there was a significant difference between the temperature of the peak hydrogen desorption rate and the start temperature of the martensitic transformation, in contrast to the findings of a previous study on fully austenitic steels. Furthermore, the silver-decoration experiment clarified that martensitic transformation caused a significant increase in local hydrogen flux and accordingly enhanced the heterogeneity of the hydrogen distribution, which originates from the transformation-induced hydrogen effusion observed in the C-TDS analysis.

Though intergranular (IG) and quasi-cleavage (QC) fractures have been widely recognized as typical fracture modes of the hydrogen-induced cracking in high-strength steels, the main factor has been unclarified yet. In the present study, the hydrogen content dependence on the main factor causing hydrogen-induced cracking has been examined through the fracture mode transition from QC to IG at the crack initiation site in the tempered martensitic steels. Two kinds of tempered martensitic steels were prepared to change the cohesive force due to the different precipitation states of Fe3C on the prior γ grain boundaries. A high amount of Si (H-Si) steel has a small amount of Fe3C on the prior austenite grain boundaries. Whereas, a low amount of Si (L-Si) steel has a large amount of Fe3C sheets on the grain boundaries. The fracture modes and initiations were observed using FE-SEM (Field Emission-Scanning Electron Microscope). The crack initiation sites of the H-Si steel were QC fracture at the notch tip under various hydrogen contents. While the crack initiation of the L-Si steel change from QC fracture at the notch tip to QC and IG fractures from approximately 10â€?μm ahead of the notch tip as increasing in hydrogen content. For L-Si steels, two possibilities are considered that the QC or IG fracture occurred firstly, or the QC and IG fractures occurred simultaneously. Furthermore, the principal stress and equivalent plastic strain distributions near the notch tip were calculated with FEM (Finite Element Method) analysis. The plastic strain was the maximum at the notch tip and the principle stress was the maximum at approximately 10â€?μm from the notch tip. The position of the initiation of QC and IG fracture observed using FE-SEM corresponds to the position of maximum strain and stress obtained with FEM, respectively. These findings indicate that the main factors causing hydrogen-induced cracking are different between QC and IG fractures.