2019 Fiscal Year Research-status Report
その場原子空孔計測手法開発による鉄系材料の水素脆化支配欠陥の決定
| Project/Area Number |
18K13980
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| Research Institution | Chiba University |
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Principal Investigator |
Chiari Luca 千葉大学, 大学院工学研究院, 助教 (20794572)
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| Project Period (FY) |
2018-04-01 – 2021-03-31
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| Keywords | hydrogen embrittlement / defects / vacancy / hydrogen / iron / stainless steel |
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| Outline of Annual Research Achievements |
The behaviour of the hydrogen-related defects in austenitic stainless steels SUS 304 and SUS 316 was investigated by positron annihilation lifetime spectroscopy. The samples were subject to tensile testing and simultaneous hydrogen charging by the electrolytic cathode reaction method in order to induce hydrogen embrittlement in the materials.
In SUS 304 stainless steel, measurements were carried out in situ at various tensile strain values at the boundary between the elastic region and the initial plastic deformation region to study the defect formation process. The measurements revealed the formation of increasingly larger vacancy clusters already at the early stages of the plastic deformation, but only in the hydrogen-charged samples. These defects are believed to originate from the agglomeration of the vacancies after hydrogen desorption at room temperature and become the trigger of the brittle fracture, upon application of larger strain.
Measurements were also carried out on SUS 316 stainless steel, which has a higher Ni equivalent and γ-phase stability compared to SUS 304. However, in this case no formation of vacancy clusters was observed in the hydrogen-charged samples subject to 10% tensile strain. Only dislocations and monovacancies were detected with a larger concentration compared to the hydrogen-free samples. This is due to the stabilization effect of hydrogen on the vacancies generated by plastic deformation. Chemical etching of the 10 μm surface layer significantly reduced the defect concentration, confirming the promotion of vacancy formation by hydrogen.
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| Current Status of Research Progress |
Current Status of Research Progress
2: Research has progressed on the whole more than it was originally planned.
Reason
The current status of this research project is evaluated to be progressing rather smoothly. The implementation of the research in the second year can be considered to have been carried out on time with small changes compared to the original research plan, as described below.
During the second year, the research focused on the detection and identification of the dominant defects in the hydrogen embrittlement of austenitic stainless steels SUS 304 and SUS 316. The research on SUS 304 followed on from that started during the first year of the project, as the results obtained at the end of the first year looked very promising. The measurements on this target followed in detail the original research plan.
On the other hand, instead of SUS 316L, SUS 316 was chosen as the best next target to study the hydrogen embrittlement mechanism in austenitic stainless steels. This is due to the high Ni equivalent and hydrogen resistance of SUS 316, just like SUS 316L, but more similar elemental composition of the other impurities to that of SUS 304. It was thought that this would allow a more direct comparison of the defect dynamics to that in SUS 304. The measurement plan was also adapted to match the experiments on SUS 304.
The results obtained so far are very satisfactory and overall confirm the outcomes expected in the project proposal.
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| Strategy for Future Research Activity |
In the next third and last year of the project, the research will be focusing on the detection and identification of the dominant defects in the hydrogen embrittlement of pure iron and austenitic stainless steels. The detailed plans for the research project will be as follows.
First of all, time-resolved measurements of the defect dynamics during room temperature aging of hydrogen-charged pure iron using the MePS positron beamline at the Helmholtz-Zentrum Dresden-Rossendorf in Germany are planned. This experiment is similar to that described in the second year of the original research plan, but the measurements are conducted using a higher-intensity beamline at the same facility. Actually, this experiment was carried out around the end of the second year of this project. However, due to the large volume of data, the data analysis is still underway and hence the results will be considered part of the third year of this project.
Moreover, measurements for the detection and identification of the dominant defects in the hydrogen embrittlement of austenitic stainless steel SUS 316 will be continued from the second year. The feasibility of measurements at higher tensile strain values to investigate the defect dynamics closer to the fracture is currently being considered.
Finally, if the time is enough, the feasibility of measurements on other austenitic stainless steels such as SUS 316L will be considered.
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| Causes of Carryover |
The reason for the carryover amount of this fiscal year lies in the presence of a carryover from the previous fiscal year. The amount allocated for this fiscal year has been almost entirely spent as planned. Hence, the carryover of this fiscal year amounts to almost the same amount as the carryover of the previous fiscal year.
The carryover amount from the previous fiscal year was planned to be used in the current fiscal year as travel expenses and personnel expenditure and remuneration to the host institution for the experiment to be carried out at the HZDR facility overseas. However, given that the beamtime was allocated at the very end of the fiscal year and the usage of the HZDR facilities turned out to be free of charge, no expenditure for this experiment was deducted from the available amount of this grant in this fiscal year.
The amount carried over to the next fiscal year is planned to be used to cover equipment and consumable costs, such as new detectors or digital storage oscilloscope. It will also be used to disseminate the research results, as travel expenses to attend international and domestic conferences scheduled in the next fiscal year.
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Research Products
(13 results)
