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Study of optimum composition of extra high purity Nb-W alloy for reprocessing condition with boiling nitric acid

再処理沸騰硝酸環境でのNb-W系EHP合金の最適組成の研究

井岡 郁夫; 鈴木 潤; 木内 清; 中山 準平*

Ioka, Ikuo; Suzuki, Jun; Kiuchi, Kiyoshi; Nakayama, Jumpei*

再処理施設用材料として、現行ジルコニウムより優れた耐環境割れ性を有する超高純度Nb-W合金(Nb-W EHP)を開発している。Nb-W EHPは、溶接性に技術的な課題がある。そこで、Wの最適組成範囲を選定するため、W量の異なるNb-W EHPの溶接継手をTIG溶接に作製した。引張試験,腐食試験,シャルピー衝撃試験により、Nb-W EHPの化学的,機械的特性へのWの影響を調べた。Nb-W EHP溶接継手の腐食速度に及ぼすWの効果がほとんどなかった。Wの増加はNb-W EHPの引張強さを向上させた。構造材として重要な延性-脆性遷移温度(DBTT)は、Wの増加に従って増加した。溶接継手のDBTTは母材に比べ約150K高かった。得られた結果より、Nb-W EHPのW量は8wt%以下に調整する必要であることがわかった。

An Extra high purity Nb-W alloy (Nb-W EHP) that is superior to current Zr for cracking in severer environment is developed for reprocessing plants. Nb-W EHP has a technical limitation in weldability for practical use. To choose an optimum composition range of W, Nb-W EHP with different content of W were manufactured. Weld joint specimens were prepared by TIG welding. Tensile, corrosion and Charpy impact tests were carried out to examine the effect of W on chemical and mechanical properties of Nb-W EHP. There was little effect of W on corrosion rate of weld joint of Nb-W EHP. The increase in W improves ultimate tensile strength of Nb-W EHP. A ductile-brittle transition temperature (DBTT) of Nb-W EHP increased with an increase in W. The increase in DBTT is a problem as a structural material. Moreover, DBTT of the weld joint was high about 150K in comparison with the base metal. It is necessary to adjust W content to 8wt% or less in Nb-W EHP from the obtained results.

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