Super ODS steels R&D for fuel cladding of next generation nuclear systems, 4; Mechanical properties at elevated temperatures

原子力システム高効率化に向けた高耐食性スーパーODS鋼の開発,4; 高温強度特性

古川 智弘  ; 大塚 智史   ; 井上 賢紀 ; 奥田 隆成*; 阿部 冨士雄*; 大貫 惣明*; 藤澤 敏治*; 木村 晃彦*

Furukawa, Tomohiro; Otsuka, Satoshi; Inoue, Masaki; Okuda, Takanari*; Abe, Fujio*; Onuki, Somei*; Fujisawa, Toshiharu*; Kimura, Akihiko*

われわれの研究グループでは、重金属冷却高速増殖炉及び超臨界水冷却高速増殖炉用の燃料被覆管として、優れた高温強度と高耐食性の両方を兼ね備えた酸化物分散強化型フェライト鋼(スーパーODS鋼)の開発を進めている。本報では、開発中のスーパーODS鋼の高温における機械的強度特性について評価した結果について報告する。

As fuel cladding material for lead bismuth-cooled fast reactors and supercritical pressurized water-cooled fast reactors, our research group has been developing highly corrosion-resistant oxide dispersion strengthened ferritic steels with superior high-temperature strength. In this study, the mechanical properties of super ODS steel candidates at elevated temperature have been evaluated. Tensile tests, creep tests and low cycle fatigue tests were carried out for a total of 21 types of super ODS steel candidates which have a basic chemical composition of Fe-16Cr-4Al-0.1Ti-0.35YO, with small variations. The testing temperatures were 700C (for tensile, creep and low cycle fatigue tests) and 450C (for tensile test). The major alloying parameters of the candidate materials were the compositions of Cr, Al, W and the minor elements such as Hf, Zr and Ce etc. The addition of the minor elements is considered effective in the control of the formation of the YAl complex oxides, which improves high-temperature strength. The addition of Al was very effective for the improvement of corrosion resistance. However, the addition also caused a reduction in high-temperature tensile strength. Among the efforts aimed at increasing high-temperature strength, such as the low-temperature hot-extrusion process, solution strengthening by W and the addition of minor elements, a remarkable improvement of strength was observed in ODS steel with a basic chemical composition of 2W-0.6Hf steel (SOC-14) or 2W-0.6Zr steel (SOC-16). The same behavior was also observed in creep tests, and the creep rupture times of SOC-14 and SOC-16 at 700C - 100MPa were greater than 10,000 h. The strength was similar to that of no-Al ODS steels. No detrimental effect by the additional elements on low-cycle fatigue strength was observed in this study. These results showed that the addition of Hf/Zr to ODS-Al steels was effective in improving high-temperature strength.