Irradiation effects on precipitation and its impact on the mechanical properties of reduced-activation ferritic/martensitic steels

照射下析出挙動とその低放射化フェライト鋼の強度特性に対する影響

谷川 博康; 酒瀬川 英雄   ; 橋本 直幸*; Klueh, R. L.*; 安堂 正己; Sokolov, M. A.*

Tanigawa, Hiroyasu; Sakasegawa, Hideo; Hashimoto, Naoyuki*; Klueh, R. L.*; Ando, Masami; Sokolov, M. A.*

代表的な低放射化フェライト鋼(F82H, JLF-1, ORNL9Cr, Niドープ材)は、300C 5dpaの中性子照射により、異なる照射硬化,照射脆化を示すことがわかっている。転位組織に大きな違いがないことから、これらを対象として照射後鉄鋼材料の析出物解析を行った。報告済みの抽出残渣解析に加えて、抽出レプリカ試料、及びTEM薄膜試料を作成し、析出物のサイズ分布、及び構造についての情報を得ることができた。その結果、照射硬化が大きな鋼(ORNL9Cr, Niドープ材)では、微細析出物の増加が顕著であった。一方、照射硬化が小さかったJLF-1では小さな析出物が消滅し、析出物が成長していることがわかった。析出物の主たる析出サイトであるパケットサイズを基準として、析出物量変化に対する硬化量を整理したところ、単純な相関関係にのることがわかった。また、上記の相関関係は、ホールペッチ則によって説明できることが示された。なお、本研究は核融合研究開発における日米協力計画として実施された。

It was previously reported that reduced-activation ferritic/martensitic steels (RAFs), such as F82H-IEA and its heat treatment variant, ORNL9Cr-2WVTa, JLF-1 and 2%Ni-doped F82H, showed a variety of changes in ductile-brittle transition temperature (DBTT) and yield stress after irradiation at 573K up to 5dpa. These differences could not be interpreted solely as an effect of irradiation hardening caused by dislocation loop formation. To address these observations, the precipitation behavior of the irradiated steels was examined by weight analysis, X-ray diffraction analysis and chemical analysis on extraction residues. The results suggested that irradiation affects precipitation as if it was forced to reach the thermal equilibrium state at irradiation temperature 573K, which usually never be achieved by aging. The details of precipitates in the irradiated RAFs were examined to determine their impact on the mechanical properties, which obtained by tensile, Charpy impact, and bend bar toughness tests. Transmission electron microscopy was performed on thin films and extraction replica specimens to analyze the size distribution, chemical composition and crystal structure of precipitates. It turned out that the hardening level normalized by square root of average packet size showed a linear dependence on the increase of extracted precipitate weight. This dependence suggests that the difference in irradiation hardening between RAFs was caused by the different precipitation behavior on packet, block and prior austenitic grain boundaries during irradiation. The simple Hall-Petch law could be applicable to interpret this dependence. Detailed analytical results will be presented and their interpretation discussed.