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原子炉圧力容器鋼の微視組織に及ぼす溶接熱及び中性子照射の影響

Effects of weld heat and neutron irradiation on microstructure of reactor pressure vessel steels

勝山 仁哉 ; 鬼沢 邦雄 ; 蔵本 明*; 野沢 康子*; 外山 健*; 永井 康介*

Katsuyama, Jinya; Onizawa, Kunio; Kuramoto, Akira*; Nozawa, Yasuko*; Toyama, Takeshi*; Nagai, Yasuyoshi*

原子炉圧力容器用低合金鋼の溶接熱影響部(HAZ)の非均質性を特徴付ける因子として、溶接熱の影響、及び中性子照射効果について、それぞれ材料特性試験やSEM・EDX等、及び三次元アトムプローブ法と陽電子消滅法により調べた。その結果、粗粒HAZは、下部ベイナイト主体のため溶接後熱処理(PWHT)前後で高靭性である。一方、細粒HAZは、PWHT前の状態ではMAが含まれるため高強度・低靭性であるが、PWHTによりMAが消失して靱性は回復し、上部ベイナイト主体の母材と同等か低靭性を示すことがわかった。また、未照射の母材・HAZ再現材の炭化物及び粒界、照射材のマトリックス中のCu析出物及び照射欠陥に関して、不純物濃度や再現熱処理による系統的な差異はなかった。以上から、RPV鋼HAZは、局所的に母材と比較して同等かやや低い靭性を示す場合があるが、母材と比べて極端に照射脆化が進む可能性は低いと考えられる。

In order to characterize an inhomogeneity of heat affected zone (HAZ) in reactor pressure vessel (RPV) steels, effects of weld heat input and neutron irradiation have been investigated by SEM/EDX, 3D atom probe and positron annihilation methods. Coarse grain HAZ with/without post weld heat treatment (PWHT) showed higher toughness compared to base metal (BM) caused by mixed structures of lower-Bainite and Martensite. Fine grain HAZ before PWHT showed much higher strength and lower toughness due to Martensite-Austenite constituent (MA). After PWHT, FGHAZ toughness became similar to BM since MA disappeared. There are no clear differences among microstructures such as carbide, grain boundary for unirradiated HAZ materials and base metal, and Cu precipitation and irradiation damage for irradiated materials. Although HAZ materials of RPV steels might show equal or lower toughness compared to BM, irradiation embrittlement of HAZ materials would not become much larger than that of BM.

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