Fracture toughness evaluation of weld-HAZ in RPV steel using Mini-C(T) specimens

RPV鋼溶接継手HAZのMini-C(T)試験片を用いた破壊靭性評価

河 侑成  ; 下平 昌樹 ; 勝山 仁哉  

Ha, Yoosung; Shimodaira, Masaki; Katsuyama, Jinya

国内の原子炉圧力容器(RPV)の溶接継手熱影響部(継手HAZ)は監視試験対象部位である。継手HAZ内には非均質に金属組織が分布し、破壊靭性のばらつきが大きいと考えられるものの、HAZ試験片の採取位置における金属組織の違いが機械的特性及び照射脆化感受性に及ぼす影響は詳細に調べられていなかった。本研究では継手HAZの非均質な組織に着目し、まず未照射材について溶接金属と母材の境界からの距離に応じて3箇所(0.5mm, 1mm及び2mm)の継手HAZ、及び母材から採取した微小試験片(Mini-C(T)試験片)を用いて破壊靭性を評価した。その結果、HAZの破壊靭性は母材と比べてばらつきが大きく、特に溶接金属と母材の境界から0.5mmのHAZにおいて破壊靭性のばらつきが最も大きい傾向を示した。HAZに対する破壊靭性参照温度は採取した3箇所の中で有意な差が得られたものの、いずれの破壊靭性参照温度も母材より低く、破壊靭性値が高いことを確認した。本研究結果により、未照射状態では母材が継手HAZを代表して破壊靭性を保守的に評価できることを確認した。

Heat-affected zone (HAZ) produced by butt-welding in reactor pressure vessel (RPV) steel is one of the representative materials for surveillance program. The fracture toughness values of HAZ may show a large uncertainty due to inhomogeneous metallurgical structures. Also, the inhomogeneous microstructure in HAZ may influence on the degree of uncertainty in the fracture toughness and the sensitivity to irradiation embrittlement. We investigated the fracture toughness in HAZ of unirradiated material with respect to its distance from the fusion line of welds, where the amount of mixed microstructures change due to the thermal history during the welding. Mini-C(T) specimens of HAZ were harvested from the crack position at 0.5 mm, 1 mm and 2 mm from the fusion line of welds. The uncertainty of fracture toughness in HAZ, from the fusion line at 0.5 mm in particular, was larger than those of base metal at a quarter thickness. From the results of fracture toughness evaluation considering the standard deviation, there was the difference of reference temperature, in each position of HAZ. in all positions of HAZ was significantly lower than that of base metal, which means the fracture toughness in HAZ was greater than that of base metal at a quarter thickness.