高温高圧純水環境下の600合金用被覆アーク溶接金属の応力腐食割れ感受性に及ぼすPの影響

抄録

The intergranular stress corrosion cracking (IGSCC) susceptibility of the shielded metal arc weld metals for Inconel alloy 600 has been investigated with particular reference to the influences of P and Nb contents on the basis of creviced bent beam (CBB) test in the pressurized hot water (corresponding to the service condition of boiling water reactor nuclear power plant). The IGSCC susceptibility of the weld metals in the as-welded state exhibited a tendency to increase with P content. When the post weld heat treatment (SR+LTA) of stress relief annealing at 893 K followed by aging at 673 K was applied, however, the influence of P content on the IGSCC susceptibility of weld metals depended on the Nb content; i.e., the IGSCC susceptibility of the weld metals containing 1.2mass%Nb was almost independent of P content, whereas the weld metal containing 4.4mass%Nb showed much decreased IGSCC susceptibility by increasing the P content from 0.012mass% to 0.031mass%. Since the double-loop electrochemical potentiokinetic reactivation (DL-EPR) test suggested that the grain boundary corrosion resistance of the weld metals of 4.4mass%Nb decreased with the rise in P content, the improved IGSCC susceptibility of the weld metals of 4.4mass%Nb and 0.031mass%P cannot be explained in terms of the corrosion resistance of the grain boundary. The weld metal containing 4.4mass%Nb was hardened significantly by the precipitation of γ' phase (Ni₃Nb) in interdendritic areas during the SR+LTA treatment. The increase in the hardness brought about by the SR+LTA treatment was depressed significantly by the rise in P content probably owing to the reduction in the precipitation area of γ' phase. It can be considered that the decreased hardness contributes to the improvement in the IGSCC susceptibility with increasing the P content, since the tensile stress applied to the specimen during the CBB test increases with the hardness.