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形態

抄録

Stress corrosion cracks (SCCs) occurring in the weld metal of a type 600 Ni base alloy during the creviced bent beam (CBB) test in pressurized pure water at 561 K have been characterized on the basis of orientation maps determined from electron back scattering pattern (EBSP) in order to distinguish the intergranular (IG) SCC from the interdendritic (ID) SCC. The specimen for the CBB test was cut from shielded metal arc weld metals of the Ni base alloy deposited using welding rods with C contents ranging from 0.03% to 0.11% and Nb contents from 1.23% to 4.39%. The CBB test was carried out on specimens in the as-welded state and those subjected to a post weld heat treatment (PWHT) consisting of tempering for 72 ks at 893 K and aging for 720 ks at 673 K. It turned out that most SCCs observed in all the weld metals except for those of high Nb content (0.076% C and 4.39% Nb) subjected to the PWHT were identified as IGSCC. Cracks identified on IGSCCs in the specimen of high Nb content receiving the PWHT were more than twice as deep as those regarded as transgranular (TG) and IDSCCs. Since the fractured surface of the IGSCC presented dendritic features, it was rather difficult to distinguish the IGSCC from the IDSCC based on the fracture morphology.