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Crの影響

Abstract

The stress corrosion cracking (SCC) susceptibility of the SMAW metals for Inconel alloy 600 to which Cr was added to 14.8-21.4mass% has been investigated on the basis of CBB test in the pressurized hot water (corresponding to the service condition of BWR nuclear power plant), since the TIG weld metal of alloy 82 involving 18-22mass% Cr possesses much better resistance to SCC than the SMAW metal of alloy 182 (Cr content = 13-17mass%). When their Cr contents were increased to the same level as those of the alloy 82, the weld metals of alloy 182 sustained only slight SCCs in the as-welded state, and no crack was detected after the post weld heat treatment (SR+LTA) of stress relief annealing at 893 K followed by aging at 673 K. These results suggest that the higher Cr content of the alloy 82 is responsible for its higher resistance to SCC than that of the alloy 182. The Cr carbides precipitated at the grain boundary during the welding and the SR+LTA treatment were also changed from M7C3 type to M23C6 type with the increase in the Cr content. Though the Cr content at the grain boundary in weld metal containing 14.8mass%Cr subjected to the SR+LTA treatment was 3mass%, the Cr content of weld metal containing 18.5mass%Cr was not less than 10mass%. The addition of the Cr to the alloy 182 increased the Cr content in the grain boundary region, suggesting that the intergranular SCC can be suppressed when the Cr content at the grain boundary is not less than 10mass%. In addition to the carbide, Ni16(Mn, Cr)6Si7 (G phase) was precipitated at the grain boundary in the alloy 182 containing 18.5mass% Cr when the SR+LTA treatment was applied. TEM-EDS analyses suggested that the G phase was enriched in P, and so could decrease the P content in the grain boundary region. Probably, the decreased P content at the grain boundary due to the precipitation of G phase contributed to the enhancement of the SCC resistance of the Cr-added alloy 182 by the SR+LTA treatment.