2009 Volume 27 Issue 3 Pages 247-260
The influence of Nb content on the SCC (Stress Corrosion Cracking) susceptibility of Inconel alloy 600 weld metals with C contents of 0.03—0.07mass% has been investigated, since in the recent nuclear power plant there is a tendency to increase the Nb addition to the weld metal to avoid the occurrence of SCC caused by the grain boundary depletion of Cr due to the preferential precipitation of Cr carbides. The SCC susceptibility of the weld metal was evaluated from the maximum depth and number of cracks occurring during the CBB (Creviced Bent Beam) test in high temperature pressurized water using plate specimens cut from shielded metal arc weld metals. For the weld metal specimens in the as-welded state, the Nb and C contents had only slight influences on the SCC susceptibility. When the specimen received a heat treatment consisting of stress relief annealing (SR) for 72 ks at 893 K and subsequent ageing (LTA) for 720 ks at 673 K, however, the significant influences of the increases in C and Nb contents on the SCC susceptibility were observed; the susceptibility of weld metals with higher C contents (∼0.07mass%) decreased with increasing the Nb content up to ∼2.6mass%, but a further increase in the Nb content enhanced the SCC susceptibility remarkably. The decreased SCC susceptibility with an increase in Nb content (less than 2.6mass%) observed in the weld metal of the higher C content can be explained as resulting from the suppression of Cr depletion layer at the grain boundary due to the formation of stable Nb carbides, since TEM-EDS analyses revealed that no Cr depletion at the grain boundary occurred at Nb contents of 2.6mass% or more. When the weld metal of the higher C content was subjected to the SR+LTA treatment, the hardness increased remarkably with the Nb content, suggesting that the higher stress was applied to the specimen during the CBB test, as the Nb content was increased. This increase in the applied stress is a possible factor that contributes to the increase in the SCC susceptibility with the Nb content in the weld metal free from the grain boundary depletion of Cr. For the weld metals with lower C contents (∼0.03mass%), the SCC susceptibility decreased with the Nb content after the SR+LTA treatment, probably owing to the suppression of the Cr carbide precipitation at the grain boundary and resulting Cr depletion.