PNNL

Abstract

Cracks on the primary-water side of alloy 182 safe-end nozzle weldments from Ringhals-3 and -4 pressurized-water reactors have been characterized by scanning electron microscopy (SEM) and analytical transmission electron microscopy (ATEM). The goal of this work was to investigate the causes of service failures in as-welded and weld-repaired regions. SEM observations showed that the cracks occurred along high-angle crystallographic boundaries in the weld metal. X-ray mapping in the SEM revealed cored structures with segregated Mn and coincident Nb borocarbide precipitates throughout the grain interiors, but no significant segregation or precipitation on the boundaries where cracking occurred. High-resolution ATEM observations confirmed the general lack of segregation or precipitation on grain boundaries leading the cracks. Fine, intergranular MC-type carbides were found in places, but not in a continuous distribution that would impact cracking behavior. High dislocation densities in the metal along cracks, duplex corrosion structures of Ni-rich metal and Cr-rich oxide along narrow cracks, the lack of detectable impurities at the crack tips were consistent with stress-corrosion cracking in primary-water. The fact that significant compositional changes were not found at boundaries where cracks propagated makes it unlikely that solidification hot cracking is the root cause for this degradation.