Microstructural Evolution of 52M Weld Metal Near the Fusion Boundary and Oxide Films Formed in Simulated PWR Primary Water

Abstract

Environmentally assisted cracking of structural materials such as dissimilar metal welds in high temperature water environments is strongly affected by the surface oxide film formed in the operating environments. In the present work, Nickel-base 52M weld metal samples are taken from two dilution zones and undiluted zone, and the microstructure was analyzed. Oxide films formed on these samples immersed in simulated PWR primary water at 325oC for 100h. The films are duplex-layered with the outer layer consisted of larger and scattered particles, and the inner layer consisted of small and dense particles. With increasing the distance from FB, the number of larger particles of outer layer decreased while the appearance of smaller particle of inner layer changed into needle-liked particles. With the increase of distance, the proportion of needle-liked particles increase, but the larger particles became less angular. The inner layer became thinner while the size of larger particles of outer layer became smaller with the distance from FB increases. The outer layer was Fe-rich while the inner layer was Cr-rich. The change of element concentration resulted in different oxidization behavior in PWR primary water. The formation of larger particles of outer layer can influence the appearance of inner oxide film. Figure 1