Effect of composition on grain boundary wetting characteristics in Ni-30Cr weld metal

Abstract

The use of Ni-30Cr components in nuclear power reactors requires a compatible filler metal to ensure reliable welded joints associated with reactor vessels, steam generators, and pressurizers. The high chromium content provides resistance to stress corrosion cracking (SCC), and niobium (Nb) additions provide ductility dip cracking (DDC) resistance. Previous work showed that solidification cracking susceptibility increases with 2–4 wt% Nb and decreases with 6–8 wt% Nb. This decrease is due to a eutectic backfilling phenomenon which “fills” freshly formed cracks when sufficient low-melting eutectic liquid is present upon solidification. A reduction in cracking susceptibility was also seen when molybdenum (Mo) was added in conjunction with high Nb levels, but the mechanism behind this effect is unknown. The objective is to address the potential effect Mo additions have on eutectic grain boundary wetting to better understand how crack healing may change with composition variance. Since Mo additions have no apparent effect on the fraction eutectic upon solidification, it is postulated that Mo may increase the “wettability” of the eutectic liquid along grain boundaries, allowing crack backfilling to occur more efficiently. Potentially, Mo additions may support more complete crack healing, thereby a further cracking susceptibility reduction, when the fraction eutectic is sufficiently high. A preliminary, qualitative approach was taken to induce local remelting and wetting of eutectic in weld metal samples with and without Mo additions. The results suggest that Mo changes the eutectic remelting and/or wetting characteristics to some degree. Fundamentally understanding molybdenum’s role in eutectic grain boundary wetting could facilitate the development of filler metals that rely on eutectic healing via backfilling to provide solidification cracking resistance.