Weldability testing to understand composition effects on eutectic backfilling in Ni-30Cr alloys

Abstract

Using Ni-30 wt% Cr in nuclear power reactors requires compatible fillers to ensure reliable welded joints. The high chromium content provides resistance to stress corrosion cracking (SCC), and niobium (Nb) additions have been made to Ni-30Cr filler metals to provide resistance to ductility dip cracking (DDC). Previous work suggested that increasing Nb from 2–4 to 6–8 wt% reduces cracking susceptibility due to crack healing supported by eutectic backfilling. Additional reduction in solidification cracking is realized when Mo additions are made in conjunction with the higher Nb levels. Based on qualitative evidence that the composition of the eutectic liquid changes grain boundary wetting characteristics and promotes backfilling, the current study was designed to quantify this behavior using a Varestraint-M test (adaptation of the original Varestraint test). The results suggest that when the Nb-rich eutectic also contains Mo, the liquid is more effective in wetting grain boundaries and promoting backfilling. This change in backfilling behavior is attributed to grain boundary wetting variability based on eutectic composition. A better fundamental understanding of the effect of Mo on grain boundary wetting of the Nb-rich eutectic phase will potentially facilitate the development of filler metals that rely on eutectic healing (or backfilling) to provide resistance to solidification cracking.