Weldability Studies of High-Cr, Ni-Base filler metals for power generation applications

Abstract

The solidification behaviour and weld solidification cracking susceptibility of high-Cr, Ni-base filler metals that are widely used, or proposed for use, in the nuclear power industry have been investigated. Two heats of ERNiCrFe-13 (filler metal 52MSS), one heat of ERNiCrFe-7A (filler metal 52M), and one heat of a modified ERNiCr-3 (filler metal 82 with higher Cr content, designated here as filler metal 52i) have been tested using both the Transvarestraint test and the Cast Pin Tear test (CPTT). The solidification behaviour in these alloys has been studied by a newly developed procedure that accurately replicates the solidification process in fusion welds of Ni-base alloys and is based on the patented technique for Single Sensor Differential Thermal Analysis (SS DTA™). Results of the solidification studies showed that filler metal 52i has the widest solidification range, followed by the two heats of filler metal 52MSS, and filler metal 52M. The filler metal 52i also has the widest eutectic temperature range. The interdendritic eutectic constituent formed in weld metal of this filler metal and filler metal 52MSS is enriched in Nb and results from the eutectic reaction of γ + L → γ + NbC at the end of solidification. Both the CPTT and the Transvarestraint test provided the same ranking of solidification cracking susceptibility among these filler metals. Both heats of 52MSS and the heat of 52i were found to be more susceptible to solidification cracking than filler metal 52M. The slightly higher resistance to solidification cracking of filler metal 52i relative to the 52MSS filler metals is attributed to crack “healing” during the final stages of solidification. This is the result of the higher fraction of eutectic liquid of filler metal 52i, as confirmed by metallographic studies. The results of this study confirm the higher solidification cracking susceptibility of high-Cr, Ni-base filler metals that contain higher Nb levels to counteract ductility-dip cracking, relative to filler metals that are Nb-free. This study has also shown that the CPTT can be used as an alternative, and reliable, tool for ranking the solidification cracking susceptibility of high-Cr, Ni-base filler metals proposed for use in nuclear power plants and other applications.