SUPPLEMENT TO THE WELDING JOURNAL, JULY 2016 Sponsored by the American Welding Society and the Welding Research Council Solidification Cracking Susceptibility of Ni30Cr Weld Metals with Variable Niobium and Molybdenum A crack healing effect was observed with additions of Nb above 4 wt% Introduction Nuclear power generation, petrochemical, and many other industries rely on high chromium (25–30 wt-%), Ni-based alloys due to the corrosion and stress corrosion cracking resistance imparted by the high chromium content, as well as good mechanical properties at elevated temperatures. Other applications include the handling of nitric/hydrofluoric acid solutions and coal-gasification units (Ref. 1). In efforts to combat the observed ductility dip cracking (DDC) that can occur when these alloys are welded, alloying additions such as niobium and molybdenum have been introduced, but may result in an increased susceptibility to solidification cracking due to a widened solidification temperature range. Compatible high-chromium welding filler metals such as 152 (ENiCrFe- WELDING RESEARCH 7) and 52 (ERNiCrFe-7) were developed for welding these high-Cr alloys, such as Alloy 690. Niobium and molybdenum have been added to this class of filler metal, in some cases to reduce susceptibility to DDC (Refs. 2–7). While this approach has been quite successful in mitigating DDC, increased susceptibility to solidification cracking has been reported, as the solidification temperature range expands and supports a eutectic reaction at the end of solidification (Refs. 6–8). It is well documented that niobium additions in Ni-based alloys result in an interdendritic eutectic reaction consisting of (fcc) and Nb-rich phases such as NbC and Laves (A2B) at the end of solidification, but little research has focused on additions in Alloy 690, due to a greater interest in corrosion resistance of the alloy (Refs. 9–12). Niobium is a common addition of choice for improving weldment quality in terms of increasing room temperature strength as well as increasing corrosion resistance, but the effect of Nb on weldability has been limited to maximum additions of up to 5 wt-% (Ref. 12). Reports regarding molybdenum suggest that additions may increase the solidification temperature range and promote the formation of a low melting eutectic phase, similar to Nb (Refs. 7, 13). Molybdenum has been shown to improve resistance to DDC in Nb-bearing filler metals such as 52MSS (ERNiCrFe-13), and there is JULY 2016 / WELDING JOURNAL 229-s BY R. A. WHEELING AND J. C. LIPPOLD ABSTRACT Ni30Cr alloys such as Alloy 690 and compatible filler metals are used extensively in the nuclear power industry to avoid problems with primary water stress corrosion cracking (PWSCC). Depending on composition, these alloys can be susceptible to both solidification and ductilitydip cracking. In this study, the effect of niobium (Nb) and molybdenum (Mo) additions to a base Ni30Cr alloy (Alloy 690) on solidification cracking susceptibility was evaluated. Both computational and experimental techniques were used to determine the effect of composition modifications on solidification behavior and cracking susceptibility. Nb additions from 0 to 8 wt% and Mo additions from 0 to 4 wt% were evaluated. The addition of Nb resulted in an increase in the fraction eutectic that forms at the end of solidification, with 8 wt% Nb resulting in over 20 vol% eutectic constituents. The addition of Mo had virtually no effect on the fraction eutectic. The cast pin tear test (CPTT) was used to measure solidification cracking susceptibility. With this test, a peak in cracking susceptibility was observed at 4 wt% Nb. The addition of Nb above 4 wt% resulted in a decrease in cracking due to a crack healing effect. The addition of Mo resulted in a decrease in cracking susceptibility in the higher Nb alloys. Characterization performed using SEM/EDS verified the crack healing effect at high Nb levels and differentiated the eutectic constituents that formed along the solidification grain boundaries. KEYWORDS • Solidification Cracking • Stress Corrosion Cracking • Alloy 690 • Nuclear Industry • Filler Metal • Cast Pin Tear Test R. A. WHEELING and J. C. LIPPOLD are with the Welding Engineering Program, The Ohio State University, Columbus, Ohio.
Welding Journal | July 2016
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