PSI - Issue 41

Paolo Ferro et al. / Procedia Structural Integrity 41 (2022) 430–438 Paolo Ferro et al. / Structural Integrity Procedia 00 (2019) 000–000

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Fig. 8. EDS line scan in FZ of as-welded (a) and PWHT joints (b,c).

According to experimental evidence, inter-dendritic corrosion (Fig. 9) is related to elements alloy segregation, say, Nb and Mo, occurring during the solidification of the weld pool. It is suggested that Nb-rich inter-dendritic regions of the solidification microstructure induced a rapid nanometric  -phase precipitation. This is supported by both the equilibrium diagram of Fig. 4b where the higher the Nb amount, the higher the  -phase thermodynamic stability, and previous literature where observations of  -phase formation along Nb-enriched inter-dendritic regions of the solidification microstructure in IN625 welds were carried out [Floreen et al., 1994; Xing et al., 2014; Ashtiani and Zarandooz, 2016]. In addition, precipitation simulations for different degrees of segregation, performed by Lindwall et al. (2019), showed that the larger the segregation, i.e., the richer the inter-dendritic regions are in Nb and Mo, the faster the  -phase (Ni 3 Nb) precipitation. Nanometric  -particles precipitation is in fact predicted to occur in Nb-enriched inter-dendritic regions by few minutes. The short-term PWHT at 980 °C, partially restored the corrosion resistance of the alloy due to a mild attempt at homogenization of the chemical composition. This is because of the short duration and low temperature of the applied PWHT not able to completely dissolve the supposed presence of nanometric  -phase (Fig. 4a). On the contrary, the temperature of 1050 °C resulted, despite the short duration of the heat treatment, effective in dissolving any detrimental secondary phases and homogenizing the