PSI - Issue 41

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

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Paolo Ferro et al. / Structural Integrity Procedia 00 (2022) 000–000

chemical composition, giving the best result in term of corrosion resistance. Such explanation however should be supported by further investigations aimed at detecting the supposed nanometric  -phase precipitation in inter dendritic zones by TEM analysis.

Fig. 9. Selective inter-dendritic corrosive attack (sample underwent PWHT at 980 °C).

4. Conclusions Short-term induction PWHTs were carried out on Inconel 625 to restore the nominal corrosion resistance of the alloy. Two temperatures, say, 980 °C and 1050 °C, were investigated with a holding time of 20 min. Corrosion tests, according to ASTM G28 A, were performed and results were correlated with welded joints microstructure and alloy elements distribution. The most important results can be summarized as follows:  The welded joints microstructure showed different morphologies of secondary phases mainly identified as carbide rich in Nb and Mo.  Key-elements such as Mo and Nb tend to segregate in the inter-dendritic zones during solidification of the weld pool.  These compositional fluctuations producing regions of locally enhanced precipitation conditions are suggested to promote a rapid  -phase precipitation of nanometric size, making therefore these areas (inter dendritic) more sensitive to corrosion.  While for thermodynamic and kinetics reasons the PWHT at 980 °C was found not sufficient to restore the corrosion resistance of the alloy, the heat treatment at 1050 °C was able to homogenize the chemical composition and improve the chemical properties of the welded joints.  The supposed corrosion mechanism will be further investigated by TEM analysis with the aim at detecting nanometric secondary phases precipitation in the inter-dendritic zones.

References

Ashtiani, H.R.R. and Zarandooz, R. 2016. Int. J. Manuf. Technol. 84, 607–619.