Evaluation of the thermal aging of δ-ferrite in austenitic stainless steel welds by electrochemical analysis

Abstract

Cr-segregation by spinodal decomposition and G-phase precipitation were observed in δ-ferrite of austenitic stainless steel welds thermally aged at 400 °C for up to 20,000 h. A reversion heat treatment (R-HT) at 550 °C for 1 h dissolved the Cr-segregation in the aged welds while some intermetallic precipitates were present. The double-loop electrochemical potentiokinetic reactivation (DL-EPR) analysis showed no significant differences among them. However, after selective etching of the austenite phase, the DL-EPR values of δ-ferrite phase steadily increased with aging time due to the growth of Cr-depleted regions by spinodal decomposition. The electrochemical behavior of δ-ferrite after R-HT condition was similar to that of unaged welds, indicating the intermetallic precipitates did not affect the corrosion resistance in this case. Overall, DL-EPR analysis of δ-ferrite phase provided better correlation with spinodal decomposition.

Influence of inconel interlayer on microstructural, mechanical and electrochemical characteristics in single-pass ATIG welding of dissimilar austenitic and duplex stainless steel

“…The tiny ferrite peaks observed in the interlayer-based weldment could come from the UMZ portion of the weldment (figures 6(b), (d)). These variations of δ ferrite in both the weldments can impact their mechanical and electrochemical properties [38].…”

Thermodynamic reasoning for colossal N supersaturation in austenitic and ferritic stainless steels during low-temperature nitridation

“…We specifically show that consideration of this, until now unrecognized phenomenon of in-situ spinodal decomposition during nitridation of ferritic or austenitic stainless steels, in calculating the N paraequilibrium solubility limit leads to a successful explanation of the reported experimental observations. It is interesting to note that significant evidence exists in literature for the occurrence of spinodal decomposition in delta-ferrite of austenitic stainless steels upon thermal aging 17 . Yet, the role of spinodal decomposition in determining the N paraequilibrium solubility limit in nitrided austenitic and ferritic stainless steels (nitrided at temperatures similar to that of the thermal aging treatments) has until now not been recognized.…”