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

Abstract

Cr-segregation by spinodal decomposition and G-phase precipitation were observed in delta-ferrite of austenitic stainless steel welds thermally aged at 400 degrees C for up to 20,000 h. A reversion heat treatment (R-HT) at 550 degrees 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 delta-ferrite phase steadily increased with aging time due to the growth of Cr-depleted regions by spinodal decomposition. The electrochemical behavior of delta-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 delta-ferrite phase provided better correlation with spinodal decomposition.