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

ABSTRACT: Cr-segregation by spinodal decomposition and G-phase precipitation were observed in ?-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 ?-ferrite phase steadily increased with aging time due to the growth of Cr-depleted regions by spinodal decomposition. The electrochemical behavior of ?-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 ?-ferrite phase provided better correlation with spinodal decomposition.

SUBMITTER: Obulan Subramanian G

PROVIDER: S-EPMC6179992 | biostudies-literature | 2018 Oct

REPOSITORIES: biostudies-literature

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

Obulan Subramanian Gokul G Kong Byeong Seo BS Lee Ho Jung HJ Jang Changheui C

Scientific reports 20181010 1

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 ...[more]

PMID: 30305663

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Project description:We describe here the relationship between grain structure, deformation mechanism and fracture characteristics in an austenitic stainless steel. This was accomplished using the novel concept of phase reversion that enabled a wide range of grain size from nanograined/ultrafine grained (NG/UFG) to coarse-grained (CG) regime to be obtained in a single material through change in temperature-time annealing sequence. In the NG/UFG structure, a marked increase in abundance of stacking faults (SFs) and twin density with strain was observed that led to a decrease in the average spacing between adjacent SFs, thus converting stacking faults into twins. Twinning in NG/UFG structure involved partial dislocations and their interaction with the grain boundaries, including SF overlapping and the coordinated nucleation of partial dislocations from the grain boundaries. The plastic zone in the NG/UFG structure resembled a network knitted by the intersecting twins and SFs. With SFE ~30 mJ/m2, the minimum stress for twin nucleation was ~250?MPa for the experiment steel and the corresponding optimal grain size (dop) wa ~120?nm. In contrast, in the CG structure, strain induced martensite formation was the deformation mechanism. The difference in the deformation mechanism led to a clear distinction in the fracture behavior from striated fracture in high strength-high ductility NG/UFG alloy to microvoid coalescence in the low strength-high ductility CG counterpart. The underlying reason for the change in fracture behavior was consistent with change in deformation mechanism from nanoscale twinning in NG/UFG alloy to strain-induced martensite in the CG alloy, which is related to change in the stability of austenite with grain size. An analysis of critical shear stress required to initiate twinning partial dislocations in comparison to that required to nucleate shear bands is presented. The appearance of striated fracture in the NG/UFG alloy suggests a quasi-static step wise crack growth process.

Dataset Information

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

Publications

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

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