Carburization and decarburization behavior of Grade 91 ferritic-martensitic steel in liquid sodium environments

This paper presents a study of carbon transfer and its effect on microstructure and tensile properties of Grade 91 (G91) ferritic-martensitic steel exposed to sodium at 550-650 ?. Sodium exposure tests were conducted in Argonne's forced convection sodium loops up to exposure times of similar to 40,000 h. Thermal aging study of G91 steel was conducted in parallel to isolate the thermal aging effect from the sodium effect. It was found that sodium exposures at 650 ? dissolved M23C6 carbides, eliminated the martensite subgrain structure resulting in excessive grain growth and reduced the tensile strength by > 50%, while sodium exposures at 550 and 600 ? had an insignificant effect on its microstructure and tensile properties. These effects were attributed to the carburization/decarburization process of G91 steel in sodium environments. Carbon concentrations in sodium were determined by a foil equilibration method. The estimated carbon concentration was in the range of 0.8-1.2 ppm in the SMT-1 loop and 0.3-0.7 ppm in the SMT-2 loop. Thermodynamic analysis of the carburization - decarburization process was conducted for G91 steels exposed in sodium environments. The carbon activity-concentration relationship for G91 was evaluated by considering four phases in G91, i.e. bcc ferrite, M23C6, NbC and VC carbides. It was found that the carburization-decarburization process in G91 steel was dictated by M23C6 carbides at high carbon activities, while NbC and VC carbides dominated the process at low carbon activities. The calculated carburization-decarburization boundary showed that G91 would undergo decarburization at 650 ? and carburization at 550 ? in the sodium loop environments, which was consistent with our experimental observations. This experimental and theoretical analysis provided a basis for predicting the effect of carbon transfer on the integrity of reactor components in sodium environments and for the design of new alloys used in sodium-cooled fast reactors.(c) 2022 Elsevier B.V. All rights reserved.

Automated summary

This study investigates the carbon transfer and its impact on the microstructure and tensile properties of Grade 91 (G91) ferritic-martensitic steel in sodium environments. Experimental and theoretical analyses reveal the carburization-decarburization process of G91 steel and its effects on grain growth and strength reduction. The findings provide important insights for predicting the integrity of reactor components and designing new alloys in sodium-cooled fast reactors.