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

Journal Article · Fri Jun 17 00:00:00 EDT 2022 · Journal of Nuclear Materials

DOI:https://doi.org/10.1016/j.jnucmat.2022.153870· OSTI ID:1909641

Li, Meimei ^[1]; Zeng, Zuotao ^[1]; Chen, Wei-Ying ^[1]

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1. Argonne National Lab. (ANL), Argonne, IL (United States). Nuclear Science and Engineering Division

Here, 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 °C. Sodium exposure tests were conducted in Argonne's forced convection sodium loops up to exposure times of ~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 °C 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 °C 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 °C and carburization at 550 °C 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.

• Research Organization: Argonne National Laboratory (ANL), Argonne, IL (United States)
• Sponsoring Organization: USDOE Office of Nuclear Energy (NE); USDOE Office of Science (SC), Basic Energy Sciences (BES)
• Grant/Contract Number: AC02-06CH11357
• OSTI ID: 1909641
• Journal Information: Journal of Nuclear Materials, Journal Name: Journal of Nuclear Materials Vol. 568; ISSN 0022-3115
• Publisher: Elsevier
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
Decarburization and carburization
G91 ferritic-martensitic steel
Microstructure
Sodium
Tensile properties
Thermodynamics