Carburization behavior and tensile properties of superalloys Inconel 617 and Incoloy 800H for very-high-temperature gas-cooled reactors under graphite dust-coated environment at 950 °C

• The Si element in Incoloy 800H exhibits a protective effect by suppressing carbon diffusion. • Revealed the difference in carburization susceptibility and failure mechanism of the two alloys. • Pre-oxidation treatment can effectively suppress carburization and restore the tensile properties of the alloy. The carburization mechanisms and changes in tensile properties of Inconel 617 and Incoloy 800H following a 100-hour corrosion exposure in an environment coated with graphite dust at 950 °C was studied in this paper. Through the microstructural analysis and mechanical property testing, it was discovered that Inconel 617 undergoes carbon atom inward diffusion, forming chromium/molybdenum carbide precipitates, which result in a reduction in ultimate tensile strength of 25.6 % and a decrease in total elongation of 64.3 %, leading to brittle fracture. In contrast, Incoloy 800H exhibits minimal degradation in mechanical properties due to the surface Si element suppressing carbon diffusion. Further research demonstrates that pre-oxidation treatment can form a dense Cr 2 O 3 layer on the surface of Inconel 617, reducing its carburization by 63.4 % and restoring its mechanical properties to levels close to the original. These findings provide critical guidance for material selection and corrosion prevention strategies in VHTR applications.