Long-term thermal aging effects in ferritic-martensitic steel HT9

The ferritic-martensitic steel HT9 is a candidate material for fuel cladding and core components in advanced nuclear reactors, such as sodium-cooled fast reactors, thanks to their high temperature mechanical properties and low susceptibility to irradiation induced swelling phenomena. However, thermal stability and elevated temperature microstructural evolution in these alloys may impact their long-term behavior and reliability. In this work, the effects of thermal aging on the microstructural and mechanical properties of HT9 have been investigated through complementary electron microscopy, synchrotron X-ray diffraction, microhardness, and thermodynamic modeling. Plates of HT9 were aged up to 50 kh at relevant sodium-cooled fast reactor operational temperatures (360 °C - 700 °C). Trends in microstructure as a function of aging time and temperature were apparent from qualitative and quantitative analysis. These observations were further supported by thermodynamic modeling of the bulk and precipitate phases. Specific phases observed include BCC Fe, FCC M23C6, HCP and FCC MX phase and Laves M2X phase. Through the application of our multi-scale and multi-modal approach, clear information on the aging mechanism of HT9 was obtained, allowing for a more informed prediction, and understanding of the long-term behavior, performance and thermal stability of ferritic-martensitic alloys exposed to elevated temperatures. [Display omitted] •Thermal aging of HT9 up to 50 kh at 360 ℃ and 500 ℃ does not lead to appreciable changes in the microstructure or hardness•Diffusion of Cr to grain boundaries and depletion of alloying elements within matrix at all aging temperatures apparent•.Formation of Laves phase precipitates was only observed after aging at 600 ℃ (at all aging times)•Deviations between the experimental and predicted weight fractions of M23C6 precipitates elevated temperatures•The chemical structures of the Laves and M23C6 precipitates were complex and found to contain Mo and W, respectively•Thermally induced softening in specimens aged at 600 ℃ and 700 ℃ due to matrix growth and reduced support of M23C6