Changes in Microstructural and Mechanical Properties of AISI Type 316LN Stainless Steel and Modified 9Cr-1Mo Steel on Long-Term Exposure to Flowing Sodium in a Bi-Metallic Sodium Loop

摘要

AISI Type 316LN stainless steel (SS) and modified 9Cr-1Mo steel were exposed to flowing sodium at 798 K (525 degrees C) for 30000 hours in a bi-metallic sodium loop. The changes in microchemical, microstructural, and mechanical properties were evaluated and compared with the as-received and thermally aged specimens. Effective carbon diffusion coefficient D-C(eff) was calculated to be 6.8 x 10(-19) m(2)/s. Depth of carburization analyzed by secondary ion mass spectroscopy technique was around 100 mu m for sodium-exposed 316LN SS. Selective leaching of nickel occurred across depth from the surface of sodium-exposed 316LN SS with the formation of 10 mu m ferrite layer, and it showed an increase in yield strength by 15 pct, reduction in ductility by 60 pct, and a decrease in impact energy by 15 pct vis-a-vis the as-received and thermally aged specimens. This reduction in ductility occurred due to extensive precipitation of sigma phase as a result of long-term thermal aging. No significant changes were observed in the sodium/modified 9Cr-1Mo steel interfacial microstructure as well as tensile properties of sodium-exposed modified 9Cr-1Mo steel. Although modified 9Cr-1Mo neither showed carburization nor decarburization on sodium exposure, it showed a drastic reduction in the impact strength, which was attributed to the presence of Laves phase, observed in X-ray diffraction patterns. (C) The Minerals, Metals & Materials Society and ASM International 2015