Influence of Sodium on the Microstructure of Laser Rapid Manufactured NiCrSiBC Hardface Alloy Coatings

To prevent high temperature wear and galling, core structural steels in fast breeder reactors are hardfaced with Ni-based alloys. To overcome cracking and dilution related issues, the hardfaced alloy with a nominal composition of Ni-13.25Cr-4Fe-4Si-2.3B-0.5C (wt. %) was deposited on 304L SS build plate by laser rapid manufacturing (LRM) process. As-deposited specimen had the following precipitates with unique morphology and microchemistry: CrB, Cr2B, Cr5B3, Cr7C3 and M23C6, which are responsible for its excellent wear resistance. Microstructure of as-deposited specimen was taken as the reference to infer changes in the microstructure due to sodium exposure. Sodium testing was done under static conditions with argon cover gas at temperature of 823 K for 500, 1000, 2000 and 4000 h. Phase identification in sodium exposed sample was done using XRD and microstructure, morphology and microchemistry of the precipitate phases were analyzed using electron microprobe. Samples in as-deposited, thermally aged and sodium exposed conditions were compared to delineate the influence of sodium from thermal ageing.