Evolution of the F82H/Cr interface after solid-state diffusion bonding below Ac1 temperature: examination of microstructures and hardness

Abstract

F82H steel and pure Cr metal were joined by solid-state diffusion bonding under high vacuum at various temperatures below Ac1 in the range 964 to 1072 K for 240 min. Bonding was successful at all temperatures and the diffusion behaviour, chemical composition and microstructure at the F82H/Cr interface and interdiffusion zone were characterised using SEM-EDS and TEM analysis. A hard, M23C6-rich interface layer formed on the Cr side of the joint due to diffusion of C from the F82H steel. The thickness of the M23C6-rich interface layer reduced with reducing temperature, from ∼1.1 µm when bonded at 1072 K, to ∼0.2 µm at 964 K. Cr diffusion into F82H was only observed at the highest temperature of 1072 K, with minor Cr enrichment detected at a depth of <1 µm into the F82H. The slow diffusion behaviour below Ac1 meant a lath martensite microstructure appears to have been retained throughout the F82H after bonding below the typical tempering temperature, and hardness testing of the F82H suggests no post-bonding heat treatment would be required. Simple models for the interface evolution of the F82H/Cr joint both above and below Ac1 are proposed, which can act as reference points for the further development of Cr-coated F82H and its likely behaviour in a range of fusion power plant operational scenarios.