Preliminary investigation of Cr-coated F82H: Evolution of microstructure and mechanical properties at the F82H/Cr interface during solid-state diffusion bonding above Ac1 temperature

Abstract

Solid-state diffusion bonding was employed to join F82H steel and pure Cr metal under high vacuum at various temperatures above Ac1 in the range 1114 to 1323 K for 60 min, and at 1323 K for 120 min. After successful bonding at all temperatures, the F82H/Cr interface and interdiffusion zone were characterised using SEM-EDS, TEM and EBSD analysis, with respect to diffusion behaviour, chemical composition and microstructure. A thin interface layer formed on the Cr side of the joint, which decreased in thickness with decreasing bonding temperature, and was rich in M23C6 with mixed columnar and particulate nature. The depth of Cr diffusion into F82H decreased with decreasing bonding temperature. At temperatures ≥1231 K a layer of ferrite grains formed in the F82H region adjacent to the interface, due to the combined stabilising effect of enhanced Cr concentration and depleted C. At the lowest bonding temperature of 1114 K, a lath martensite microstructure was retained throughout the F82H due to a significantly lower extent of Cr and C diffusion. The diffusion coefficient of Cr in F82H was determined at all temperatures examined, and the activation energy of Cr diffusion in F82H >Ac1 was approximated as 225 KJ mol−1. Finally, hardness testing of the interface and interdiffusion zone suggests the F82H properties were retained after bonding at the lowest temperature of 1114 K, however at higher temperatures the newly formed ferrite region was softer. Further research is required to understand the impact of the hard M23C6-rich interface layer on mechanical properties of the joint, and whether microstructural change can be minimised, or even eliminated, below the Ac1 temperature.