Microstructural characterization of surface layers formed on a low alloy steel during erosion oxidation in a fluidized bed

Low alloy steel specimens were subjected to erosion-oxidation in a simulated fluidized bed environment in the range 100–600° C. Transmission electron microscopy was carried out on the surfaces which experienced particle impacts, with specimens prepared by backthinning. At low temperatures (≤ 200° C) a dense, protective layer of bed particle fragments formed on the steel surface due to fragmentation of particle asperities and their subsequent comminution to a very fine size (< 10 nm). At intermediate temperatures thin oxide films developed on the wear scar surfaces; these were predominantly magnetite with a fine grain size. There was rapid material loss with particle impacts removing the oxide and some metal below, but the thin oxide rapidly regrew due to the fine grain size, absence of a haematite layer and the mechanical damage during particle impact. At high temperatures the oxide became sufficiently thick to be mechanically protective. Erosion occurred within the fine grained surface haematite layer, while there was grain growth in the lower magnetite layer.