This paper describes the influence of normalising temperature on metal carbonitride (MX) precipitation behaviour and mechanical property of high chromium (Cr) ferritic steel for fast breeder reactor (FBR) structures. Generally, vanadium (V) and niobium (Nb) play a role of precipitation strengthening elements as MX particles in high Cr ferritic steels. Precipitation behaviour of MX particles depends not only on the amounts of V and Nb but also on heat treatment conditions. In order to obtain fine and dense MX precipitations and superior mechanical properties, optimal heat treatment conditions were investigated. The microstructures were observed using optical and electron microscopy. Extracted residue analysis was conducted to identify the precipitated elements. It was clarified that fine and dense MX precipitations and superior strength were obtained by normalising at higher temperatures. This mainly resulted from Nb which had dissolved in matrices in normalising at higher temperature and then precipitated as fine MX particles in tempering process. In addition, M_2X precipitations were also found to be fine and dense by normalising at higher temperatures. This might contribute to improve the strength of the steel. It can be concluded that the normalising process at high temperature enables MX precipitations to be fine and dense because Nb can be dissolved as much as possible in matrices by the process. Further, since FBR components are used at elevated temperatures for very long periods, the long-term stability and efficiency of strengthening mechanism brought about with such MX particles needs to be investigated.
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