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Abstract
Precipitation behaviour during normalizing heat treatment has been investigated on modified 9Cr – 1Mo steel. Heterogeneously distributed spherical MX particles and platelet M3C were observed in the as-normalized condition. The number of the precipitates decreased with increasing normalizing temperature and no precipitates was observed after normalizing at 125°C. The size of MX increased with an increase in normalizing temperature up to 1200°C. For MX, not only size, but also composition of metallic elements was influenced by the normalizing temperature. Since equilibrium composition of MX depends on temperature, an MX particle with non-equilibrium composition dissolves and precipitation of it takes place with its equilibrium composition at the normalizing temperature. A phase field diagram of NbX–VX quasi binary system in modified 9Cr –1Mo steel was experimentally determined. It has been supposed that precipitation of M3C takes place during cooling from normalizing temperature in the surrounding area of MX particles where the concentration of niobium and vanadium in matrix is poor.
