Influence of Normalizing Heat Treatment on Precipitation Behavior in Modified 9Cr-1Mo Steel

摘要

Precipitation behavior during normalizing heat treatment has been investigated on modified 9Cr-1Mo steel. Heterogeneously distributed spherical MX particles and platelet M_3C were observed in the as normalized condition. Number of the precipitates decreased with increasing normalizing temperature and no precipitates was observed after normalizing at 1250℃. The size of MX increased with increase in normalizing temperature up to 1200℃. For MX, not only size, but also composition of metallic elements was influenced by normalizing temperature. Since equilibrium composition of MX depends on temperature, 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 M_3C 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.