Phase Equilibrium between Austenite and MX Carbonitride in a 9Cr-1Mo-V-Nb Steel

Abstract

Precipitation behavior during normalizing heat treatment has been investigated on a 9Cr-1Mo-V-Nb steel. Heterogeneously distributed spherical MX (M=Nb, V, Cr and X=C, N) carbonitride particles and platelet M3C (M=Fe, Cr) carbide 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°C. Although the size of M3C was almost constant independent of normalizing temperature, that of MX increased with increase in normalizing temperature up to 1200°C. For MX carbonitride, not only size, but also composition of metallic elements was influenced by normalizing temperature. Since equilibrium composition of MX carbonitride 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 a 9Cr-1Mo-V-Nb 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.