Two-phase Separation of Primary MX Carbonitride during Tempering in Creep Resistant 9Cr1MoVNb Steel

Abstract

Precipitation behaviour during heat treatment has been investigated on 9Cr1MoVNb steel. Coarse primary and fine secondary MX carbonitrides are observed in the as normalized condition. On the other hand, coarse primary MX is not detected after tempering at 1 038 K, and Nb-rich and V-rich fine secondary MX carbonitrides and M23C6 carbide are observed in the as tempered condition. Variations of metallic elements concentrations of Nb-rich MX and V-rich MX in the as tempered condition are within the relatively small ranges. Average diameter of 34 nm for V-rich MX is slightly larger than that of 24 nm for Nb-rich MX in the as tempered condition. Both size and metallic elements concentrations of the fine secondary MX observed in the as normalized condition are almost the same as those of Nb-rich MX observed in the as tempered condition. A composition of metallic elements of the coarse primary MX varies along the line between those of Nb-rich MX and V-rich MX in a Cr-Nb-V ternary phase diagram. It has been considered that, at normalizing temperature of 1 323 K, Nb-rich MX and V-rich MX forms solid solution and composition of the primary MX varies along the tie line between those two phases. Two-phase separation of primary MX into Nb-rich MX and V-rich MX may be caused by the miscibility gap in primary MX and another two phases at the tempering temperature of 1 038 K.