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 M 23 C 6 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.

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

“…10(a)), distribution of compositions in a range of niobium from 50 to 90 mass% was observed along a line between 100 mass% Nb and 80mass%V-20mass%Cr. This result was similar to that reported by Suzuki et al 10) on a 9Cr-1Mo-V-Nb steel in the as normalized condition, although low niobium particles less than 50 mass% were not observed in this study.…”

“…Inoue et al has investigated phase equilibrium between austenite and (Nb, V)(C, N) complex carbonitride in Fe-Nb-V-C-N alloys, and binodal curve and phase separation behaviour in quasi binary system of NbC-VC have been reported. 9) Suzuki et al 10) investigated on precipitation behaviour of MX carbonitride during normalizing and tempering heat treatment in 9Cr-1Mo-V-Nb steel and observed a distribution of chemical composition of MX carbonitrides along the line between 100 mass% Nb and 85mass%V-15mass%Cr in a Cr-Nb-V ternary triangle, similar to the results in the present study (Fig. 10).…”

“…11) Recently, Inoue et al reported that two-phase separation behavior of primary MX into Nb-rich MX and V-rich MX was caused by the miscibility gap between primary MX and another two phases at the temperature. 9) Such two-phase separation behavior has been clearly demonstrated by experimental results during tempering heat treatment at 765°C in a 9Cr-1Mo-V-Nb steel by Suzuki et al 10) Since MX particles are major strengthener, precipitation behavior in cooperate with two-phase separation of those during normalizing and tempering heat treatment strongly influences on creep strength. However, it has not yet been clearly understood, and presence of cementite (M 3 C) in the as normalized condition in 9-12Cr ferritic creep resistant steels [12][13][14][15][16][17] has not yet been authorized.…”

“…[2][3][4] Many researches have been conducted on features and role of MX carbonitrides. 2,[5][6][7][8][9][10] A possibility of further improvement in creep strength by controlling MX carbonitride has been also investigated. 11) Recently, Inoue et al reported that two-phase separation behavior of primary MX into Nb-rich MX and V-rich MX was caused by the miscibility gap between primary MX and another two phases at the temperature.…”

The Influence of Thermal Cycles on the Microstructure of Grade 92 Steel

“…[5,30] It is worth noting that the conventional definitions of the HAZ microstructure, primarily in low alloy steel welds, involves a further classification of the regions with a peak temperature above the Ac 3 temperature into the so-called coarse-grain and fine-grain regions, related to whether the grain structure was significantly influenced by second-phase particles. [1,2,14] However, it was observed in this case that the rapid weld thermal cycle results in the Ac 3 temperature being close to the temperature at which the pre-existing secondary precipitates are completely dissolved.…”