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 (MϭNb, V, Cr and XϭC, N) carbonitride particles and platelet M 3 C (MϭFe, 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 1 250°C. Although the size of M 3 C was almost constant independent of normalizing temperature, that of MX increased with increase in normalizing temperature up to 1 200°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 M 3 C 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.KEY WORDS: ferritic creep resistant steel; 9Cr-1Mo-V-Nb steel; MX carbonitrides; cementite; phase equilibrium; precipitation behavior; two-phase separation; normalizing.quency vacuum induction furnace. The ingot was heated to 1 150°C for 1.5 h and hot rolled in a range of temperatures from 1 150°C to 900°C into bar with a diameter of 16 mm. Normalizing heat treatment conditions are shown in Table 2, together with prior austenite grain size number and Vickers hardness in the as normalized condition. Normalizing heat treatments were performed for 600 s in a range of temperatures from 1 050 to 1 250°C and for 3 600 s at 1 100 and 1 200°C, followed by air cooling. Prior austenite grain size of the steel coarsened with increase in normalizing temperature, but hardness of about HV400 was almost the same independent of normalizing temperature. Presence of d-ferrite was not detected for all the normalizing temperatures investigated.Distribution of precipitates was observed on carbon extracted replica under Field Emission Transmission Electron Microscope (FE-TEM). Each precipitates were analyzed by Energy Dispersion X-ray Spectroscopy attached to FE-TEM (TEM-EDS). Mechanically polished specimen surface was etched in a saturated solution of picric acid in ethyl alcohol with 1 % hydrochloric acid (Villela's reagent). The carbon was deposited on the etched surface, and the carbon film was detached from the specimen surface in a Villela's reagent. The carbon extracted replica was cleaned in ethyl alcohol and collected on copper grids for TEM examination. Precipitates were identified by means of X-ray diffraction analysis on an electrolytically extracted residue. Figure 1 shows a TEM micrograph of the carbon extracted replica prepared from the steel in the as received condition. Figure 1(b) is a high...

Z-phase Formation during Creep and Aging in 9–12% Cr Heat Resistant Steels

“…The volume fraction of d-ferrite was evaluated to be about 5 %. 20) Creep tests were performed at 873 K under a constant load in air using specimens 6 mm in gauge diameter and 30 mm in gauge length. The microstructures were observed using a conventional TEM and an energy-filtered TEM (EF-TEM) at 200 kV.…”

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

“…The EDX spectra that were obtained from these particles indicate a chemical composition that is close to the M 3 C, cementite, precipitate. [24] Nb-and V-rich particles were both observed with a size of less than 100 nm, with round and elongated shapes for the Nband V-rich particles, respectively. The EDX spectra that were obtained from the V-and Nb-rich particles indicated a chemical composition close to the MX precipitates.…”

“…The EDX spectra that were obtained from the V-and Nb-rich particles indicated a chemical composition close to the MX precipitates. [24,25] Cr-rich particles were found with variable sizes from less than 100 nm to over 400 nm. The chemical composition of the Cr-rich particles, as indicated by EDX, is similar to that expected from M 23 C 6 carbides.…”

Heterogeneous austenite grain growth in martensitic 9cr steel: Coupled influence of initial metallurgical state and heating rate

“…% V~50 % and higher) in T91 steel samples normalized 10 min. at 1050 and 1100 ºC [44], and V-rich precipitates (wt. % V~80 %) in experimental Fe-Nb-V-C-N alloys austenitized at 1000 ºC or higher temperatures for several hundred hours [45].…”