Effects of Nb and v on the Precipitation Phases of MX in Q345E Applicable to Cryogenic Flange

Abstract

Thermodynamic software (Thermo-Calc) is utilized to Q345E applicable to cryogenic flange. The aim is to study the equilibrium precipitation phases between 400 and 1600 C, and the solubility of Nb and V in austenite. The effects of alloying elements, such as C, N, Nb and V, on these equilibrium precipitation phases are also discussed. Besides theoretical calculation, transmission electron microscopy (TEM) is used to observe these precipitation phases. The results show that main equilibrium precipitation phases are M7C3, MX#1 (Nb-rich), MX#2 (V-rich) and AlN. Among them, the MX#1 phase is composed of major elements (Nb, C, and N) and minor elements (V and Cr). For MX#2 phase, its major elements are V, C, Nb, and N, and minor element is Cr. The precipitation amount and temperature of MX#1 phase were controlled by Nb content, and little affected by the contents of C, N and V. TEM results reveal that the size of precipitation phases varied from several nanometers to more than 100 nm. Large precipitation particles belong to MX#1 phase (Nb-rich), and their morphology tends to be cubic. On the other side, the small precipitation particles are attributed to MX#2 phase (V-rich), and their morphology is spherical or elliptical. These results are consistent with the thermodynamic calculation results about phase composition and precipitation temperature of MX#1 and MX#2.