Effect of Carbides Formed in 9Cr-1Mo-V-Nb Weld Metals on Elevated Temperature Tensile Strength

Abstract

Alloy design of 9Cr-1Mo-V-Nb weld metals was carried out through thermodynamic simulation, and elevated temperature tensile testing was performed on the designed alloy. Based on the thermodynamic simulation of the 9Cr-1Mo-V-Nb weld metal, 0.09 to 0.10 wt% C was added to increase Cr-rich M23C6. Nitrogen was reduced from 0.04 to 0.02 wt% to suppress the formation of the Z-phase. Elevated tensile tests at 600 degrees C showed that the 0.10C specimen, in which a large amount of Cr-rich M23C6 was formed, produced a higher tensile strength and elongation than the 0.09C specimen. Below the fracture surfaces after the elevated temperature tensile test, the microvoids located in the 0.10C specimen were smaller than those in the 0.09C specimen. In the 0.10C and 0.09C specimens, the microvoids generated by oxide inclusions were mostly located in the grain boundaries. Cr-rich M23C6 precipitates in the weld metal were mainly located on the grain boundaries, and the (V, Nb)-rich MX precipitates were located in the grains. In the 0.10C weld metal, a large amount of Cr-rich M23C6 precipitates, which were coherent with the grain, were distributed at the grain boundaries. The coherent Cr-rich M23C6 precipitated along the grain boundary probably inhibited the mobility of dislocations and grain boundaries, and the propagation of microvoids generated by oxide inclusions, therefore increasing the high temperature strength and elongation.