Ductility-dip cracking susceptibility of Inconel 690 using Nb content

Abstract

A specimen with a newly designed composition was produced using shielded metal arc welding by changing the Nb content. To evaluate the susceptibility of ductility-dip cracking (DDC), tensile test and hardness measurement were performed, and to examine the behaviors of precipitates during heat treatment, the microstructures were observed using an optical microscope and a scanning electron microscope. As the Nb content increased, the hardness of the specimen improved through solution hardening, and as the hardness increased, the yield strength in the tensile test results increased. Furthermore, as the Nb content increased, the amount of Cr-rich carbide precipitated at the grain boundary decreased and the amount of transgranular Nb-rich carbides increased. This increased the susceptibility of DDC and the critical strain at which the DDC begins to appear at tension increased. The microstructure analysis of a specimen heat-treated at 715 °C revealed that the intergranular Cr-rich carbides became continuous and coarse through the diffusion of Cr, and the tensile test results showed that DDC occurred at a strain lower than that of the as-weld specimen. In the specimen heat-treated at 1100 °C, the intergranular Cr-rich carbides were melted and the intergranular precipitates were removed.