Effect of heat treatment on intermediate-temperature embrittlement, microstructure, and tensile properties of an electron beam welded grade-6 Ni-Co-based superalloy

Electron beam welded joints of high-alloyed and precipitation-strengthened wrought superalloys are susceptible to intermediate-temperature embrittlement (ITE), which severely limits their use in engineering. To address this issue, this study optimized heat treatment process to regulate the microstructure of the welded joints. The results showed that the ITE of the alloy and joints mainly originated from stress-accelerated grain-boundary oxidation due to the precipitation of a large number of carbides at the grain boundaries (GBs). By making modifications to the heat treatment process that resulted in fewer grain boundary carbides, the ductility of the alloy welded joint increased from approximately 6% at 800 °C to over 12%, while simultaneously maintaining a stable tensile strength of 860 MPa. The improvement in performance was attributed to a stable oxidation and a significant reduction in stress concentration at the GBs, enhanced coordinated deformation capability between the GBs and the grain interiors, and the formation of high-density micro-twins within the grains. This drastically narrowed the performance gap between the welded joint and the base material, which serves as a viable means to improve the intermediate temperature reliability of welded superalloy components.