Preventing solidification defects in large superalloy castings used in advanced electric power systems

Abstract

The need for clean and affordable power has resulted in great demand for high-efficiency gas turbine power systems. To improve energy generation efficiency, and lower CO 2 emissions, the new generation of land based gas turbines needs to be larger and operated at a higher temperature. Consequently, it is required the use of Nb-containing superalloys such as Inconel 718 for the rotor disk, to avoid overaging of the microstructure during the forging process, but its applicability is being limited by difficulties in the reliable production of large Vacuum Arc Remelting (VAR) ingots (33 to ~35 in. diameter) without macrosegregation defects such as freckles and center segregation. These defects impair the mechanical properties of the material and are believed to be caused by convection in the mush layer. Because they cannot be corrected by subsequent treatment, the affected ingots should be scrapped highly increasing the production costs.