DE2659168

A porous high-silica core is disclosed for use in directional solidification casting processes having exceptional thermal stability at temperatures above 1650 DEG C. and containing mineralizers which promote the formation of cristobalite. The cores may be made by mixing at least 75 parts of essentially pure fused silica particles with 1 to 25 parts of activating particles containing a mineralizer, such as an alkali metal or alkaline earth metal compound, may be fired at a temperature of 1000 DEG to 1300 DEG C. until they contain 35 percent or more of cristobalite and may then be cooled to room temperature. They may thereafter be incorporated in a shell mold in accordance with the "lost-wax" process and preheated with the shell mold at a temperature of 1300 DEG to 1600 DEG C. to provide a cristobalite content of 60 to 85 percent or more within a short period of time, such as 10 to 30 minutes, and before a molten superalloy is allowed to flow into the mold. The core has exceptionally high thermal stability and may be maintained at a temperature of 1550 DEG to 1600 DEG C. or higher for one hour or more during the directional solidification process without substantial deformation. The high ultimate use temperature of the core makes it possible to produce better castings by carrying out the D.S. process at a temperature 50 DEG to 100 DEG centigrade higher than previously used.