| 摘要 |
Partially devitrified silica cores with exceptional high temperature strength are disclosed for foundry use in sand casting of ferrous alloys. Precision cored holes of small size and long length, which heretofore could not be cast accurately or had to be machined because of limitations in existing core technology, are formed using accurately machined extruded porous silica cores fired to eliminate combustibles and partially devitrified (e.g., 15 to 30%) to develop a strong crystalline phase bond that resists viscous flow at a temperature of 1500 DEG C. and to increase the refractoriness of the vitreous silica grains, whereby the permeability and thermal shock resistance are such that the core can be heated very rapidly by molten steel to above 1500 DEG C. without spalling, cracking, sagging, breaking, or loss of integrity and without gas holes, scabs, fissures or other serious casting defects. The cores are made from an extrudable composition containing vitreous silica, a mineralizer, an organic binder, and a plasticizer and/or tempering fluid. High-purity vitreous silica grains are mixed with a mineralizer and large amounts of finer vitreous silica particles (e.g., below 10 microns) which bond the silica grains together to provide extremely high strength after firing. The extruded cores are fired to form cristobalite in major amounts in the bond region between grains and in minor amounts in surface portions of the silica grains. The sand casting process of the invention can employ hollow porous thin-wall cores having small diameters, such as 0.4 to 2 centimeters or less, and length-to-diameter ratios, such as 20:1 or more. The problem of casting defects due to gassing is solved by causing the expanding gases to move radially inwardly to the central passage of the hollow core.
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