ARGON SUBSTITUTING METHOD FOR MOLD

PURPOSE:To substitute the inside of a mold thoroughly with Ar in good yields, by evacuating the inside of the mold and holding the reduced pressure thereof in a vacuum vessel and feeding gaseous Ar into the vessel under the reduced pressure. CONSTITUTION:A mold 11 which is completed of molding is charged into a vessel 12 for vacuum, and the inside of the vessel 12 is held at reduced pressure (for example, about 1 Torr degree of vacuum) by an evacuating system 13. In this way, the oxygen partial pressure in the cavity parts in the mold 11 including a casting part 14, a riser part 15, and a sprue part 16 as well as the oxygen partial pressure in the voides among the sand grains in a sand part 11' are reduced. Gaseous Ar is fed through an introducing pipe 17 into the vessel 12 which is completed of the pressure reduction and holding, whereby the gaseous Ar is filled in the cavity parts and the voids among the sand grains in the mold 11. After the substitution with the Ar is completed in the above-mentioned manner, special covers are put in the parts of the sprue 16 and the riser 15 open to the atmosphere and the outside circumferential part of the mold 1 is also sealed to prevent the releasing of the gaseous Ar, and in this state molten steel is charged into the mold.