PURPOSE: To effectively prevent the quenching crack and overhardening of a product of a material in which the quenching crack is readily caused by filling molding sand for a CO2 process into a part of the product in which the quenching crack is probably caused and by quenching the product.

CONSTITUTION: When the product of a material in which the quenching crack is readily caused is quenched, the molding sand for a CO2 process consisting of about 100pts.wt. silica sand and about 7pts.wt. water glass is filled into a part of the product in which the quenching crack is probably caused or a part not to be overhardened. The part may be coated with the molding sand. After the molding sand is solidified by blowing gaseous CO2, the product is water-quenched. Thus, the quenching crack and overhardening of the product are provented.