LOCAL HARDENING METHOD OF CAST IRON

PURPOSE:To harden locally a cast iron without generating chill by casting an oridinary cast iron into a casting mold, admitting air or a fluid mixture of air and water into the casting mold under cooling and increasing the cooling speed of only the required part of the casting. CONSTITUTION:An ordinary cast iron is cast into a casting mold 2 and a casting 1 having a prescribed shape is formed. Then the casting 1 undergoes the process of solidification and cooling and a fluid mixture of air and water is admitted into an air hole 4 provided through a master mold 2 and a core 3 at the point of the time when the temp. of the casting 1 attains 720-1,150 deg.C. The admission is ceased when the temp. of the casting 1 attains 300-700 deg.C. The admission of the refrigerant can be started at the point of the time when the temp. falls to the solidifying temp. of the cast iron or below and said admission is required to be started at the eutectoid transformation point or above. Air or a fluid mixture of air and water is optimum as the refrigerant and the water alone is not preferable as the water penetrates in the casting mold. The cooling speed of the casting varies with the flow rate of the refrigerant and the Brinell hardness and tensile strength thereof vary as well.