SPHEROIDAL GRAPHITE CAST IRON MEMBER AND MANUFACTURE THEREOF

PURPOSE:To obtain a spheroidal graphite cast iron member having a double structure of a surface layer part somewhat deteriorated in hardness and improved in elongation and an inner part having high strength and in general, showing excellent mechanical strengths. CONSTITUTION:The spheroidal graphite cast iron member is a one having a surface layer part with at least 1mm thickness substantially consisting of a ferritic phase and an inner part consisting of a pearlitic phase and a ferritic phase, and in which the ferritizing rate of the surface layer part is regulated to >=70% which is the value higher than the ferritizing rate of the inner part at least by about 15%. Molten metal is poured into a mold and knock-out is executed in a state in which the temp. of the approximately whole body of the casting is still lies in the A1 transformation point or above after the completion of the solidification of the molten metal. When the temp. difference between the surface layer part and inner part of the obtd. casting reaches 40 to 60 deg.C, the casting is charged to a soaking furnace held to 750 to 900 deg.C and is held in the soaking furnace for the time in which the same ferritizing rate is attained, and the casting is cooled at 15 to 100 deg.C/min cooling rate.