DIFFERENTIAL DEVICE

PROBLEM TO BE SOLVED: To provide a rotary body which is harder and has high resistance against wear by forming it by a spheroidal graphite cast iron material obtained by setting a carbon equivalent amount to a proper range, reducing an amount of Cu, reducing an average particle size of graphite, and forming a base around graphite into a martensite phase. SOLUTION: A spheroidal graphite cast iron material is used to form a rotary body of a differential device. A martensite phase of its base structure has high fatigue strength when compared with a pearlite phase and a ferrite phase. When its carbon equivalent amount (CE=C+1/3 (Si)) is 4.53%, or less, its fatigue strength exceeds 400 N/mm , and average grain size of graphite is below 20 μm at this time. Moreover, the Cu content in which pearlite scarcely exists in the structure is less than 0.25% to ensure the tensile strength and elongation after heat treatment. Furthermore, tempering is done at the fatigue strength of 400 N/mm or more and the hardness of HRC 50 or more and at a temperature of 200 to 350 deg.C by taking the tensile strength, elongation, and an impact value into account.