PURPOSE: To produce the cast-ion casting parts excellent in impact resistance against external stress at the time of heat-treating the parts having a joint by hardening and tempering the part respectively under specified conditions.

CONSTITUTION: The cast-iron casting parts having a joint (e.g. steering knuckle for automobile) is heat-treated. In this case, the joint is firstly kept at 900-1000°C for ≥5sec by induction heating and then hardened. The joint is held at ≥600°C for ≥10sec by induction heating and tempered. Consequently, the hardness of the joint is controlled to about 20-40 HRC, the ferrite area of the base structure is adjusted to ≤ about 5%, the remainder consists of tempered martensite, the region other than the joint is formed with spheroidal graphite cast iron consisting of a ferrite base structure, and the casting parts excellent in impact resistance is obtained.