PURPOSE: To obtain spheroidal graphite cast iron which permits easy machining by maintaining the spheroidal graphite cast iron contg. C, Si, Mn, etc. at a high temp. then subjecting the same to air cooling then to machining followed by heating to an eutectic transformation temp. section to austenitize the base and cooling quickly the same.

CONSTITUTION: The spheroidal graphite cast iron consisting, by weight %, of 3.0W4.0% C, 1.5W3.0% Si, ≤0.7% Mn, 0.02W0.1% Mg, 0.03W0.09% Mo, 0.3W2.0% Cu and/or Ni, and the balance substantially Fe is heat treated as follows: This spheroidal graphite cast iron is subjected to the primary heat treatment by holding the same for 5minW2hr at 750W800°C then air cooling the same, whereby the base is made into the two-phase texture of ferrite and pearlite. The above- described cast iron subjected to the primary heat treatment is subjected to machining such as cutting and is then heated to an eutectic transformation temp. section to austenitize the base, whereafter the cast iron is quickly cooled and is held for 0.5W4hr at 200W400°C. As a result, the spheroidal graphite cast iron having good machineability and excellent wear resistance and resistance to fatigue is obtd.