The effect of heat treatment on the ultrasonic sound velocity in flaky, CV and spheroidal graphite cast iron was investigated and the following results were obtained. (1) Sound velocity in cast iron decreases by heat treatment. The decrease becomes apparent either when heating temperature is higher than the upper eutectoid temperature, heating time is prolonged or cooling is rapid. These effects are more apparent in flaky graphite cast iron than in CV or spheroidal graphite cast iron. (2) The decrease of the velocity is caused by the decrease of the effective area fraction of matrix A_ef, as a result of formation of voids and gaps on or in graphite through diffusion of carbon atoms from graphite to austenite. However, the decrease of the A_ef below eutectoid temperature is considered to be related to formation of cracks induced by thermal stress and to increase of the amount of graphite during heat treatment. (3) Young's modulus E (kgf/mm²) of a heat treated cast iron is approximated by equation (1) and (2)
      E=21500A'_ef+1000(1−A_ef) ………… (1)
      A'_ef=A_ef−x …………………………… (2)
where A'_ef is the effective area fraction of matrix in the heat treated cast iron, A_ef is the effective area fraction of matrix in the as–cast state and x is the area fraction of void and crack formed during the heat treatment. These equations are the same as the ones applied to cast iron under stress.