The variation of mechanical properties due to the mass effect in cast iron was examined on the specimens cut from several parts of a gray cast iron flywheel by subjecting them the repetition of heating and cooling, and of annealing. The conditions of heat treatment were as follows: (1) from 800°C to 600°C, (2) from 650°C to 350°C, and from 350°C to 150°C, and (3) from 800°C, 650°C, or 350°C to the room temperature, respectively.
In the case (1), the change in mechanical properties of each part decreased rapidly in the early stage of heat repetition, and every part yielded almost the same strength after heating cycles of more than 50 times.
On the other hand, their microscopic structures showed that the area percentage of graphite and ferrite increased with increasing number of heat repetition, and a good correspondence existed between the changes in the structure and in mechanical properties. In the case (2), the same tendency with the case (1) could be recognized between the changes in mechanical property and microscopic structure, but since these changes were small, a larger number of heat cycles was necessary for the homogenization of mechanical properties of every part. And, in the heat cycles at low temperatures (350°C_??_150°C), the changes in mechanical properties and microscopic structures were not detected even after 100 cycles. In the case (3), that is, in the repetition of annealing, the changes in mechanical properties and microscopic structures revealed almost the same tendency with those in the case of the heat treatment mentioned above.
