2010 Volume 51 Issue 1 Pages 176-185
Although the thinning of spheroidal graphite cast iron castings has been promoted to reduce the weight of the castings, the thinning tends to cause chilling. Due to the chilling, the required mechanical properties can not be obtained. The addition of certain elements is a way to solve this problem. In this study, the spheroidal graphite cast iron melt containing minor Bi, 3.3 to 3.7 mass%C and 2.0 to 3.2 mass%Si was poured into a stepped plate mold to obtain the thin wall castings, and observation of their graphite and matrix microstructure, thermal analysis during the solidification process of the melt in the mold and the qualitative analysis of elements inside the spheroidal graphite by FE-EPMA were carried out.
It was found that an increase in the Si/C mass ratio in the spheroidal graphite cast iron was effective for decreasing the amount of cementite (chill) in the matrix, and the chill was further inhibited by adding 0.01 mass% Bi even for the thin wall castings of 2 mm. Amounts up to 0.01 mass%Bi promoted refinement of the graphite, increased the graphite nodule, and promoted ferritizing of the matrix. It was also found that a high Si/C mass ratio in the spheroidal graphite cast iron promoted the effects of Bi. The temperature of the eutectic start and that of the eutectic solidification end increased due to the 0.01 mass%Bi. The temperature of the eutectoid transformation start increased and the stability eutectoid transformation of the thin wall castings was promoted by containing a minor amount of Bi. It was confirmed that substances including Bi and Mg existed in the graphite containing Bi. These results lead to the conclusion that the Bi compound and the Mg compound acted as heterogeneous nuclei of the graphite, and the nuclei promoted the crystallization of the graphite, and then the graphite nodule increased.
