| Summary: | 碩士 === 國立臺灣大學 === 機械工程學研究所 === 97 === The primary purpose of this research is to study the effects of C and Si contents, method of post inoculation and the employment of chills on the (surface) microstructure and hardness of low-alloy ductile cast irons.
The results show that, at a fixed CE, increasing Si content (from 1.15%Si to 1.68%Si) increases nodule count, but reduces carbide content. In addition, late inoculation exerts more effect on irons with higher Si than with lower Si. On the other hand, at a fixed Si content, irons with a higher C content and/or were late inoculated, have higher nodule count and less carbide content. In addition, late inoculation promotes bainite formation rather than pearlite, while no significant difference in matrix structure was obtained for irons without late inoculation. Regarding the hardness, casting AX has the highest hardness value HRC 50-54, among the four castings studied. Surface chilling significantly increases the nodule count, promotes uniform distribution of carbides and also refines carbide phase, and increases hardness.
Finally, thermal analyses were performed to attain the cooling curves at different locations in the castings and correlated the cooling curves with the continuous cooling transformation diagram of similar compositions to predict the matrix structure formed.
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