| Summary: | 博士 === 國立臺灣科技大學 === 機械工程系 === 100 === This dissertation is aimed to study the effect of the cooling rate on solidification behaviors and microsegregation in ductile iron castings. The water-chilled copper mold was used to mastering the direction of heat transfer Relationship between cooling rates and microsegregation during solidification was established. In addition, the effect of cooling rate on the alloy carbides and the precipitates in microstructure and macrostructure of end-chilled Si-Mo ductile irons was investigated.
The results showed that with chilled copper mold the nodular size increases as the distance from copper mold increasing, in the slowest solidification zone nodular graphite appearing deterioration apparently, the nodular degradation is even more obvious in ductile iron containing Ti. Casting cooling rate affects significantly alloying elements effective segregation coefficient keff and hardness distribution. Solidification parameters were obtained from the results of thermal analysis, it shows that the moving rate of liquidus depending on the local cooling time, but the moving rate of solidus keeping almost in the same. In the mid-chilled casting, the solidification pattern is different, additional liquid disturbance induced by thermal gradient and gravity which affects directly the cooling curve, the elemental segregation and nodular size in various regions. For high-silicon molybdenum ductile iron, the results showed that the eutectic carbide rich molybdenum M6C phase, as a result of segregation, in the final liquid region of eutectic composition (austenite + eutectic carbides) Mo average total content of about 19 wt% of in line with the C-Fe-Mo ternary equilibrium phase diagram of eutectic composition, but this alloy carbides in addition to the outside of C, Fe, Mo, also contains high concentrations of Si. This is the replacement of Si and M6C carbides Mo, which results after cooling in the unit cell carbide around the ferrite containing Si decreased, which may affect the unit cell between the antioxidant capacity and mechanical properties.
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