This paper investigates the effects of silicon content above 3 wt% and different cooling rates on the graphite morphology and the mechanical properties of ductile cast iron. To research and determine these effects, three grades of high-silicon ductile iron were cast with silicon content of 3 wt%, 3.5 wt% and 4.5 wt%. In order to investigate the effect of different cooling rates during solidification, four sections with different thicknesses were cast in the same mold, i.e., cube with wall thickness 180 mm and standard Y2, Y3 and Y4 keel blocks. Microstructure analysis and tensile test were performed on the as-cast samples. In the second step of analysis, detailed metallographic observations on specially prepared samples were performed in order to investigate the risk of graphite degeneracy appearance, especially chunky graphite formation due to high silicon content and its effect on mechanical properties. The results showed that graphite content, ferrite fraction, yield strength and tensile strength were increased by increasing the silicon content. On the other hand, elongation decreases with increasing silicon content. Also, based on the metallographic observations of the present work, the risk of chunky graphite formation increases with increase in silicon content and with decrease in the cooling rate. Chunky graphite formation affects negatively mechanical properties, especially elongation in ductile cast irons with silicon content higher than 3.5 wt%.

中文翻译：

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冷却速度对高硅球墨铸铁中石墨形态和力学性能的影响

本文研究了硅含量超过3 wt％和不同的冷却速率对球墨铸铁的石墨形态和力学性能的影响。为了研究和确定这些效果，铸造了三种等级的高硅球墨铸铁，硅含量分别为3 wt％，3.5 wt％和4.5 wt％。为了研究凝固过程中不同冷却速率的影响，在同一模具中铸造了四个厚度不同的部分，即壁厚为180 mm的立方体和标准的Y2，Y3和Y4龙骨块。对铸态样品进行了微结构分析和拉伸试验。在分析的第二步中，对特殊制备的样品进行了详细的金相观察，以研究石墨变质外观的风险，特别是由于高硅含量及其对机械性能的影响而形成的块状石墨。结果表明，随着硅含量的增加，石墨含量，铁素体分数，屈服强度和拉伸强度均得到提高。另一方面，伸长率随着硅含量的增加而降低。而且，基于本发明的金相观察，形成粗大的石墨的风险随着硅含量的增加和冷却速率的降低而增加。块状石墨的形成会对机械性能产生不利影响，特别是硅含量高于3.5 wt％的球墨铸铁的伸长率。通过增加硅含量来提高屈服强度和拉伸强度。另一方面，伸长率随着硅含量的增加而降低。而且，基于本发明的金相观察，形成粗大的石墨的风险随着硅含量的增加和冷却速率的降低而增加。块状石墨的形成会对机械性能产生不利影响，特别是硅含量高于3.5 wt％的球墨铸铁的伸长率。通过增加硅含量来提高屈服强度和拉伸强度。另一方面，伸长率随着硅含量的增加而降低。而且，基于本发明的金相观察，形成粗大的石墨的风险随着硅含量的增加和冷却速率的降低而增加。块状石墨的形成会对机械性能产生不利影响，特别是硅含量高于3.5 wt％的球墨铸铁的伸长率。