Formation of Surface Solidification Layer in Spheroidal Graphite Cast Iron

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  • 球状黒鉛鋳鉄の表面凝固層生成について
  • キュウジョウ コクエン チュウテツ ノ ヒョウメン ギョウコソウ セイセイ ニ ツイテ

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The study of solidification mechanism is very important in the elucidation of various properties such as toughness and strength as well as defects such as shrinkage cavities, pinholes, etc. of the spheroidal graphite cast iron. This is why solid density, liquid density, relation between volume of molten liquid and temperature, type of eutectic solidification and expansion-shrinkage at solidification have already been studied on the spheroidal graphite cast iron. The formation of surface solidification layer on hyper-eutectic composition spheroidal graphite cast iron and flake graphite cast iron was studied in this experiment, because this is closely related to the solidification mechanism as with the above mentioned studies. The most important result obtaind through this investigation is that there is a remarkably different type of formation and growth of surface solidification layer between the two kinds of cast iron which clarifies the fact that the two solidification mechanims are very different. <br>  The results are summarlized as follows. <br>  1) On the spheroidal graphite cast iron, a very thin film is formed along the wall of the mold when the molten metal reached melting point in the mold. The growth of the thin film at the first stage is very slow with the growth rate at about 0.004 mm/sec. The second stage start at the end of the first stage, the growth rate being 30 times larger than the first stage. The formation and growth of the first stage is the result of the change in graphite shape induced by external gas, but since the film is very thin at this stage, it is probable that the real solidification of this cast iron begins at the second stage. <br>  2) The formation and growth of surface solidification layer in flake graphite cast iron is similar to the spheroidal graphite cast iron but the formation rate at the first stage is about 6 or 7 times larger than that of the spheroidal graphite cast iron.

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