球状黒鉛鋳鉄の共晶凝固について

  It is very important to know the types of eutectic solidification in order to clarify the properties of spheroidal graphite cast iron. Some examples are shrinkage cavity formation and expansion or shrinkage at solidification. In this paper, the type of eutectic solidification of the spheroidal graphite cast iron treated with Ca or Mg spheroidizer and flake graphite cast iron was studied through CRF value t₂/t₁; t₁ is difference of starting time of eutectic solidification between surface and center part in the casting sample and t₂ is time of eutectic solidification for center part of same sample. </br>  Many researchers have studied the eutectic solidification of cast iron qualitatively and have found that the solidification type of this alloy is mushy. However, quantitative studies had not been done. </br>  The authors have quantitatively measured the mushiness of this alloy and following results were obtained. </br>  (1) Eutectic solidification type of spheroidal graphite cast iron and flake graphite cast iron are very different, especially in hyper-eutectic composition alloy, the former is the severe mushy type but the latter is not so. CRF value of spheroidal graphite cast iron is 20∼30, but it is only about 5∼6 in flake graphite cast iron; the former being 5∼6 times larger than the latter. CRF expresses the degree of mushiness and skin type. </br>  (2) We consider that both solidification types in hypo-eutectic composition alloys are almost identical because the CRF value, eutectic solidification time and expansion mode at solidification of both alloys are nearly the same. </br>  (3) The eutectic solidification time of hyper-eutectic composition spheroidal graphite cast iron is longer than the flake graphite cast iron of the same composition. The expansion at solidification of spheroidal graphite cast iron is great because of this longer eutectic solidification time and thus affects the mold strength, and graphitization.