黒鉛球状化機構に関する研究

抄録

  So many theories for the formation of spherulitic graphite in ductile cast iron have been proposed, and thereby we have deepened our understanding of this form of graphite. Spherulite is well known among mineralogist and the spherulites of many organic and inorganic compounds were refered to with respect to the formation of spherulitic graphite in cast iron. A spherulite named “nodular troostite” in steel has been also well known among metallurgist. The nodular troostite is formed when high carbon steel is mildly quenched where A₁ transformation is depressed from 60°C to100°C under 723°C of normal eutectoid transformation point of Fe-C alloys.
  The present research was undertaken to study the noduralization of graphite in Ni-C and related Fe-Ni-C alloys, where no carbide is introduced, in their structune as in the case of cast iron, where double equilibrium γ and Fe₃C in one side and γ and graphite in the other are comming into coexistence. In Ni-C alloys the graphite spherulite is simply formed when thery are melted in CO gas atmosphere and then cast in steel mold, where the eutecic point of 1318° is under-cooled to a temperature of 1285°, without any addition of magnesium.
  Ni-C alloys having eutecic or slightly hyper eutectic composition were used as base metals. They were produced in alundum crucibles, using electrolytic nickel and pulverised charcoal under the cover of a mixture of calcium carbonate and charcoal powder. They were melted in a glover type electric furnace, heating up to a temperature of from 1550° to 1600°C, and then remelted again under the similar condition to get possibly homogeneous alloys. About 20 grams of these alloys were melted in the same furnace without covering flux in a smaller Tammann tube made of alumdum or of carbon electrode, embeded in charcoal powder retained in a some-what larger crucible. Within a few minutes they were molten and attained a temperature of 1550°C, where they were held 2 min. and cast in various molds, i.e. baled core mold, green sand mold, steel mold and copper mold of the same dimension. Thermal analysis of these catings were made with an electro-oscillograph, and the depression of eutectic freezing point and the structure of each castings were observed. Thus we confirmed that melts in the steel mold have an undercooled eutectic point at 1285°C, having a super cooling of 33°C, and complete spherulitic graphite structure all over the specimens.
  The influence of the addition of third elements on the graphite structure were then studied. An addition of manganese up to 2% and silicium up to 1% respectively have no influence on the form of graphite nodules. The influences of calcium and magnesium were then studied. The addition of these elements shew the structure consisting of larger nodules provably fo primary graphite and small nodüles provably of eutectic graphite. Besides they have no influence on the graphite stucture as the residual calcium content is less then 0.12% and the residual magnesuim content is less than 0.098%.
  The addition of titanium up to 0.15% was effective for the refinement of graphite nodules. With titanium content over 0.15%, the structural change was apparent owing to the formation of titanium carbide eutectic. Finally we studied the effect of iron, adding Fe-C eutectic alloy to Ni-C alloys. Up to 10% of iron the graphite structure was not changed, but if the iron content was increased over 10%, the graphite structure changed gradually forming incomplete nodüles and finally all flaky graphite of on alloy containing 50% of iron.
  From all these experiments the neccessary conditions for the formation of spherulitic graphite in Ni-C and Fe-Ni-C alloys may be summerised as the following.
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