Formation Mechanism of Pearlite-ring in Decarburized Spheroidal Graphite Cast Iron

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  • 脱炭処理した球状黒鉛鋳鉄におけるパーライトリングの生成機構
  • ダッタン ショリ シタ キュウジョウ コクエン チュウテツ ニ オケル パーライトリング ノ セイセイ キコウ

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Abstract

  During the cooling of a spheroidal graphite cast iron specimen to room temperature after decarburization at 973 to 1373 K, a decarburized layer and pearlite layer were found to form parallel to the surface of the specimen. In the case of rod shaped specimens, the pearlite layer showed a ring-like shape in the cross-section of the specimen, and so was termed “pearlite-ring”. The width of the pearlite-ring increased with increasing depth of the decarburized layer, and also with increasing decarburizing temperature and decarburizing time. The width had a tendency to increase with increasing cooling rate after decarburization. From the results of EDS analysis and microstructure observation on formation behavior of the pearlite-ring, the formation mechanism is considered as follows : In the decarburized layer, there are some voids without any graphite, other voids with residual graphite and a distribution of dissolved carbon concentration decreasing from the inner side to surface side in the specimen. Since the γ austenite structure during decarburization transforms to a α ferrite structure with low solubility for dissolved carbon concentration, during cooling from γ to α, the supersaturated dissolving carbon atoms should precipitate as Fe3C in the matrix or as C on the residual graphite in the voids. In the decarburized region having suitable dissolved carbon concentration and voids without any residual graphite, the carbon atoms in the matrix precipitate as Fe3C to form pearlite structure. Therefore, the forming position of the pearlite-ring strongly depends on the cooling rate after decarburization in connection with the diffusion rate of carbon atoms in the matrix.

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