球状黒鉛鋳鉄の逆変態中の膨張について

抄録

  Recently, the improvement of the toughness of iron and steel by reverse transformation is attracting attention. The mechanism of reverse transformation of spheroidal graphite cast iron, however, is considered to be different from steel because of high carbon and silicon content. In fact, during the reverse transformation of ferritic spheroidal graphite cast iron, a remarkable expansion was observed, which was never found in steel. This paper intends to elucidate the mechanism of expansion during reverse transformation of ferritic spheroidal graphite cast iron. The specimen was annealed to obtain a fully ferritic matrix, heated rapidly and held isothermally at the reverse transformation temperature between 800-900°C, and was measured dilatometrically. Metallurgical experiment and colorimetric analysis of carbon cantent dissolved in austenite were also performed. The experimental results were compared with the calculated results from a simple diffusion model.
  Dilatometric behavior during reverse transformation is complicated, but several points, where dl/dt changes remarkably, correspond to the volume fraction of austenite detected by the metallurgical experiment. In the early stage of reverse transformation, a little contraction is observed. This results from the dissolution of a small amount of residual cementite and the decrement of vacancies formed by Kirkendall effect. The remarkable expansion in a later stage of reverse transformation is observed only in ferritic spheroidal graphite cast iron. This phenomenon results from the fact that the density of the austenite matrix with about 0.52-0.53%C dissolved is smaller than that of the ferritic matrix at a given temperature, and that probably, in addition, the graphite becomes porous during reverse transformation.