Fractal Analysis of Solidification Microstructure of High Carbon High Alloy Cast Roll Manufactured by Centrifugal Casting

The fractal dimension analysis was applied to the evaluation of primary dendrite and several eutectic structure morphologies in the high speed steel type cast iron for rolls which have been produced by the centrifugal cast technique. Initially, the affine parameter of the dendrite morphology of transparent organic alloy was evaluated based on the self-affine fractal theory. Primary γ dendrite and several eutectic structures crystallize in the high carbon high alloying element roll specimen. The microstructural morphology of sliced γ dendrite was analyzed by the self-similar fractal technique. The fractal dimension of each part of two-dimensionally-sliced dendrite varies from 1.05 to 1.13, and that of aggregation of dendrite's parts increases to 1.30. The fractal dimension of the primary dendrite among eutectic phases varies depending on the analysis area, and decreases with in decreasing of volume fraction of primary dendrite. In the specimen contained relatively high alloying elements, the round shape of sliced dendrite and the reduction of area fraction of primary dendrite cause to the difficulty of analyses using secondary dendrite arm spacing and fractal dimension of primary dendrite. Therefore, the fractal dimension of primary γ and γ+MC eutectic structure were evaluated. The fractal dimension analysis reveals that the morphology of dendrite and eutectic structure in segregated region slightly increases in comparison with the normal microstructure, and decreasing with the cooling velocity. Lower equilibrium partition coefficient of alloying elements and centrifugal force could cause the micro segregation of outer dendrites in roll specimen and complicated primary dendrite and γ+MC eutectic structure.