Objective Description of Graphite Morphology in Ductile Cast Iron by Numerical Geometric Parameters
摘要
As an innovative evolution of conventional cast iron, solid solution-strengthened ductile iron (SSDI) with its excellent ductility and strength stands out from compared to commonly used standard grades. The spheroidal graphite precipitates in the present metal matrix are produced by a magnesium treatment of the cast iron melt and are decisive for the excellent mechanical properties of the cast material. Deviations of the graphite particles from the spheroidal form, up to degenerations such as chunky graphite, lead to a deterioration of the mechanical properties. The current standards ISO 945-2 and 4 mention nodularity and roundness as important quantities to describe different graphite morphologies. Furthermore, analysis software for graphite description uses probability formulas containing the two-dimensional geometric parameters compactness, convexity, and roundness to distinguish between the different graphite forms I to VI. However, there is no objective description of degenerated graphite, like chunky graphite or exploded graphite. To avoid different interpretations of the same graphite morphology and to provide a more objective analysis, a logistic regression/data-driven approach is suggested. Different two-dimensional geometric parameters are measured to describe each graphite precipitate in a microsection, namely, roundness, convexity, Feret-ratio, nearest-neighbor distance, compactness, graphite particle area Areal, and sphericity. The individual geometric parameters are evaluated using a logistic regression model on the basis of their empirical distribution (e.g., mean value, first and third quantile, etc.). The present results show that an objective quantification of the change in graphite morphology toward degeneration is possible using this statistical approach.