Relationship between chemical microheterogeneity of gray cast iron and the structure of flake graphite

The relationship between chemical microheterogeneity of gray cast iron castings, revealed by the method of color etching in sodium picrate, and the structure of flake graphite was studied. The results of microstructural studies indicate that the crystallization of gray cast iron is accompanied by the emergence of several levels of chemical and structural microheterogeneities of the crystallizing melt, affecting the formation of graphite. Microstructural features indicate that the cast iron melt, which is close in carbon content to the eutectic composition, acquires a dendritic and cellular structure before the formation of graphite. The intensity of chemical microheterogeneity and its topology are hereditarily related to the location, size and shape of graphite inclusions. Flake graphite formed inside and on the periphery of melt cells has different structure and thickness of plates. The enrichment of areas of cast iron melt with silicon leads to the formation of dendrites of primary austenite and primary graphite of the largest size in them. The microhardness of ferrite in the dendrites of former primary austenite is approximately 1.5 times greater than the microhardness of ferrite in areas adjacent to inclusions of primary graphite. A close inverse correlation (r = –0.93) was established between the microhardness of ferrite in areas with different liquation intensities and the thickness of graphite flakes located in these areas. The greatest spread of microhardness values and width of graphite flakes is observed in the most liquated areas enriched with silicon in the center of the cells, corresponding to the location of eutectic colonies. In the spaces between the cells, graphite is observed with the smallest thickness of the flakes, which are located at the smallest distance from each other. It is necessary to continue research into the hereditary influence of the structure and physical and chemical properties of high-carbon iron melts on the mechanism of graphite formation and the features of its structure in cast irons.