A study of the fine structure of graphite inclusions in gray cast irons by means of the scanning electron microscope

Abstract

1. Direct examination with a scanning electron microscope of gray cast iron with graphite inclusions differing in shape from lamellar to spheroidal indicates that the lamellar graphite is characterized by platelets located at the intersection of two crystals of the metallic base; spheroidal graphite inclusions are located at the intersections of more than two such crystals. 2. Graphite inclusions (the center and branches) are essentially similar — they consist of separate crystallites of different sizes oriented at certain angles, with a block structure and approximately circular shape; at the junction of the branch and the center the mosaic structure is rebuilt, with orientation of the blocks along the branches and some change in their shape, but essentially the same elements. 3. With increasing depth of ionizing etching additional details of the structure are revealed, showing the block structure of the crystallites in the graphite inclusion. The thickness of the blocks is 200–700 A, which corresponds to the dimensions of graphite crystals actually existing in the liquid melt; the width of the blocks varies from ∼0.5 to ∼1.5 μ, and the length from ∼1 to ∼3 μ. 4. All the details of the block structure observed show signs of the complete inclusion as one approaches the intersection of the crystals with the matrix under the influence of force — parallel for platelets and manysided for spheroidal inclusions. The structure of the outer layers of the graphite inclusions shows signs of their diffusional growth in the solid solution with retention of the mosaic block structure. 5. The results of the study make it possible to predict the structure and properties of cast irons after treatments in the liquid and solid states.