The increasing performance and durability of cutting tool inserts have created metallurgical challenges for production foundries to produce near-net shaped castings within strict dimensional tolerances. In order for foundries to take full advantage of the increased cutting speed capabilities, it becomes necessary to reduce machining allowances and produce much more stable casting surfaces. To accomplish this, a better understanding of the complex microstructures formed within the first 0.120 in. (3 mm) of the mold/metal interface (as-cast surface) is necessary. The goal of the work presented here was to examine the microstructures formed in the near-surface region of gray iron castings, determine what was responsible for formation, and how these microstructures behaved during the machining process. A series of experiments were performed to evaluate the effect of graphite flake morphology, matrix microstructure, and alloying elements on near-surface machinability. Three-dimensional cutting forces, quantitative metallography, and high-speed photographic measurements were used to evaluate the behavior of flake graphite, ferrite, coarse/dense pearlite, steadite, and carbides during the machining process. Data from the experiments also indentified the importance of inoculation practice, cooling rate, and mold sand properties on the final near-surface microstructure/machinability behavior. A case study was then performed for industrial brake rotor castings produced from class 35 gray cast iron, in which diagnosis of a machinability problem proved to be near-surface microstructure related. It was found that a combination of mold sand properties and inoculation practice were responsible for surface free-ferrite/graphite morphology microstructural defects --Abstract, page iii
