Effect of increased mechanical strength on the machinability of graphite cast irons in face milling

Abstract

Due to the low cost and high vibration damping capacity, gray cast irons are commonly used in machine tool bases, in addition to applications with noise restrictions, such as engine blocks, housings, and brakes. The matrix’s graphite, sulfides, and ferrite/pearlite ratio are some of the most important parameters governing the machinability of the gray cast irons. This work aims to evaluate the machinability of high-resistance gray cast irons of the FC 300 grade, in two versions, with the addition of molybdenum (FC 300 (Mo)) and with refined graphite and addition of molybdenum (FC 300 (Mo+RG)), for use in cylinder heads and engines blocks, compared to materials that have been used for this purpose, gray cast iron FC 250 and the compacted graphite cast iron FV 450. The materials were characterized according to the cementite interlayer spacing and microhardness of the perlite matrix, the number of eutectic cells, and the distribution of manganese sulfide inclusions, with those characterizations being correlated with the machinability results. The face milling process was chosen for the tests with uncoated cemented carbide tools without cutting fluids; the cutting speed and feed rate varied in addition to the type of cast iron. Analysis of tool life, wear mechanisms, and machined surfaces’ quality (Ra roughness parameter) were the output variables considered. Using harder materials such as FC 300 (Mo) or FC 300 (Mo+RG) for the engine would bring benefits such as increased durability, reduced weight, and improved efficiency, but it has a cost in machining. These harder materials have a shorter tool life compared to the commonly used gray cast iron FC 250, but the difference in performance is less pronounced under more extreme cutting conditions, such as high cutting speed and feed rate. When analyzing the surface roughness, FC 250 produced a smoother surface at low cutting speeds but performed worse than FC 300 (Mo) and FC 300 (Mo+RG) at higher cutting speeds.