Relationship between mechanical and metallurgical properties with machinability when drilling high-strength cast irons

The search for greater energy efficiency in internal combustion engines has led to the development of high-strength cast irons. The superior properties of these materials do, however, lead to lower machinability, making the final product less competitive. This paper aims to investigate, in the drilling process, the correlations between machinability and both mechanical and metallurgical properties of high-strength cast irons. Three grades of gray cast irons were evaluated: the standard grade 250 and two high-strength grades 300, one with refined graphite and other with molybdenum addition. The compacted graphite cast iron grade 450 was also tested for comparisons, as this material is also a promising candidate for such application. The output variables considered were the tool life, with analysis of the tool wear mechanisms, thrust force, torque, and quality of machined holes (roughness, roundness, and cylindricity). For correlation with the machinability, the materials were characterized according to the interlamellar spacing and microhardness of the pearlite matrix, the number of eutectic cells, and distribution of the manganese sulfide (MnS) inclusions. Test results generally showed a close correlation between the machinability and the mechanical and metallurgical properties of the materials, especially between the gray cast irons.