Influence of Si content on the machinability of ductile iron

Abstract

The development of new chemical compositions of cast iron contributes to increasing the competitiveness of foundry and manufacturing companies that always seek to optimize the properties of these materials. Previous works detected machinability variation along the cross-section of nodular cast irons (DI) bars produced by continuous casting due to the variation of the microstructure and mechanical properties presented, with decreasing machinability from the periphery to the core. This variation imposes complications, as it implies different ideal (or optimal) cutting conditions along the cross-section of the round bars. Assays with ductile irons (DI) with varying silicon content showed that the reduction in silicon decreased the microhardness of ferrite and increased the percentage of pearlite in the matrix, thus generating a decrease in the ductility of the material. Thus, this work investigates the effect of silicon content on machinability variation along the cross-section of nodular cast iron bars in the turning process. Three different types of nodular cast iron alloys were used, differing by the amount of silicon in the composition. Material characterization and machining tests were carried out in three regions: the periphery, the intermediate region, and the core. The hardness, metallography, size of graphite nodules, and amount per region were analyzed in the characterization tests and compared to the evolution of wear and tool life machining force and surface roughness. The addition of different percentages of silicon did not avoid the differences in behavior between the three regions of the same material, but it changed the behavior of the three alloys studied. The silicon content affects the material’s microstructure, which, together with the cooling rate, favors the differentiation of graphite nodules in different sizes, arrangements, and spacing. This differentiation contributes to the difference in the material’s behavior in the machining between the regions along the cross-section. Strengthening in the core region was a trend in all three alloys. There was a statistical difference in roughness between the regions of the material. Cast iron with medium silicon content showed lower machining force and roughness values but close to those of high silicon. As with high-silicon cast iron, the forces remained at a more stabilized level, without significant variations between regions of the material.