Correlation between machinability and chip morphology of austempered ductile iron

In this work, macrostructure, microstructure, and work hardening of as-cast and austempered ductile iron chips produced by different cutting speeds were investigated. Cutting forces, surface roughness, and tool wear were also studied. After austenitizing at 900°C for 90 min, samples were austempered in a salt bath at 360°C for 90 min. Machinability tests were conducted with uncoated cemented carbide tools on a CNC lathe. The tests were carried out without using any lubricant. In these tests, the cutting speed was varied in the range 50-200 m/min, while the feed rate (0.12 mm/rev) and depth of cut (2 mm) were kept constant. The tangential, feed, and radial cutting force components acting on the tool holder were measured with a three-component dynamometer. The chips obtained during machining were polished by embedding into epoxy resin, examined with optical microscope and micrographs were taken. The microhardness values were measured on each chip sample. As the cutting speed was increased, deformation strengthening of the microstructure was increased, which decreased the tool life. It was also detected that during the machining, with the effect of cutting forces and stress, spheroidal graphites were broken off in the chip and lost their sphericity, so the chip became fragile and unstable and the graphite displayed a lubricant feature. At low cutting speed, a built-up edge was observed. However, as the cutting speed was increased because of heating, built-up edges were unstable.