Surface abuse when machining cast iron (G-17) and nickel-base superalloy (Inconel 718) with ceramic tools

Single-point continuous-turning tests were carried out on a G-17 cast iron and a nickel base, Inconel 718, alloy using round and rhomboid-shaped pure oxide (Al2O3 + ZrO2) and mixed oxide (Al2O3 + TiC) ceramic tools to study the extent of damage on the machined surfaces under optimum cutting conditions. The test results show that improved surface finish and lesser damage were obtained when machining the work materials with round inserts of both tools tested. Machining with the mixed-oxide ceramic tools generally produced better surface finish than with the pure oxide ceramics because of their improved hot hardness, fracture toughness and wear resistance. Hardness values of the outer layer of the machine surfaces (up to 0.20 mm deep) show that the cutting conditions chosen produced significant variation and values well above the average hardness of the work materials because of the high rate of work hardening, increased compressive stresses and plastic deformation, particularly for the nickel base, Inconel 718, alloy. The hardness value of the surface layer was also increased by prolonged machining. Metallographic examination of the machined surfaces revealed minimum surface alteration after machining grey cast iron, unlike the very severe damage (tearing and microstructural changes) observed after machining the nickel alloy. Surface damage on the nickel alloy only became severe after machining with the round inserts for three minutes, unlike the rhomboid-shaped inserts which produced poor surfaces even at the start of machining.