SURFACE ABUSE WHEN MACHINING CAST-IRON (G-17) AND NICKEL-BASE SUPERALLOY (INCONE-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-sh aped pure oxide (Al2O3 + ZrO2) and mixed oxide (Al2O3 + TiC) ceramic t ools to study the extent of damage on the machined surfaces under opti mum cutting conditions. The test results show that improved surface fi nish and lesser damage were obtained when machining the work materials with round inserts of both tools tested. Machining with the mixed-oxi de ceramic tools generally produced better surface finish than with th e 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 condit ions chosen produced significant variation and values well above the a verage hardness of the work materials because of the high rate of work hardening, increased compressive stresses and plastic deformation, pa rticularly for the nickel base, Inconel 718, alloy. The hardness value of the surface layer was also increased by prolonged machining. Metal lographic examination of the machined surfaces revealed minimum surfac e alteration after machining grey cast iron, unlike the very severe da mage (tearing and microstructural changes) observed after machining th e nickel alloy. Surface damage on the nickel alloy only became severe after machining with the round inserts for three minutes, unlike the r homboid-shaped inserts which produced poor surfaces even at the start of machining.