Surface damage during machining of annealed 18% nickel maraging steel Part 1 — Unlubricated conditions

The effect of cutting speed and tool wear land length on the surface damage produced during the machining of annealed 18% nickel maraging steel under dry orthogonal conditions was determined. Machined test pieces were examined with a scanning electron microscope and an optical microscope. Surface roughness was determined with a profilometer.The results of the investigation show that during machining a wide variety of different forms of surface damage is generated. The machined surfaces show extended regions where both coarse and fine scale surface damage have occurred at cutting speeds up to 0.1 m s^?1, whereas at cutting speeds greater than 0.1 m s^?1 the surfaces show evidence only of fine scale surface damage. It is suggested that the regions of coarse scale surface damage are associated with the phenomenon of partially discontinuous chip formation and the nucleation of cracks in the vicinity of the tool nose region. Several mechanisms of crack nucleation and propagation, which are thought to account for the occurrence of many aspects of the surface topography observed, are presented and discussed. It is suggested also that the regions of fine scale surface damage are associated with the phenomenon of continuous chip formation and interaction between the tool nose region and the freshly machined workpiece surface.It is shown that scanning electron microscopy is more indicative of the true condition of the surface than surface roughness measurements.