Surface Integrity of Die Material in High Speed Hard Machining, Part 1: Micrographical Analysis

The effects of cutting conditions and tool wear on chip morphology and surface integrity during high speed machining of D2 tool steel (60-62 Hrc) are investigated experimentally and analytically in this paper. Polycrystalline Cubic Boron Nitride (PCBN) tools are used in this investigation. The chips and the subsurface of the workpiece are examined using optical and scanning electron microscopy. Microhardness measurements are performed on the surface and subsurface of the workpiece. The X-ray diffraction technique is used to measure the residual stresses induced in the machined surface. The paper is divided into two parts. Part 1 presents the results obtained from the micrographical analysis of the chips and the surfaces produced. Part 2 deals with microhardness and residual stresses of the machined surface. The micrographical analysis of the chips produced shows that different mechanisms of chip formation exist depending on the magnitude of the cutting pressure and tool wear. Saw toothed chips are produced during the machining of D2 tool steel if the cutting pressure exceeds approximately 4000 MPa. The metallographic analysis of the surface produced illustrates the damaged surface region that contains geometrical defects and changes in the subsurface metallurgical structure. The types of surface damage are dependent on the cutting conditions, tool geometry and the magnitude of the wear lands.