An experimental investigation of work material microstructure effects on white layer formation in PCBN hard turning

Abstract

White layers formed in machining of hardened alloys are known to be very hard and resistant to standard etchants used in metallographic studies. Many studies have been performed on this subject, but only with little progress showing definite results concerning the actual effectiveness of white layer formation. Hence, the basic question that remains unanswered is: are the white layers a tribological advantage for the manufacturing industry producing parts/components from hard alloys? The focus of this study is to investigate the evolution of white layers produced during progressive tool flank wear in dry hard turning with CBN (cubic boron nitride) tools, and to correlate this with the surface integrity of the machined surface. The following four materials were machined: X160CrMoV12 cold work steel (AISI D2), X38CrMoV5 hot work steel (AISI H11), 35NiCrMo16 high toughness steel and 100Cr6 bearing steel (AISI 52100). Samples of chips were metallographically processed and observed under an electronic microscope to determine whether white layers are present or not. More specifically, chip-forms/shapes were studied to determine how they developed during machining with potential appearance of white layers, with a view to correlating the chip-forms/shapes with the white layer formation. Finally, by using scanning electron microscopy and EDS techniques on these chip samples, properties and microstructures of white layers were deduced in order to verify some of the prevalent theories.