Tool wear and machining dynamics when turning high chromium white cast iron with pcBN tools

Abstract

High chromium white cast irons (HCCI) with high hardness and abrasiveness owing to a particular combination of hard primary and eutectic carbides were used as workpiece materials for longitudinal turning with binderless cBN (bcBN) and high cBN (hcBN) tools. The materials with low and high Si content were used in as-cast and quenched states. Binderless cBN is prone to cratering and flank wear, while hcBN is more prone to significant cutting edge rounding. Atypical behavior was found when machining high-Si material where the increase in cutting forces was accompanied by suppression of vibrations. The observed formation of a tool protection layer (TPL) on the tool–chip interface was found to be responsible for the improved dynamic stability. Electron microscopy and EDX analysis revealed that the TPL consists of nanocrystalline Al2O3 with SiO2 inclusions, both present in the workpiece materials. The stability of TPL was found to depend on both workpiece and tool materials. The absence or removal of the layer resulted in the development of self-excited vibrations with a frequency of 380Hz, which affected the machined surface.