Chip formation mechanism in dry hard high-speed orthogonal turning of hardened AISI D2 tool steel with different hardness levels

Abstract

Serrated chip formed in dry hard turning is considered one of the major chip types. In this paper, the main objective was to understand how the crack initiation and propagation, and thermo-plastic instability and pressure from force contribute to the formation mechanism of serrated chip in dry hard high-speed orthogonal turning (DHHOT) of the hardened steel with different hardness levels at cutting speed with 50, 450, and 850 m/min. The influences of the cutting speeds (50, 450, and 850 m/min) and workpiece hardness (40, 45, 50, 55, and 60 ± 1 Rockweel hardness (HRC)) on chip morphology, segment spacing, degree of segmentation, chip deformation coefficient, shear angle, and chip segmentation frequency also were experimentally investigated. Experimental results showed that the very high strain in the shear band does give rise to the high temperature in higher hardness material and at higher cutting speed and this makes the high-speed slip of the shear band much easier happen along existing micro-crack. The critical chip is produced at a cutting speed of 50 m/min and a hardness level of 50 ± 1 HRC. The strain rate increases with the increments of the cutting speed, which increases brittleness, and thus induces acceleration of the crack propagation speed in shear band. Moreover, the increments of the quenching hardness can increase the brittleness of the workpiece and thus lead to the large damage in shear band. The microstructure of the material within the bottom of chip showed that the elongated grains do appear due to thermo-mechanical effect between the chip back and the rake face of the cutting tool.