Mechanism of the crack propagation in the chip root in dry hard orthogonal turning of the hardened steel

Abstract

Analytic model of the crack-tip field in the cutting tool tip located in the chip root in dry hard orthogonal turning (DHOT) of the hardened steel was established in this paper according to the metal cutting principle and fracture mechanics theory. The relationship between the crack inclination angle ψ to the direction of the tensile stress σ and the crack propagation angle θ0 in I-II mixed model also was proposed. The control equation, displacement function, and boundary condition of crack dynamic propagation in the chip root were derived. The mathematical model for the relationship between the shear angle ϕ and the crack propagation angle θ0 also was presented. Furthermore, formation mechanisms of the serrated chip in DHOT of the hardened steel Cr12MoV (40–60 ± 1HRC) by utilizing the Polycrystalline Cubic Boron Nitride (PCBN) inserts were experimentally investigated. The results showed that the cutting speed and quenching hardness of workpiece have great influence on the crack propagation angle θ0. The shear angles ϕ calculated by the mathematical model are basically in agreement with the experimental results. The thermal softening chip squeezed from the cutting tool rake face begins to shear and slip at high speed along the direction of the crack propagation angle θ0 in the chip root and then the serrated chip forms.