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Abstract
Thermo-mechanical loads during hard turning lead to the formation of so-called White Layers on the machined surface. Characterized by a very fine microstructure and high hardness, White Layers have a negative effect on the fatigue life of a component. The fundamental mechanism for the White Layer formation is the dynamic recrystallization (DRX). Therefore, in the current work, two different DRX models, Helmholtz free energy and Zener-Hollomon, are implemented into Abaqus/Explicit to predict the thickness of the White Layer when hard turning quenched/tempered AISI 4140 and the results are compared with each other. For the simulation of the machining process a Finite Element Method (FEM) model based on the Coupled-Eulerian-Lagrangian (CEL) method is built up. Although both DRX models achieved a very good match between predicted and measured White Layer thickness and grain size evolution on the workpiece rim zone, the Zener-Hollomon model produced more closer agreement.
Highlights
Cr/Mo-alloyed steels are mainly used in the automotive and aviation constructions due to their high strength combined with high toughness
Hard turning is known for the induced thermo-mechanical loads that lead to altered microstructures, hardness, residual stresses, dislocation densities and grain sizes at the surface layers
This paper focuses on Finite Element Method (FEM) based modelling of White Layers in the hard machining of AISI 4140 steel, which can be used to determine the process parameters where the formation of White Layers is reduced or does not occur at all
Summary
Cr/Mo-alloyed steels are mainly used in the automotive and aviation constructions due to their high strength combined with high toughness. Caruso et al applied the mentioned hardness-based flow stress and coupled it with the Zener-Hollomon model to predict the grain size [23] They showed that the increase in cutting speed causes a thicker White Layer, while the thickness of the Dark Layer decreases with increasing cutting speed. Buchkremer and Klocke have presented a thermodynamically motivated DRX model based on the variation of Helmholtz free energy to predict the dynamically recrystallized grain size during the orthogonal cutting of AISI 4140 with SFTC Deform® -2D [21] Their experimental and simulative results showed that the austenitizing temperature is not reached or exceeded for cutting speeds vc = 50–150 m/min and undeformed chip thicknesses h = 0.05–0.2 mm. Two different DRX models, Zener-Hollomon (empirical) and Helmholtz free energy (physics-based), were investigated and implemented in a Finite Element model and the results were compared and validated with different set of process parameters
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