Microstructure Impact on the Machining of Two Gear Steels. Part 1 : Derivation of Effective Flow Curves

A multiscale approach is presented here to investigate the effect of the ferrite-pearlite microstructure after annealing on the subsequent machining process of steel gears. The case-hardening steel 18CrNiMo7-6 and a cost efficient alternative with reduced Cr and Ni content have been studied. Based on an advanced microstructure characterisation, three different scales are introduced: the nano-scale with pearlite, built of ferrite-cementite bi-lamellas, the micro-scale, which corresponds to a RVE of the ferrite/pearlite microstructure and the macro-scale. In order to derive the effective flow behaviour of pearlite, virtual uniaxial tensile and shear tests of the ferrite/cementite bi-lamella are performed at the nanoscale. The flow behaviour of the ferrite phase is described there by an extension of the Kocks-Mecking law suitable for large machining strains. Moreover, at the nanoscale, the effective flow curve of the ferrite matrix having either small MnS or NbC inclusions is determined. At the microscale, effective flow curves for both steel grades are derived from virtual tests on 3D RVE’s of both steel microstructures and compared with experimental measurements