Milling is a well-established manufacturing process that allows not only allow the creation of desired geometries, but also enables the generation of surface morphologies. High manganese TWIP steels are known for their excellent combination of strength and ductility due to TWinning Induced Plasticity (TWIP). The properties of the material lead to a pronounced hardening during milling processes. Since fatigue crack initiation frequently occurs at the surface, a hardened surface layer can potentially increase fatigue strength.

In the present work, surface morphologies were produced by up (UM) and down milling (DM). In order to distinguish the influence of different surface morphology characteristics on the fatigue properties, two different surface morphologies were created by electrolytically polishing of the up milled surfaces, removing the near-surface material to a depth of approximately 4 µm (EP 4) and 50 µm (EP 50). In addition, the rolling skin (RS), which represents the initial surface condition of the sheet material, was investigated. The surface morphologies were firstly analysed in terms of topography using confocal microscopy and SEM. Secondly, the near surface grain structure was characterised using FIB techniques. Thirdly, the near-surface residual stress was measured using the sin²(ψ)-method. Fourthly, nanoindentation was used to study the differences in the near surface hardness gradient after milling.

Stress controlled single step tests with a load ratio of R = -1 at stress amplitudes of σa = 360 MPa, σa = 340 MPa and σa = 320 MPa were performed at ambient temperature. To avoid any influence of temperature on the deformation behaviour due to specimen self-heating, the temperature was measured in the gage length of the specimens. Furthermore, frequencies of f = 1 Hz, f = 2 Hz and f = 5 Hz were chosen, resulting in a specimen temperature increase of < 10 K. In addition to fatigue life, the cyclic deformation behaviour was analysed by measuring the stress-strain hysteresis. Despite the identified large differences in several characteristics of surface morphologies, milling did not affect either the cyclic deformation behaviour or the fatigue life. This result was further investigated by focusing on the crack initiation site and the near surface residual stress evolution as a function of cycles at a stress amplitude of σa = 340 MPa.