Characterization of the milling-induced hardness gradient in the near-surface material volume of high manganese TWIP steel 1.7401 by nanoindentation

Abstract

The hardness gradient induced by up and down milling in the near-surface material volume of high-manganese TWIP steel 1.7401 was characterised up to a surface distance of 50 μm using nanoindentation. Hereby, even the influence of various small indentation depths was such pronounced that the indentation size effect influenced the measured hardness significantly and was, hence, studied. In a first step, the effect of uniaxial, quasi-static deformation on hardness was investigated, which serves as a comparison to hardening by milling which is associated with a complex, multiaxial deformation. Subsequently, suitable parameters for characterization of the milling induced hardness gradient were identified through variation of indentation depths and indentation depth-to-indent spacing ratios. Additionally, the near-surface hardness gradient was examined transversely and longitudinally to the feed direction to analyse whether possible process-induced differences occur due to intermittent cutting. Furthermore, the hardness in the nanocrystalline layer was examined in detail and correlated with the respective microstructures, observed through FIB cutting and ion beam imaging. Finally, the hardness gradients after up and down milling were compared, the effect of electrolytic polishing on the near surface hardness gradient was analysed and the hardness in the rolling skin, representing the initial state, was studied. Milling results in a maximum increase in hardness of approximately 50 % in comparison with the base material, and the hardness decreases degressively with increasing distance to the surface up to a depth of approximately 20 μm. Hardness increases in the milling-induced near-surface material which can be attributed to higher dislocation density. The near-surface layer, measuring 1–2 μm in depth, consists of fine-grained material, and the transition to coarser grains corresponds with a change in hardness slope. No significant hardness gradient was detected along the feed direction, and up and down milling results in similar hardness gradients.