Influence of the surface morphology on the cyclic deformation behavior of HSD® 600 steel

Abstract

Abstract. The presented research work investigates the fatigue properties in the low cycle fatigue (LCF) regime of the high manganese metastable austenitic High Strength and Ductility (HSD ® ) 600 TWIP steel dependent on its surface morphology. The steel features, according to its chemical composition following the alloying concept Mn-Al-Si-C and heat treatment, a fully austenitic microstructure that shows deformation induced twinning at ambient temperature. Due to this microstructural deformation mechanism, HSD ® 600 steel has an outstanding combination of strength and formability. Besides monotonic deformation behavior, characterized by tensile tests, cyclic deformation behavior was investigated with varying the surface morphology of fatigue specimens. In order to create different surface morphologies, flat fatigue specimens were excised from larger sheets by waterjet-cutting. Depending on the surface morphology, further climb milling or up-climb milling in the gauge length was performed. The three investigated morphologies (asreceived with rolling skin, climb milled and up-climb milled) differed in roughness, initial residual stresses and initial phase compositions. For all variants, total strain controlled fatigue tests with stepwise increasing load amplitudes as well as total strain controlled single step tests were performed in the low cycle fatigue regime with a load ratio of Rε = -1 and a frequency of f = 0.2 Hz. Beside stress-strain hystereses, the changes in temperature ΔT and the magnetic properties ξ were measured. The magnetic properties directly correlate with the transformation from paramagnetic γ-austenite to ferromagnetic α'-martensite. The cyclic deformation behavior of the HSD ® 600 steel in the LCF regime was characterized by cyclic softening until fracture at low total strain amplitudes but changed with increasing total strain amplitudes into initial cyclic hardening followed by cyclic softening. This initial cyclic hardening became more pronounced when the total strain amplitude increased. Furthermore, single step tests at lower total strain amplitudes showed a saturation state before fracture. A comparison between the monotonic and cyclic deformation behavior showed a significant difference of the stress levels at the same amounts of plastic deformation respectively. Nevertheless, the different surface morphologies led to different lifetimes at high total strain amplitudes but to similar lifetimes at lower total strain amplitudes.

Surface Morphology and Its Influence on Cyclic Deformation Behavior of High-Mn TWIP Steel

“…Flat specimens with longitudinal axis transverse to rolling direction were taken by water-jet cutting. Note that the monotonic properties given in Reference [53] do not differ between longitudinal and transverse direction. To achieve different surface morphologies, the gage length surfaces were milled with the vectors of feed and cutting speed against each other (up milling) and with both vectors in the same direction (down milling), respectively.…”

“…Additionally, the magnetic properties were measured in situ with a Feritscope ® "MP30E" Helmut Fischer (Helmut Fischer GmbH, Sindelfingen, Germany) for specimens with the as-received morphology, i.e., with a so-called rolling skin, because this rolling skin showed phase transformation from γ-austenite to α'-martensite during fatigue tests while the austenitic microstructure of the bulk material was stable against phase transformation. The rolling skin was removed in case of the other investigated morphologies, see Reference [53] for further details.…”

“…The monotonic deformation behavior of the HSD ® 600 TWIP steel in comparison to total straincontrolled strain increase tests is shown in Reference [53]. In the present work, total strain-controlled fatigue tests were performed at specimens with all four investigated surface morphologies to characterize the cyclic deformation behavior in the LCF regime.…”

“…Furthermore, to obtain a reference state, some of the down milled specimens were electrolytically polished after the milling process until the milling induced residual stresses were removed and a smooth surface was achieved. Including the two machined surfaces, in this study four surface morphologies were investigated: unmachined with a so-called rolling skin of about 30-40 µm [53] (M RS ), after up milling (M UM ), after down milling (M DM ) and as a reference state after polishing (M PO ) ( Figure 3). These four morphologies have different properties concerning topography and roughness, initial residual stresses and initial phase fractions.…”

Effect of Thermomechanical Treatment on Structure and Functional Fatigue Characteristics of Biodegradable Fe-30Mn-5Si (wt %) Shape Memory Alloy

“…The accumulation of plastic deformation resulting from the action of cyclic mechanical stress can lead to the initiation and propagation of fatigue cracks in the regions of stress concentration. In [ 25 , 26 , 27 , 28 , 29 , 30 ], the fatigue behavior of the Fe-Mn-Si engineering alloys was studied at relatively small deformations; it is shown that the mechanocycling process is accompanied by an initial hardening followed by softening and secondary cyclic hardening.…”