Abstract
Due to pronounced work-hardening induced by the complex interplay of deformation mechanisms such as dislocation slip, twinning and/or martensitic phase transformation, high-manganese steels represent a class of materials well-suited for mechanical surface treatment. In the present study, the fatigue behavior of a high-mangsanese steel showing twinning-induced plasticity (TWIP) effect at room temperature (RT) was investigated after deep rolling at 550 °C. Results are compared to a former study discussing the behavior after RT deep rolling. Evolution of the near-surface microstructure was analyzed by scanning electron microscopy (SEM), microhardness measurements and residual stress depth profiles obtained by X-ray diffraction (XRD). Both uniaxial tensile tests and uniaxial tension-compression fatigue tests have been conducted in order to rationalize the macroscopic material behavior. Following deep rolling at 550 °C, SEM measurements employing electron backscatter diffraction (EBSD) revealed a heavily deformed surface layer as well as localized deformation twinning. Specimens showed inferior hardness and residual stress depth profiles when compared to RT deep rolled counterparts. Tensile tests indicated no difference between the conditions considered. Fatigue properties however were improved. Such behavior is rationalized by a more stable residual stress state induced by dynamic strain aging.
Highlights
Due to their improved formability and crash performance, advanced high-strength steels (AHSS) have been in the focus of the mobility sector for more than two decades
In addition to nanoparticle-hardened, martensitic transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) steels, as well as the subgroup of density-reduced light-weight steels realized by adding high aluminum contents, TWIP and TRIP steels in particular have been in focus of scientific and commercial interest in the group of high-manganese steels (HMnS) due to their outstanding mechanical properties under monotonic loading [3,5]
HMnS TWIP and TRIP steels are characterized by a lower density of about 6.8–7.3 g/cm3 compared to conventional steel counterparts [9,10], eventually allowing for the development of weight-reduced crashrelevant body structures leading to a reduction in fuel consumption and CO2 emissions
Summary
Due to their improved formability and crash performance, advanced high-strength steels (AHSS) have been in the focus of the mobility sector for more than two decades. The crack growth behavior as well as the fatigue properties in different regimes, including low-cycle and high-cycle fatigue (LCF/HCF) regimes, have been investigated in several studies focusing on the influences of twinning and martensitic phase transformation, i.e., TWIP- and TRIPeffect, as well as the influence of grain size, strain rate, deformation temperature and pre-deformation on the fatigue behavior. Following deep rolling at RT and 200 ◦C, twinning in the near-surface area was detected to a similar extent, indicating that the increase in temperature and, in the SFE was not high enough to suppress the TWIP-effect. The martensitic phase transformation induced by cryogenic deep rolling was shown to have a negative impact on the fatigue performance, at least at relatively high loading amplitudes This was rationalized by an increased plastic deformation as well as microstructural notch effects in the two-phase region established in the near-surface area. The residual stress stability at half-life for conditions deep rolled at room and elevated temperature was studied
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