Einfluss der Hochverformung auf das Ermüdungsverhalten eines austenitischen Stahls

Translated title of the contribution: The effect of severe plastic deformation on the fatigue behaviour of an austenitic stainless steel

Research output: ThesisDiploma Thesis

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Abstract

Ultra-fine grained and nanocrystalline materials produced by severe-plastic deformation represent a new class of materials. These materials offer extraordinary properties and their mechanical properties can be varied over a wide range. The increasing interest of metals produced by severe plastic deformation is reflected by the increasing amount of published papers on this topic. Larger specimens now permit, the investigation of the toughness or the fatigue properties of severely deformed metals. In the last years several studies on pure metals like copper, titanium or aluminium offered a basic understanding of fatigue mechanisms in such fine-grained materials. However, only a few studies on structural materials, such as austenitic stainless steels, exist. In this diploma thesis the fatigue behaviour of a severely deformed 316L austenitic stainless steel was investigated. Disk shaped samples were deformed in a high-pressure torsion (HPT) tool. By different processing parameters and a post heat treatment, four different ultra-fine grained and nanocrystalline structures were obtained, with grain sizes from 50 nm to 800 nm. The analyses of these different structures should allow a better understanding of the microstructural influence on the fatigue properties of the 316L steel. After the HPT process, samples for tensile and fatigue testing were prepared. Mechanical tests were conducted on the different severely deformed structures as well as on the 10 µm grain size starting material to show the effect of severe plastic deformation. Despite the small sample size 3 mm gauge length, the strain was directly measured on the sample surface. Along with the tensile tests, strain (LCF region) as well as stress-controlled (HCF region) fatigue experiments were carried out. Tensile tests clearly showed that the ultimate tensile strength could be enhanced by a factor of three up to 1900 MPa for the ultrafine-grained samples, but their ductility was inferior compared with the ductile starting material. This fact leads also to a significantly lower life time in the LCF regime, whereas the fatigue limit of the severely deformed structures (600 MPa) was doubled compared to the starting material. In stress controlled tests the ultra-fine grained structures exhibited a significant higher number of cycles to failure for all amplitudes. For strain controlled tests, the performance of the severely deformed samples strongly depends on the applied strain amplitude. The main advantage of the ultra-fine grained austenitic structures compared with heavily cold-rolled austenitic steels is, apart from their higher stress levels, their excellent cyclic stability. Investigations in the Scanning Electron Microscope showed that the damage and fracture mechanisms of coarse grained grained materials changes in the ultra-fine grained and nano structures. Shear banding and strain localisation in these shear bands play a major role for the damage process during cyclic as well as monotonic loading in the severely deformed samples. Further research is needed to better understand the mechanisms for the development of this shear bands.
Translated title of the contributionThe effect of severe plastic deformation on the fatigue behaviour of an austenitic stainless steel
Original languageGerman
QualificationDipl.-Ing.
Supervisors/Advisors
  • Pippan, Reinhard, Supervisor (internal)
Award date16 Dec 2011
Publication statusPublished - 2011

Bibliographical note

embargoed until null

Keywords

  • fatigue
  • 316L
  • ultrafine grained
  • nanocrystalline
  • high-pressure torsion

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