Severe Plastic Deformation of Ferritic and Austenitic Steels

Stephan Günther Scheriau

Research output: ThesisDoctoral Thesis

Abstract

The effect of severe plastic deformation on technical relevant steels is of great interest because they have been found to exhibit excellent properties over their coarse grained counterparts. The present work is assigned to the fundamental understanding of severe plastic deformation of ferritic and austenitic single phase steels with respect to the processing temperature. The two types of steel have been chosen as examples for mechanical and magnetic application, respectively. In the first part the microstructural refinement of typical soft magnetic steels such as Fe-3wt% Si and Fe-17wt% Co is studied in detail by transmission electron microscopy. The physical properties of the aforementioned steels are examined particularly with regard to the coercive force Hc and the saturation magnetization Ms. In several studies before it is shown that the grain size substantially affects the magnetic properties of soft magnetic materials. A dramatic decrease in Hc is observed when the grain size D is in the order of the Bloch wall thickness (D smaller than around 100 nm). In the second part a modified 316L austenitic stainless steel is subjected to High Pressure Torsion. The microstructural evolution is studied in detail with regard to the deformation temperature. The experiments were performed in a temperature range between -196°C (0.04 Thom) and 720°C (0.58 Thom). Depending on the HPT temperature different deformation mechanisms such as simple dislocation glide, mechanical twinning and transformation induced martensite formation govern the severe plastic shearing at high pressure The thermal and chemical stability of severely deformed austenitic microstructures is analyzed in the third part of the thesis. It is observed that the defect density and the distribution of possible nucleation sites for recrystallization is homogeneously distributed when the disc is conducted to cyclic shearing. At the end of the thesis the stress corrosion cracking (SCC) behaviour of the severely deformed 316L austenitic steel is investigated in 45wt% MgCl2 chloride solutions and compared to inert glycerine. For samples with coarse grained microstructures (D >1m) the fracture surface shows fan shaped transgranular characteristics. Dislocation activity and slipping is dominating the crack propagation. In the fine grained microstructures (D
Translated title of the contributionHochverformung von Ferritischen und Austenitischen Stählen
Original languageEnglish
QualificationDr.mont.
Supervisors/Advisors
  • Clemens, Helmut, Assessor B (internal)
  • Pippan, Reinhard, Assessor A (internal)
Publication statusPublished - 2009

Bibliographical note

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Keywords

  • severe plastic deformation
  • high pressure torsion
  • ferritic steels
  • austenitic steels
  • soft magnetic materials
  • micro/nanostructures

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