Macro- and Microscopic Properties of Gradient Ultra High Strength Thermo-Mechanically Rolled Seamless Steel Tubes

Gerald Winter

Research output: ThesisDoctoral Thesis

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Ultra high strength thermo-mechanical rolled tubes (UHSTMRT) used as structural and design elements in various engineering applications are expected to withstand precedent service conditions including cyclic loading and extreme thermal as well as chemical loads. The further development of UHSTMRT can be based only on a detailed understanding of structure-property gradients across the tube wall and on employing those gradients to adjust the tube overall functional properties. The applied water cooling at the tube surface directly after the last production step results in the formation of complex microstructural, residual stress and phase gradients across the wall. In order to reveal complex structure-function-relationships, advanced multi-scale and multi-method investigations must be applied in order to understand how the cooling intensity influences the variation of the properties across the wall and then scales with macroscopic tube toughness, ultimate tensile strength and resistance to fatigue. In this thesis, advanced analytical techniques are used to investigate structural and mechanical property relationships in UHSTMRT produced under well-controlled thermal conditions by characterizing the overall properties of tubes as well as by analysing the local mechanical, microstructural and phase gradients. Laboratory and synchrotron X-ray diffraction techniques are used to analyse two new weldable, low carbon alloying concepts fabricated using different thermo-mechanical (TM) treatments. The overall macroscopic properties were determined by testing tubes as full wall sections specimens and secondly as micro-sized samples. The micro-sized samples for tension and impact testing are machined from individual tube regions across the wall. Complementary, residual stresses and the microstructure across the tube wall were characterized using X-ray diffraction, Moessbauer spectroscopy (MS) and transmission electron microscopy (TEM) as well as light optical microscopy. Using this multi-method and multi-scale approach gave the possibility to determine a whole set of parameters like phases, grain morphology, dislocation densities, residual stress profiles, tensile strength, fracture strains as well as toughness and hardness profiles across the tube wall. Finally the data were correlated with the conditions applied during tube production in order to understand the formation of gradients as well as their influence on mechanical qualities. The process of the (surface) spray water cooling after the stretch reducing was modelled by finite element (FEM) software DEFORM HT. In the model, experimental data were used to validate and improve the prediction quality for new steel grades.
Translated title of the contributionMakro- und mikroskopische Eigenschaften von Ultra Hochfesten Thermo-Mechanisch gewalzten nahtlosen Stahlrohren
Original languageEnglish
  • Keckes, Jozef, Assessor B (internal)
  • Buchmayr, Bruno, Assessor A (internal)
Publication statusPublished - 2015

Bibliographical note

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  • macro- and microscopic properties
  • gradient seamless steel tubes
  • ultra high strength
  • thermo-mechanically rolled
  • synchrotron
  • XRD
  • residual stress
  • X-ray diffraction
  • moessbauer spectroscopy
  • transmission electron microscopy
  • spray water cooling
  • thermomechanical treatment
  • structure function relationships
  • structure property gradient
  • multi scale
  • multi method

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