Influence of Microstructure on Material Behaviour of WC-Co Hard Metals under Static and Cyclic Loading Conditions

Thomas Klünsner

Research output: ThesisDoctoral Thesis

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Abstract

WC-Co hard metals are so far mainly utilised for wear applications and metal machining. In recent years the property spectrum of WC-Co has been widened significantly, so today new hard metal grades are available with improved hardness, bending strength and/or fracture toughness. The main achievements are based on WC grain refinement, leading to fine and ultrafine grained hard metal grades. To facilitate the reliable design of fatigue resistant tools or structural components via e.g. finite element methods (FEM), a sound knowledge of the materials’ behaviour under static and cyclic loading conditions is required. Unfortunately, material data such as yield strength and fatigue strength for WC-Co hard metals is scarce in literature today, especially for grades of ultrafine WC grain size. There is increasing interest in the application of hard metals in components and tools, varying widely in dimensions, e.g. cutting matrices at the decimetre sizes, fine blanking tools at centimetre sizes or printed circuit board drills with diameters even below 200 µm. Since there exists a significant influence of loaded volume on tensile stress sustained in service, part design also requires some basic understanding of their relation. To date, the dependence of tensile strength data on specimen size has not been thoroughly investigated for ultrafine grained WC-Co hard metals in a wide range of specimen sizes. In the present work, detailed information about a material testing procedure for the reliable determination of the stress-strain response of WC-Co hard metals under uniaxial loading conditions, in tension and compression, is presented. Material data such as Young’s modulus and yield strength were determined under uniaxial loading conditions, along with the influence of microstructural parameters such as WC grain size, binder content and binder matricity on flow behaviour. In addition, the dependence of tensile fracture behaviour on specimen size was examined for a vast dimensional spectrum, in which loaded volumes vary over a wide range depending on specimen size and stress distribution, revealing the intrinsic ultimate strength of ultrafine grained WC-Co hard metals in tension. Furthermore, the cyclic stress-strain response, including ratchetting behaviour, was studied under uniaxial loading conditions, as well as the cyclic fracture behaviour, which was investigated via S N curves determined under uniaxial and bending loading conditions. The threshold of the stress intensity factor range at which fatigue crack growth starts from inhomogeneities was determined for different stress ratios. Results of investigations on the effects of surface finish, residual stresses and overloads on the S-N behaviour of WC-Co hard metals are discussed.
Translated title of the contributionGefügeeinfluss auf das statische und zyklische Materialverhalten von WC-Co Hartmetall
Original languageEnglish
QualificationDr.mont.
Supervisors/Advisors
  • Pippan, Reinhard, Assessor A (internal)
  • Ebner, Reinhold, Assessor B (internal)
Publication statusPublished - 2011

Bibliographical note

embargoed until 30-05-2016

Keywords

  • WC-Co
  • hard metal
  • mechanical properties
  • micromechanical experiments
  • size effect
  • matricity
  • ratchetting
  • overload

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