Einflussfaktoren auf den Eigenspannungszustand in Hartmetall

Kayser, Wolfgang Andreas; Broeckmann, Christoph (Thesis advisor); Klocke, Fritz (Thesis advisor)

Düren / Shaker Verlag (2019) [Book, Dissertation / PhD Thesis]

Page(s): 1 Online-Ressource (XVII, 188 Seiten) : Illustrationen, Diagramme

Abstract

During the manufacturing process of cemented carbides, severe internal stresses form between the cobalt binder and tungsten carbide phase of cemented carbides. The cause for internal stresses is found in strain incompatibilities between the constituent phases caused by the different mechanical and thermophysical properties of the constituents. The main cause for strain incompatibilities is the difference in coefficient of thermal expansion of cobalt and carbide. Further incompatibilities could be caused by the re-precipitation of elemental tungsten and carbon solved in the binder at existing grain boundaries of the carbide. The objective of this thesis is to generate a finite element model, which is capable to reflect the temperature-dependent type II residual stresses in cemented carbides and to account for the causes mentioned above. Therefore geometric models are derived from EBSD data of WC6Co and WC20Co cemented carbides, which are discretised by finite elements in a subsequent step. For each cemented carbide grade five models are generated. The material description of the finite element models is based on a constitutive law, which describes the cobalt as elastic-viscoplastic material. In contrast to cobalt the tungsten carbide phase is modelled as purely orthotropic-elastic material. Within the constitutive law the temperature dependence of the model parameters is accounted for. Based on the assumption that solely cobalt underlies plasticity, the temperature-dependent internal stresses are determined for different cooling conditions, leading to the characteristic residual stress state at room temperature. Therefore only the cobalt binder is responsible for the compensation of strain incompatibilities. Within a first numerical study, the influence of the cooling conditions on the residual stress state is investigated for all available models. The re-precipitation effect is neglected at first. In a subsequent study the qualitative effect re-precipitation is incorporated in one model of each of the cemented carbide grades under investigation. For the implementation of the re-precipitation effect the results of CALPHAD simulations, describing the solution and re-precipitaiton kinetics of tungsten and carbon in cobalt, are integrated into the constitutive description. By means of comparing the numerical results achieved with and without considering the re-precipitation effect, the order of magnitude of the influence of re-precipitation on the residual stress state is determined. The study is closed with the comparison of numerical and experimental neutron diffraction data, to add information to the numerical results regarding the residual stress state in cemented carbides.

Identifier

  • ISBN: 978-3-8440-7017-0
  • REPORT NUMBER: RWTH-2019-10477