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Fatigue Strength of Machined and Shot Peened Grey Cast Iron: Influence of Graphite Morphology
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, The Institute of Technology.
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, The Institute of Technology.
Base Engine & Materials Technology, Volvo Powertrain, Gothenburg, Sweden.
Testing Methods and Testing, Scania CV AB, Södertälje, Sweden.
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2013 (English)Manuscript (preprint) (Other academic)
Abstract [en]

A common opinion is that cast iron, especially grey cast iron, is not as notch sensitive as steel and has therefore not been treated by shot peening to suppress crack initiation. For a heterogeneous material that also is brittle, just like grey cast iron, the shot peening parameters needed to induce beneficial surface residual stresses can be problematic to identify. Fatigue testing under uniaxial loading with an R value of -1, on mechanically polished and shot peened specimens, has been performed to determine the fatigue strength at 107 cycles as well as complete Wöhler-curves. Two different types of specimen geometries were tested, one smooth and one notched specimen having kt equal to 1.05 resp. 1.33. With large shots and high peening intensity (heavy shot peening) the fatigue strength clearly decreased whereas small shots and low peening intensity (gentle shot peening) might have lowered the fatigue strength. A short annealing at 285° after gentle shot peening increased the fatigue strength. The results are discussed and explained based on x-ray diffraction measurements, i.e. residual stress and full width at half maximum profiles, as well as microstructural investigations using scanning electron microscope (SEM).

Place, publisher, year, edition, pages
2013.
Keyword [en]
Fatigue strength, grey cast iron, residual stresses, shot peening
National Category
Engineering and Technology Materials Engineering
Identifiers
URN: urn:nbn:se:liu:diva-99841OAI: oai:DiVA.org:liu-99841DiVA: diva2:658364
Available from: 2013-10-21 Created: 2013-10-21 Last updated: 2013-10-21Bibliographically approved
In thesis
1. Residual Stresses and Fatigue of Shot Peened Cast Iron
Open this publication in new window or tab >>Residual Stresses and Fatigue of Shot Peened Cast Iron
2013 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The complex geometry of cylinder head in heavy-duty diesel engine makes grey cast iron or compact graphite iron a perfect material choice due to its castability, thermal conductivity and damping capacity. To increase the efficiency of the engine, the fatigue property of the material needs to be improved. Shot peening is often used to increase the fatigue strength of components. The benefits are associated with the compressive stresses induced and with surface hardening. In this research project, these effects on grey and compact iron have been analyzed for different shot peening parameters using XRD, SEM and fatigue testing methods. The ultimate aim of the project is to increase the fatigue strength of cast irons by optimization of residual stresses.

The XRD measurements and SEM examinations revealed that the shot peening parameters including shot size and peening intensity had significant influences on the resulted residual stresses and strain hardening while changing the coverage made little difference. Also differences in the peening results between the two materials were observed, which were ascribed to an effect of the different graphite morphology. Nevertheless, a residual stress profile similar to the one general considered to improve the fatigue strength in steels could be obtained in both grey and compact iron after shot peening.

The axial fatigue testing with R=-1 on the grey iron showed that peening using large shot size and high peening intensity (heavy shot peening) resulted in a fatigue strength reduction of 15-20% in comparison with the mechanically polished surface. The negative effects are likely related to surface damage and relatively high tensile residual stresses in subsurface induced by the heavy peening. Grey cast iron has low ductility in tension and therefore tensile residual stresses may promote multiple cracking and crack networking during cyclic loading.

Shot peening using much smaller shots and lower intensity (gentle shot peening) which resulted in a much smaller residual stress field gave no significant changes in fatigue strength. However, a short time annealing at 285°C of specimens being gently shot peened increased the fatigue strength roughly by 10%. The improvement could be an effect of precipitates formed due to the heat treatment, which lock the dislocation movement under cyclic loading.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2013. 45 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1622
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-99842 (URN)10.3384/lic.diva-99842 (DOI)LiU-TEK-LIC-2013:56 (Local ID)978-91-7519-501-8 (ISBN)LiU-TEK-LIC-2013:56 (Archive number)LiU-TEK-LIC-2013:56 (OAI)
Presentation
2013-10-24, ACAS, Hus A, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2013-10-21 Created: 2013-10-21 Last updated: 2013-11-12Bibliographically approved

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Lundberg, MattiasPeng, RuJohansson, Sten

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