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Residual Stresses and Fatigue of Shot Peened Cast Iron
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, The Institute of Technology.
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: urn:nbn:se:liu:diva-99842DOI: 10.3384/lic.diva-99842Local ID: LiU-TEK-LIC-2013:56ISBN: 978-91-7519-501-8 (print)OAI: oai:DiVA.org:liu-99842DiVA: diva2:658377
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
List of papers
1. Residual Stresses in Shot Peened Grey and Compact Iron
Open this publication in new window or tab >>Residual Stresses in Shot Peened Grey and Compact Iron
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2013 (English)Manuscript (preprint) (Other academic)
Abstract [en]

In this paper grey and compact cast irons with a pearlitic matrix are investigated after shot peening with twelve unique combinations of parameters, namely shot size, intensity and coverage, followed by residual stress measurements and evaluations. Cylindrical test samples were cut out from real cylinder heads and polished on the top flat surface to decrease effects from cutting. Residual stresses and the affected depth from the different peening conditions varied between -245 to -565 MPa and from 280 μm to 770 μm in depth. By an increase in shot size or intensity or coverage resultant surface compressive stresses will decrease whereas the affected depth increases with increasing intensity. The increased affected depth is as a result from the increased extent and magnitude of plastic deformation. The compacted cast iron was more affected than the grey cast iron, meaning that the same shot peening parameters resulted in both higher compressive stresses and larger deformation depth.

Keyword
Residual stress, FWHM, cast iron, shot peening, x-ray diffraction
National Category
Engineering and Technology Materials Engineering
Identifiers
urn:nbn:se:liu:diva-99837 (URN)
Available from: 2013-10-21 Created: 2013-10-21 Last updated: 2013-10-21Bibliographically approved
2. Shot Peening Induced Plastic Deformation in Cast Iron: Influence of Graphite Morphology
Open this publication in new window or tab >>Shot Peening Induced Plastic Deformation in Cast Iron: Influence of Graphite Morphology
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2013 (English)Manuscript (preprint) (Other academic)
Abstract [en]

Graphite morphology in cast irons strongly affects their mechanical properties like the Young’s modulus, tensile/compressive strength and cyclic mechanical behaviour. In this paper, pearlitic compact graphite iron (CGI) and grey cast iron (GI) have been studied in a scanning electron microscope (SEM) before and after being shot peened under different conditions. The plastic deformation behaviour of CGI and GI under different shot peening conditions is analysed using electron backscattering diffraction (EBSD) and electron contrast channelling imaging (ECCI). EBSD and ECCI revealed that plastic deformation around graphite inclusions depends on the size and shape of the graphite. The different response of CGI and GI to shot peening is explained by the different damping properties of the graphite and the matrix capability for plastic deformation.

Keyword
Shot peening, cast iron, graphite morphology, EBSD, residual stress
National Category
Engineering and Technology Materials Engineering
Identifiers
urn:nbn:se:liu:diva-99839 (URN)
Available from: 2013-10-21 Created: 2013-10-21 Last updated: 2013-10-21Bibliographically approved
3. In-situ SEM/EBSD Study of Deformation adn Fracture Behaviour of Flake Cast Iron
Open this publication in new window or tab >>In-situ SEM/EBSD Study of Deformation adn Fracture Behaviour of Flake Cast Iron
2013 (English)Conference paper, Published paper (Refereed)
Abstract [en]

Cast irons’ position as an important engineering material is un-disputed. They are widely used in many important industrial applications such as the automotives and workshop machinery. Nevertheless, fracture mechanisms in cast irons are not fully understood. In this study the fracture path and non-linear elastic behaviour of a fully pearlitic flake cast iron under uniaxial tensile loading have been investigated in a Scanning Electron Microscope (SEM) equipped with an Electron Backscattering Diffraction (EBSD) detector. The tensile load was applied via a specially made sample stage. Under uniaxial tensile loading the graphite flakes act as notches or cracks and therefore the fracture process starts at one or many graphite tips. The crack can propagate in many different ways, at the graphite and matrix interface, through the graphite, at the interface between cementite and ferrite or through the pearlitic grains. At the point where the stress strain curve deviates from its linear path plastic deformation at graphite tips can be noticed. Interface cracking between graphite and the matrix also starts at this point.

Keyword
high-temperature, ageing, toughness, austenitic stainless steel, nickel base alloy
National Category
Engineering and Technology Materials Engineering
Identifiers
urn:nbn:se:liu:diva-95430 (URN)
Conference
13th International Conference on Fracture ICF13, June 16-21, Beijing, China.
Available from: 2013-07-03 Created: 2013-07-03 Last updated: 2013-10-21
4. Fatigue Strength of Machined and Shot Peened Grey Cast Iron: Influence of Graphite Morphology
Open this publication in new window or tab >>Fatigue Strength of Machined and Shot Peened Grey Cast Iron: Influence of Graphite Morphology
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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).

Keyword
Fatigue strength, grey cast iron, residual stresses, shot peening
National Category
Engineering and Technology Materials Engineering
Identifiers
urn:nbn:se:liu:diva-99841 (URN)
Available from: 2013-10-21 Created: 2013-10-21 Last updated: 2013-10-21Bibliographically approved

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