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The Effect of Graphite Fraction and Morphology on the Plastic Deformation of Cast Irons
Daros Piston Rings AB.
Division of Component Technology, Jönköping University, Jönköping, Sweden.
2007 (English)In: Metallurgical and materials transactions. A, ISSN 1073-5623 (print) 1543-1940 (online), Vol. 38, no 4, 840-847 p.Article in journal (Refereed) Published
Abstract [en]

The plastic deformation behavior of cast irons, covering the majority of graphite morphologies, has not been comprehensively studied previously. In this investigation, the effect of graphite morphology and graphite fraction on the plastic deformation behavior of pearlitic cast irons has been evaluated. The investigation is based on tensile tests performed on various different cast iron grades, where the graphite morphology and volume fraction have been varied. Pearlitic steel with alloying levels corresponding to the cast irons were also studied to evaluate how the cast iron matrix behaves in tension without the effects of the graphite phase. It is concluded that as the roundness of the graphite phase increases, the strain hardening exponent decreases. This demonstrates that the amount of plastic deformation is higher in the matrix of lamellar cast iron grades compared to compacted and nodular cast iron grades. Furthermore, this study shows that the strength coefficient in flake graphite cast irons increases as the graphite fraction decreases due to the weakening effect of the graphite phase. This study presents relationships between the strain hardening exponent and the strength coefficient and the roundness and fraction of the graphite phase. Using these correlations to model the plastic part of the stress-strain curves of pearlitic cast irons, we were able to calculate curves in good agreement with experimentally determined curves, especially for gray cast irons and ductile iron.

Place, publisher, year, edition, pages
2007. Vol. 38, no 4, 840-847 p.
National Category
Engineering and Technology
URN: urn:nbn:se:liu:diva-14419DOI: 10.1007/s11661-007-9115-8OAI: diva2:23471
Available from: 2007-04-20 Created: 2007-04-20 Last updated: 2010-05-31
In thesis
1. Influences of the Graphite Phase on Elastic and Plastic Deformation Behaviour of Cast Irons
Open this publication in new window or tab >>Influences of the Graphite Phase on Elastic and Plastic Deformation Behaviour of Cast Irons
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The amount and morphology of the graphite phase largely controls the resulting properties of cast iron. For instance, in flake graphite cast irons the mechanical properties are low while the thermal conductivity is high. This is in contrast with spheroidal graphite cast irons where the mechanical properties are high and the thermal conductivity is low. These differences are due to the different graphite morphologies and must be accounted for in the design work and material selection of cast iron components. In this work the influence of the graphite phase on the elastic and plastic deformation behaviour of cast irons has been studied.

The material grades studied originate from castings for marine diesel engine piston rings with different chemical analyses. Two groups of pearlitic cast iron materials were studied; one with differences in graphite morphology and one with grey irons that differed in graphite content. For these different material grades the mechanical properties were correlated to microstructural parameters. In addition to standard uniaxial tensile tests, acoustic emission measurements were used for the study of deformation.

When studying the modulus of elasticity of the cast iron it was found that the modulus of elasticity of the inherent graphite phase depends on the roundness of the graphite particles and is due to the strong anisotropy of the graphite phase. A linear correlation between nodularity and the modulus of elasticity of the graphite phase was derived. This correlation made it possible to account for the anisotropy of the graphite phase in the model used. By applying the linear function when modelling the effective modulus of elasticity, a high accuracy between experimental and theoretical values was achieved.

Another factor affecting the elastic response when subjecting a cast iron component to tensile load was found to be the plastic deformation that actually occurs at very low strains for all of the studied cast iron grades. It was observed that the plastic deformation in the low strain elastic region, quantified by using acoustic emission measurements, increased linearly with decreasing modulus of elasticity. These measurements showed that the amount of plastic deformation in the elastic region was largely controlled by the graphite morphology. It was concluded that as the roundness of the graphite particles increases, the plastic deformation activity in the elastic region decreases.

The plastic deformation activity continued linearly into the pronounced plastic region of the tensile tests. A decrease in roundness or increase in graphite fraction resulted in an increase of the amount of plastic deformation and the strain hardening exponent. A

dependence between strength coefficient and graphite fraction was observed. Models for the flow curves for pearlitic cast irons were developed and shown to accurately reproduce the observed experimental curves.

The surveys performed and conclusions from this thesis will be helpful in the design of new cast iron materials.

Place, publisher, year, edition, pages
Institutionen för ekonomisk och industriell utveckling, 2007
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1080
Cast iron, Flake graphite, Compacted graphite, Spheroidal graphite, Elastic deformation, Plastic deformation, Modulus of elasticity, Graphite modulus of elasticity, Strength coefficient, Strain hardening exponent, Acoustic emission
National Category
Other Materials Engineering
urn:nbn:se:liu:diva-8776 (URN)978-91-85715-52-7 (ISBN)
Public defence
2007-05-25, sal E1405, Ingenjörshögskolan i Jönköping, Jönköping, 10:00 (English)
Available from: 2007-04-20 Created: 2007-04-20 Last updated: 2009-03-13

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