ISSUE 4/2008

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Dan Larsson, Guldsmedshyttan/Jönköping, Torsten Sjögren, Jönköping/ Borås and Ingvar L. Svensson, Jönköping, Sweden
Modelling the plastic deformation behaviour of cast irons

An evaluation of two different equations/models for modelling the plastic deformation behaviour of different cast iron grades has been conducted. The study includes: spheroidal (SGI), compacted (CGI) and lamellar (LGI) graphite cast iron. The Hollomon equation σ = Kεn, frequently used to model the plastic behaviour, and a modified Hollomon equation called the Ludwigson relation σ = K1εn1 + Δ were evaluated. The Ludwigson relation provided the best fit for both the SGI and CGI materials. This is largely due to its ability to capture the two-slope plastic deformation behaviour (in a log-log plot of the true stress-true plastic strain curve) that the SGI and CGI materials commonly exhibit. For the LGI materials, the Hollomon and the Ludwigson equations provided equally accurate models of the material deformation behaviour.
The effects of the microstructural parameters, such as pearlite content, nodularity and roundness on the constants n1, K1, n2 and K2 in the Ludwigson relation of the SGI material was examined and linear relationships were determined. From these linear equations it was possible to accurately model the true stress and true plastic strain curve.
Likely causes of a two-slope deformation behaviour in certain cast irons is also discussed.

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