Important parameters describing the metallic matrix are the Ferrite grain size dF
Materials and the pearlite share. A selection of
basic graphite morphology parameters which will be used is given in the following discussion. The shape of graphite particles can be denoted by the shape factor f. The shape factor is defined by the term 4πA/U² where A is the cross section area of the graphite particle and U its perimeter. If the particle is a sphere, f equals unity. The more a particle deviates from a sphere, the lower the f values become. The shape factor can be used to approximately describe the notch effect caused by the graphite particles. The parameter λ, the mean distance between the graphite particles (nearest neigh- bor distance), plays a major role within the discussion of the fracture behavior. λ indirectly denotes the size of the matrix areas between the graphite particles. For a given graphite volume fraction, λ is related to the particle count per area, NA
. Furthermore, the size of the graphite particles can be de- fined by their mean diameter dG
eters in different directions.
Digital image analysis systems have made significant prog- ress in quantitatively analyzing the microstructure of DCI. Digital imaging techniques may require specialized employ- ee training. Nevertheless, the preparation of metallographic microsections and the binary image preparation remain of vital importance in the procedure. A selection of recent ref- erences22, 23
report in more detail on issues of graphite mor- phology characterization with DCI. as the mean value of diam-
Results of three different investigation programs on DCI materials of the European grade EN-GJS-400 will be dis- cussed here.24-26
vermicular graphite were investigated.25
Additionally, DCI grades with compacted The designations of
the materials and the quasi-static tensile properties at room temperature are listed in Table 1.
Table 2 gives a survey on microstructural parameters. The materials were cut from heavy castings,24- 26 rately cast samples,25
Y-shaped sepa- cast plates24, 26 mm and generic DCI weldments.25
Influence of Microstructure, Strain Rate and Temperature on Mechanical Properties of DCI
The influence of microstructure, strain rate and temperature on the mechanical properties of DCI is demonstrated as fol- lows. This background is of special concern because appro- priate mechanical properties are vital not only for the com- ponent design but also as input data for fracture mechanics testing and analysis.
Figs. 1-4 demonstrate the influence of strain rate on the true stress-strain curves of two DCI materials (Tables 1-2) with varying pearlite contents.24
It can be con- cluded that strength moderately increases with decreas- Table 1. Designations and Quasi-Static Tensile Properties of the DCI Materials Under Investigation at Room Temperature (Note: Internal Material Designations). with a thickness of 160
International Journal of Metalcasting/Volume 8, Issue 2, 2014
27
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