Figure 28. Effect of nodule count on the intercellular segregation of Mn and Si in ductile iron. Nodule count: 1-55 mm,-2
2-125 mm,-2 3-356 mm.-2
Influence of Cell or Nodule Count on the Mechanical Properties
A reduction in the size of eutectic cells (or an increase in cell count) lowers the plane of weakness within iron. From the tensile strength point of view the most desir- able structure is medium size Type A graphite with a pearlitic matrix9
count. Hence, a high cell count is associated with a high tensile strength in cast iron59
which possesses a relatively high cell (Fig. 29a). Yet, the inter-
connection and sharpness of the flake graphite edges can lead to easy fracture, as well as areas of high stress concentrations. In contrast, ductile iron contains non- interconected nearly spheroidal graphite particles thus minimizing the planes of easy fracture. As a result, high strength (Fig. 29b) and good ductility (Fig. 29c) are found in ductile iron which easily surpass the tensile properties of gray iron.
According to the literature, a moderate increase in nod- ule count in normal wall parts, from 100 to 250 mm,-2 leads to significant increases in strength and ductility, as a result of a uniform distribution of fine nodules and a homogeneous micro-structure obtained by austemper- ing.60
In particular it has been shown13 that the rolling
contact fatigue resistance of high nodule count aus- tempered ductile iron (ADI) samples is over five times higher than that obtained in normal nodule count ADI of similar hardness. In contrast, it has been observed that the wear rates under abrasive conditions increases as the nodule count increases.13
(c)
Figure 29. Effect of eutectic cell count on (a) the tensile strength of gray cast iron and (b, c) effect of nodule count on the UTS, yield strength and elongation of ductile iron.
(b)
(a)
54
International Journal of Metalcasting/Summer 10
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