(a)


(b)


(c)


Figure 24. Schematic description and exhibited graphite structures for hypoeutectic cast iron. (Ia,b,c) cast iron solidification and the effect of cell count on the structure of the graphite skeleton and, as well as, on the distribution and size of flake, (II) A, D and E type graphite on a metallographic section graphite and (III) SEM micrographs of A, D and E type graphite.


There is very little information on the influence of nodule count on the austempering times. From the published literature,58


with a cast iron with 1304 nodules per mm2 the


time needed to obtain an ausferrite matrix in a cast iron with 2546 nodules per mm2


is far shorter (25 min) when compared (approx. 90 min).


Figure 25. ASTM graphite size number relation to cell count and graphite type.


Modelling of microsegregation including microanalysis in duc- tile iron shows that by increasing the graphite nodule count, sol- ute segregation to the intercellular boundaries can be reduced significantly (Fig. 28). The exhibited matrix microstructure can be a function of intercellular segregation. In particular, it is known that Si is a ferrite stabilizer and that Mn is a pearlite and carbide promoter. Hence, in as-cast iron, the regions near the graphite boundaries should develop increasing amounts of fer- rite when compared with the intercellular regions. The segrega- tion effects on heat-treatment are also important. If the purpose of heat treatments is to achieve full matrix ferritization, the heat treatment times should be increased in cast irons with a high degree of segregation as Mn is a pearlite stabilizer.


52 International Journal of Metalcasting/Summer 10


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