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more clearer. Moreover, the previous research showed in- creased surface nodule count with higher oxygen activity in the spheroidal graphite range.8


spheroids generally have lower nodularity.21


Fewer but larger sized Döpp also


found that a high amount of pig iron (> 75%) also low- ered nodule count and decreased nodularity.22


no Ce was added. However, as Finally, a


small Ce content improves nodularity too. In the present and previous research8,9


already mentioned before, the lower values for the nodu- larity is mainly due to the image analysis technique used and associated parameters. The method used by the pres- ent authors is in the first place intended to quantify the quality of graphite nodules as well as the compacted / lamellar graphite structure with a single method. The pro- nounced maximum for the nodularity might be interesting for the production of ferritic thick wall castings. Maximal nodularity and ferrite content favor impact energy which is sometimes difficult to obtain in the as cast condition. Figure 2 shows some data which are close to the com- pacted graphite iron range. It led to a new research which aimed to answer the question whether oxygen activ- ity could also provide information in case of compacted graphite iron. This idea was raised before by Kusakawa23 and Hummer.24


The first results published for compacted graphite cast iron were carried out at similar sulfur content. In these circum- stances, a well defined oxygen activity (300 ppb) which forms the upper limit of compacted graphite cast iron was found. This critical value is valid for ferritic and pearlitic compositions. Two different inoculants give the same re- sults. The experiments show that oxygen activity and its relation with graphite structure are independent of the way magnesium wire is added. For a certain oxygen activity, the associated graphite structure does not depend on the way the magnesium wire was added; the total length all at once or in several parts. The present research is based on many more experiments, with special interest for the effect of different sulfur levels.


Experimental Procedure


The goal of the present experiments is to examine if a use- ful relation can be established between oxygen activity and the production window for compacted graphite cast iron; more specifically, the minimal mechanical proper- ties as required by the ISO 16112 standard, the transi- tion from compacted graphite to lamellar graphite and the 20 percent nodularity level. For this purpose, various mechanical properties (tensile strength, proof strength, elongation, hardness) as well as the graphite and matrix structure have been determined.


Figure 3. Graphite structure in iron with lowest oxygen activity from Fig. 2 (060295 S1, oxygen activity at 1420ºC = 53 ppb, Mg 0.077%). The picture was taken at magnification 100x and covers 1.2 x 0.89 mm.


Figure 2. Nodularity versus oxygen activity (at 1420ºC) for ferritic ductile iron.8


28


Figure 4. Graphite structure in iron with optimal oxygen activity from Fig. 2 (060295 S1, oxygen activity at 1420ºC = 74 ppb, Mg 0.025%). The picture was taken at magnification 100x and covers 1.2 x 0.89 mm.


International Journal of Metalcasting/Spring 10


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