colored as a function of the sulfur content, present in each series at the transition from compacted to lamellar graph- ite. For each series, the data point in the Figure with highest oxygen activity, corresponds to the Y-block poured at maxi- mal oxygen activity which is still free of lamellar graphite. Figure 18 confirms the results of Figure 13: the transition from compacted to lamellar graphite moves to higher oxy- gen activity when the sulfur content of the melt drops.
Figure 18 has been used to determine the oxygen activity which corresponds to 20 percent nodularity, wherever this was possible. Results show considerable dispersion (Figure 19). This might be due to the inherent dispersion on the graph- ite structures itself or on varying inoculation efficiency during manual addition of the inoculant. Better inoculation increases nodularity in compacted graphite cast iron. On average, at an oxygen activity of 200 ppb, 20 percent nodularity occurs, a value in line with Figure 18. The Figure suggests that at lower sulfur content, compacted graphite can be produced in a wider range of oxygen activities since the distance to the red line increases. In Figures 13 and 19, the matrix has no influence. Consequently, the pearlite promoting elements used here, Mn, Cu and Sn, have in their present content, no influence on the graphite transition from compacted to lamellar nor on the po- sition of the 20 percent nodularity value.
Two Step Method In a previous publication,29
it was mentioned that a two step
method would be most appropriate for industrial production control of compacted graphite cast iron when using oxygen activity measurements. However, the proposal was not de- tailed. Here, a well described procedure will be given al- though it must be underlined that the validation has been obtained in laboratory conditions.
Figure 16. Example of graphite structure from sample with 8% elongation in Fig. 14. The picture was taken at magnification 200x and covers 0.59 x 0.44 mm. Nodularity results for this picture: NFA04-197 39.1%; ISO16112 17.3%; Le/Th
First of all, we need to clarify why a one-step method is not recommended when producing compacted graphite cast iron. In a one-step method, magnesium is added to the melt, the iron is inoculated and poured. The problem arises from the fact that the base iron contains sulfur and oxides. Reduction of oxides as well as the formation of MgS con- sume magnesium. Hence, sufficient magnesium is required before magnesium becomes active in the melt. In chemi- cal terms, magnesium activity19
needs to be large enough
and sulfur and oxygen activities small enough to induce a transition from lamellar graphite to compacted graphite. When using quality pig iron, as in the present laboratory experiments, the sulfur content of the base melt is known in advance. However, industrial charges are composed of scrap which may lead to uncertainties with respect to the
Figure 15. Example of graphite structure from sample with highest elongation in Fig. 14 (22%). The picture was taken at magnification 200x and covers 0.59 x 0.44 mm. Nodularity results for this picture: NFA04-197 94.0%; ISO16112 60.3%; Le/Th
36
Figure 17. Example of graphite structure from sample with 1% elongation in Fig. 14. The picture was taken at magnification 200x and covers 0.59 x 0.44 mm. Nodularity results for this picture: NFA04-197 6.7%; ISO16112 0.2%;
Le/ThInternational Journal of Metalcasting/Spring 10
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