examine the results. It would be better if starting point (A1) and end point (A2) could be plotted. This is possible by tak- ing the logarithm of the simplified equation Eq. 6:
log (w) + log (aO ) = log(constant) Equation 10 REFERENCES
Using a double logarithmic plot, all points should fit on a line. This is shown for all the experiments carried out for the two step method (Figure 22). Also the theoretical curve cor- responding to Eq. 9 is plotted. Each line connects the starting point with the end point, with starting point: measured oxy- gen activity, corresponding to a fictitious Mg-wire length given by Eq. 9, and end point: measured oxygen activity and effective Mg-wire length added.
The results seem quite good. Moreover, in the laboratory con- ditions, relatively short Mg-wire is added manually because of the small amount of liquid iron, typically, 240 kg. Industrial conditions using larger ladles and machine controlled, single wire additions will likely produce better results.
Summary
A new sensor, previously tested for ductile iron, has been used to examine the relation between oxygen activity and various features of compacted graphite cast iron. The experi- ments show that for a given sulfur content, a well defined oxygen activity exists which forms the upper limit of com- pacted graphite cast iron. In a short range above this value, compacted graphite remains, but mechanical properties as listed in the ISO standard, are not met anymore. For still higher oxygen activities, lamellar graphite occurs. Lowering the sulfur content of the iron, increases the transition values for the oxygen activity. The oxygen activity corresponding to 20 percent nodularity seems to be constant although some dispersion is noted. Because the sensor produces a result in about 12 seconds, the measurement allows ‘a real time’ characterization of the melt during production. A theoretical
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6. Sillen R. V., “Process Control Methods for Production of Castings in Compacted Graphite Iron”, 66th World Foundry Congress Istanbul, pp 1017-1030 (2004).
7. Mampaey F., “Acoustic Resonance Analysis for Examining the Graphite Shape in Cast Iron,” AFS Transactions, vol. 115, paper no. 07-129 (2007).
8. Mampaey F., Habets D., Seutens F. and Plessers J., “The Use of Oxygen Activity Measurement to Determine Optimal Properties of Ductile Iron During Production,” International Foundry Research / Giessereiforschung, vol. 60, no. 1, pp 2-19 (2008).
9. Mampaey F., Habets D., Plessers J., Seutens F., “The Use of Oxygen Activity Measurements to Determine Compacted Graphite Structure,” 2008 Keith Millis Symposium on Ductile Cast Iron, American Foundry Society, pp 116-127 (2008).
10. Speer M.C., Parlee N.A.D., “Desulfurization Reactions of Magnesium Vapor in Liquid Iron Alloys,” Cast Metals Res. Journal, vol. 8, no. 3, pp 122-128 (1972).
11. Fray D.J., “The Use of Solid Electrolytes in the Determination of Activities and the Development of Sensors,” Metallurgical and Materials Transactions B, vol. 34B, pp 589-594 (2003).
12. HEN, Heraeus Electro-Nite Celox-Foundry, CF 10700692.
13. Mampaey F., “Image Analysis of Graphite Particles by a Mathematical Description of the Particle Contour,” AFS Transactions, vol 113, paper no 05-155 (2005).
Figure 22. Change of oxygen activity due to a second Mg-wire addition. Each line connects oxygen before and after addition. The red curve corresponds to Eq. 9.
International Journal of Metalcasting/Spring 10
14. Riposan I., Datcu M., Chisamera M., Manolescu E., “Control of the Degree of Graphite Compactness in Vermicular and Coral Graphite Cast Irons with the Help of a Romanian System of Automatic Digital Image Analysis, “FOCOMP’86, pp 269-278, Foundry Res. Inst. Poland (1986).
39
relation between magnesium wire added and oxygen activ- ity has been derived. This relation is the basis for a two step method for compacted graphite production. Experiments confirm the proposed procedure.
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