chemical principles, predicts increasing oxygen activities when magnesium activity — or equivalently wire length — approaches zero. The oxygen activity of a base melt without any magnesium added varies for the present experimental conditions between 900 and 1200 ppb. Taking 1000 ppb as the maximal value, Equation 6 becomes
Equation 7 where w1 is the wire length consumed to reduce free oxygen.
The second refinement concerns the reduction of oxides and MgS formation. Previous research showed that the reduction of oxides occurs gradually when magnesium is added and oxygen activity drops. Only when oxygen ac- tivity becomes 72 ppb, representing optimal conditions to produce ductile iron, solely MgO is formed.29
Accordingly,
we assume that for oxygen activities between 1000 ppb and 72 ppb, oxides present in the melt as well as sulfur, gradu- ally combine with magnesium
Equation 8
is the wire length consumed to reduce oxides and to bind sulfur. Combining Equations 7 and 8, relates the total wire added w and oxygen activity
where w2 Equation 9
Figure 20 shows the experimental relation between the length of wire added in a single step and resulting oxygen activity. When a large length of Mg wire is added (about 7.5 m per 100 kg melt) the efficiency of magnesium addi-
tion drops. These experiments were carried out in order to create a high initial magnesium content (0.070 – 0.080%). For the theoretical curve of Eq. 9 shown in Figure 20, next data have been used: K1 = 250 and K2 = 4x10-4
.
Note, however, that variation of the initial oxide and sul- fur content will modify the constants in Equation 9. When adding more wire, a departure from the curve is noted for several reasons. First of all, the solubility of magnesium in liquid carbon saturated iron is limited.10
The higher the
magnesium in solution, the higher the vapor pressure and the faster the element leaves the bath. More importantly, in the experimental conditions where we intended to add excessively high magnesium, two connected wires were added simultaneously which is rather inefficient. How- ever, also for small additions (
The practical implementation of the two step method is quite simple and illustrated schematically in Figure 21. After a first addition (e.g. during a pre-treatment), oxygen activity is measured (e.g. A1 tious wire length wA1 oxygen activity A2 wA2 wA1
) which corresponds to a ficti-
. The constants in Equation 9 will be different for each foundry since it depends on the efficiency of the wire treat- ment. Even more simply, each foundry has to measure oxy- gen activity before and after a second treatment, and relate the difference between both, to the length of wire added. This is the only input needed to calibrate the method. Fig- ure 21 is based on the principle of ‘additivity’: a certain oxygen activity is obtained for a wire length as given by Equation 9, whether this length is added in one step or in several steps, remains the same.
.The additional length of wire to be added is then wA2
The two step method has been examined experimentally on a laboratory scale. However, Figure 20 is not really suited to
(Eq. 9). The wire length for the target is calculated with Equation 9, giving –
Figure 20. Magnesium wire added in a single step versus oxygen activity. The curve corresponds to Eq. 9.
38
Figure 21. Additivity principle of two step method for production control of compacted graphite cast iron.
International Journal of Metalcasting/Spring 10
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