initial sulfur content of the melt. Even if sulfur content is known, this is not the case for the amount of oxides pres- ent in the melt. Total oxide content and distribution varies considerably, even in the same pig iron.30,31
The only alter-
native for a one-step method is to add more magnesium so that in all cases enough active magnesium is present. This is the current practice when producing ductile iron. In case of compacted graphite cast iron, with its small production window, a one-step treatment will regularly result in over treated melts with too much nodular graphite.
Hence, the idea of a two-step method. After the first addition giving too low a magnesium content to produce compacted graphite, oxygen activity is measured. The extra magnesium addition needed should be calculated from the difference be- tween the measured and the target oxygen activity. Indeed, because all reducible oxides have disappeared and sulfur is combined with magnesium, any further magnesium addition will directly raise magnesium activity (i.e. free magnesium) and equivalently lower oxygen activity.
First of all, it would be helpful to get an idea about the rela- tion between the magnesium added and the oxygen activity. The chemical reaction
Equation 1 satisfies at equilibrium Equation 2 where ∆G0 is the Standard Gibbs energy for the reaction, K
is the equilibrium constant, R is the gas constant and T is the absolute (Kelvin) temperature. At a given temperature, the right hand side is constant, giving
Figure 18. Nodularity as determined by length – thickness method in Y-blocks as a function of oxygen activity at 1420°C. Only structures without lamellae are shown. Symbols are colored according to sulfur content at the transition compacted graphite to lamellar graphite for ease of examination: red S ≥ 0.014%; black 0.007 ≤ 0.007%; blue S ≤ 0.002%.
Equation 3
As a first approximation, we assume that a relation exists between magnesium activity and the amount of magnesium added, here expressed as the length of wire added per 100 kg melt (w)
Equation 4 where w0
represents magnesium wire needed to reduce ox- ides and to combine with sulfur. Assuming that the activity of MgO is constant, gives
Equation 5
tionship (solubility product): w aO
In case w0 = constant is zero, we retrieve the well known chemical rela- Equation 6
Figure 19. Oxygen activity at 1420°C and sulfur content for which 20 percent nodularity occurs. Red and blue lines are taken from Figure 13.
International Journal of Metalcasting/Spring 10
Crucial for the two-step method is the correct prediction for the second wire addition. However, to derive the govern- ing equation, some refinements need to be included which concern the limited oxygen solubility and the reduction of oxides and MgS formation. Equation 6, which is based on
37
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