The previous Figure reveals how oxygen activity and vari- ous properties change with time after magnesium addition. However, magnesium fade — and because of its relation with oxygen, also oxygen activity — may be discontinuous as a function of time. Indeed, when the melt in the furnace is calm, magnesium fades very slowly. Once the power is turned on to keep the metal in the furnace at a constant tem- perature, stirring of the liquid begins and magnesium fade increases substantially. The most important conclusion from this behavior pertains to the use of time as a production con- trol factor. Magnesium fade depends on many factors; in this case the magnesium fade is affected by the power supply intensity and length of time. Hence, magnesium content may decrease at an irregular rate and will not be reproducible as a function of time. The measurement of oxygen activity yields values which are reproducible and independent of the rate of magnesium fade.
Consequently, all properties examined need to be plotted as a function of the oxygen activity. The moment of the pour- ing of a keel block differs from the time oxygen activity is measured. Hence, the oxygen activity value for a keel block is linearly interpolated as a function of the time using the oxygen activity measured just before and after the moment of pouring a keel block.
Several series of experiments similar to those shown in Fig- ure 5 have been carried out. For all these experiments, tensile strength, proof strength, elongation and nodularity will be plotted as a function of the oxygen activity. Ferrite content and hardness do not provide additional information and will not be considered further. Figure 6 shows the tensile strength as a function of the oxygen activity valid for 1420°C (2588F). Points labeled with E correspond to an extra magnesium wire
Figure 6. Tensile strength for a ferritic matrix as a function of the oxygen activity at 1420°C. The red line corresponds to the minimal strength required by the ISO Standard (Table 2).
Figure 5. Change of various properties during holding of melt 070839-S2. After 31 minutes extra magnesium was added. The vertical dashed line shows the end of the compacted graphite formation. The vertical arrow illustrates the sudden drop of elongation. Oxygen activity at 1420ºC, ferrite percentage, tensile strength, proof strength are shown in red and pertain to the left scales. Nodularity, elongation and Brinell Hardness are shown in blue (right hand scale).
International Journal of Metalcasting/Spring 10
Figure 7. Proof strength for a ferritic matrix as a function of the oxygen activity at 1420°C. The red line corresponds to the minimal strength required by the ISO Standard (Table 2). Tensile bars not presenting a proof strength anymore have been given a zero value in order to illustrate the transition from a compacted to a lamellar graphite structure.
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