Magnesium Evaporation from Ni-base Alloys during Vacuum Induction Melting
Abstract
Magnesium is an element that plays a significant role in the nickel-base alloys. Therefore it is necessary to realize base peculiarities of the magnesium behavior at vacuum induction melting.
All the laboratory heats were melted in a 25-kg VIM furnace. Once the charge is melted down and heated, a sample was taken and the furnace was filled with argon. In experiments with the virgin charge the melt was preliminary deoxidized by aluminum. Thereafter 0.2 wt pct magnesium was added to the melt and it was held for 30-60 min.
During this period 5-7 samples of metal were drawn without any break of seals, the time of sampling being accurately fixed. The melt temperature was measured by Pt/Pt-Rh 6/30 immersion thermocouple; the accuracy of thermocouple was preliminary checked by the crystallization point of Ni. The thermocouple was fixed together with the sample mould at a common device and was introduced in the furnace through the lock chamber. This arrangement ensures, that every sampling is accompanied by temperature measurement.
To avoid the mistakes from extra magnesium loss with Mg-bearing inclusions the first sample was taken 8 min after magnesium addition.
The reduction of magnesium by carbon is also hindered: the equilibrium CO pressure above nickel melt at MgO reduction does not exceed 0.1 kPa. Therefore, in the experiments with argon atmosphere magnesium reduction is impossible, and in vacuum it may occur only to the depth not more than 0.5 mm, which corresponds to 0.3% of all reaction surface of 25-kg crucible.
The influence of stirring intensity on the magnesium evaporation process was investigated on commercial scale 6-t VIM furnace. The experiment procedure was similar to that in 25-kg furnace: after the melting of the charge and heating of the liquid metal the argon pressure of 13.3 kPa was built up and 0.2 wt pct of magnesium was added to the melt. Samples of molten metal were taken during subsequent hold. The inductor active power during the hold was different for two heats: 100 kWt and 500 kWt. Respectively was quite different the stirring intensity of metal.
Mg reduction from the lining of 6-t periclas crucible was ignored on the same reasoning, as for laboratory scale unit. Besides that, account should be taken of alloy composition: it is known, that in the presence of Al and Ti on the "metal-lining" boundary stable compounds of: MgO*AI203 and (Mg, Cr, AI, Ti)xOy-type occur.
Experimentally and computationally it is found that the magnesium evaporation process in vacuum is controlled by mass transfer in metal, and in argon atmosphere – by mass transfer in metal and in gas simultaneously. The rate of evaporation constant is affected by argon pressure and also by metal streams rate in crucible and their turbulence. The influence of several alloying elements, such as chromium, molybdenum, aluminum and iron, on the magnesium evaporation constant was investigated experimentally. It is found that these elements differently affect the constant of magnesium evaporation from nickel: aluminum lowers, and chromium, molybdenum, and iron increase it.
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