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Use of numeric simulation to prevent molten metal splatter in coreless induction furnaces

On high-powered medium-frequency furnaces (approx. 1000 kW per tonne of furnace capacity) used for melting cast iron, metal spatter from the bath surface will occasionally be observed in the melt superheating phase. This phenomenon has been found to occur mainly when the furnace is full, i.e., at bath levels significantly above the top edge of the induction coil.

One cause of such metal splatter is the boiling reaction that is a function of temperature and carbon and silicon concentrations of the melt.

1.     C  +  O    _       ( CO )
2.   2C + (SiO2)   Si + 2{CO}

This CO gas formation may set in quite vigorously as soon as the boiling temperature is reached, particularly with thin-walled or rusty charge material which drives up melt oxygen levels. To gain a more accurate understanding of this process that will help in devising countermeasures, the energy, heat and material transfer phenomena taking place inside a medium-frequency furnace were studied in greater detail with the aid of coupled numeric flow and temperature field computing.

It was found that high flow velocities in the upper part of the crucible and the presence of a pronounced meniscus can prevent the occurrence of metal splatter. Presumably, under normal conditions,  the CO bubbles formed in the melt are dragged away by the strong bath flow, escaping into the atmosphere via the pronounced meniscus. Permanent melt degassing will thus take place without any interference by metal splatter.

If, on the other hand, the melt is prevented from continuously degassing in this manner, larger gas bubbles will form and will ultimately develop enough buoyancy to rise abruptly to the surface which they will then penetrate with force, causing the melt to splatter.  The correctness of this description is confirmed by tests and observations made on coreless induction furnaces in several foundries.

 

Calculation of the flow velocity in m/secCalculation of the temperature field in °C

The study has laid the groundwork for the creation of designs and operating regimes for coreless induction furnaces whereby metal splatter from high-oxygen melts can be reduced.  In planning new installations, the furnace rating, operating frequency and coil arrangement can thus be optimized accordingly.  It is also possible to run the furnace at a lower frequency in the superheating phase through the use of a frequency changeover feature (multi-frequency technology). This will result in a more pronounced meniscus and more vigorous surface flow, thus preventing molten metal splatter.

A similar challenge is encountered in melting down galvanized steel scrap, where the problem lies in the associated formation of zinc vapour bubbles in the melt.  Here, too, a "custom" control of the bath movement can help in the management of outgassing phenomena.

 


 

 

 

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