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Metal Casting Technologies : December 2010
24 www.metals.rala.com.au TECHNICAL FEATURE decomposition of CaCO3. The powders were added in small batches and allowed to melt. When the melt attained sufficient fluidity it was tapped in a preheated clay graphite crucible and allowed to cool down. The solidified slag mass removes easily due to the non wetting behaviour of slag with graphite crucible. The slag is crushed and sieved into fine powder. This slag being hygroscopic was safely stored in a dessicator. The ESR process was carried out in a water cooled mould of with electrode connected to DC positive power. A dummy steel starting block was welded on a 1 mm thick steel square plate. The cast electrode was welded to the electrode holder and the process was started by initiating an arc between starting block and the base of the electrode. Slag was preheated at 9000C for 6 hours to avoid any moisture pickup. The slag was added into the arc zone immediately. Water flow to the mould and base part is maintained at 35 litres/ min and 20 litres/min. A pure aluminium wire of 3 mm diameter is tied on the electrode length for slag deoxidation purpose. The electrode was fed into the molten slag smoothly through a reduction gear box and screw arrangement. The full slag addition took around 3 to 4 minutes. The electrode drops get refined when they pass through the liquid slag. The process current and voltage were maintainedat750-780Aand25±2 V. The collected droplets solidify at the mould base. The slag melting point should be less than the electrode metal and it should be electrically conducting. Image 4 shows the ESR Unit. The molten slag temperature is about 18500C. Alumina increases the slag resistivity whereas Lime addition increases the desulphurization. As a result of ESR process the properties of the ESR ingot are increased to a great extent. The smaller inclusions which are not removed get uniformly distributed throughout. The ESR ingot is removed from the mould and later on characterized. The length of the ESR ingot is 240 mm. Image 5 shows the ESR ingot. Characterization and results The ESR ingot was annealed at 9000C in an electric resistance furnace. The ingot was cut for chemical analysis, mechanical testing and microstructure samples. Image 6 shows the sound defect free ingot. Heat treatment was carried out in a tubular furnace. The austenitizing Image 4. The ESR Unit. Image 5. The ESR ingot. Image 6. The sound defect free ingot.