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Metal Casting Technologies : March 2007
www.metals.rala.com.au 32 low power even a rough surface is obtained. Figure 10 shows the ingot surface with constrictions. Ferro alloys like Fe - Si can be added during the process. The crushed ferro alloy powder along with dry slag is added into a hollow tube and tapped to remove any air pocket. The tube ends are made flat by hammering on an anvil. The tube is welded on the electrode length surface. As arcing takes place and slag is added the tube flat end is melted and ferro alloy powder passes into the liquid metal. The whole powder can fell down when its end is opened. To avoid this, the passage of powder is reduced by tapping the full tube by a hammer at intervals of about 1 inch length. The passage of electrode into the mould should be straight. If during the process electrode movement is eccentric and it touches the mould wall shorting can occur. This can cause the water passing through the mould jacket to rush inside the mould. This can cause a big accident and the molten slag and metal can be splashed out. Regular eccentricity of electrode with respect to mould has to be checked during the ESR process. The base should have a provision for XY moving arrangement. Water at about 35 litres per minute (lpm) is passed into the mould jacket and about 22 lpm is passed into the base. The outlet water from the mould and base is collected in a big tank and is cooled in a cooling tower and again recirculated. This avoids wastage of water. After standardization of ESR process with mild steel electrodes, trials were carried out silicon steels with low carbon and preparing alloy steels with Cr, V, Mo and Ni. Silicon steels are used in electrical applications. The main problem encountered was oxidation of silicon. Steel with 4 % Si was melted in a coreless induction furnace with a magnesia lining and poured in cast iron moulds of 40 mm diameter and 600 mm height. These as cast silicon steel electrodes were subjected to ESR. It was found that there is oxidation of silicon and final ingot had some 2.5 %Si. The slag was quiet and not foamy during the process. Trials were done by adding more silicon during air induction melting to compensate the loss of Si during the ESR. Results showed good Si composition. The electrode can be coated with a thin layer of graphite or magnesia with sodium silicate as binder. The electrode can be melted in inert atmosphere to avoid Si oxidation. The alloy steels prepared displayed high strength and toughness. The static ESR limits the ingot height and is not feasible in industrial applications. Continuous ESR experiments were carried out in a water cooled copper mould of 160 mm length and 50 x 50 mm square cross section. 30 x 30 mm square cross section steel electrodes were remelted in a continuous operation. The square ingot built up is withdrawn from the bottom of the mould. Care has to be taken that there is no metal leakage when the ingot is being withdrawn. Ingot has to be withdrawn slowly and matched to melt deposition rate. The slag cap weight at the end of the process was 300 gms. With continuous withdrawal no slag shell was observed on the ingot surface whereas in static ESR slag skin thickness ranging from 0.7 - 0.9 mm was obtained. In the case of continuous ESR the ingot surface had dull appearance and was uneven at some places. VARIATIONS IN ESR ESR process can be done as a static or a continuous operation. Slag cap forms at the top and ingot is formed at the bottom so the final ingot length is lesser. The static operation has a fixed mould height. The ingot built is of fixed height. A longer mould means more metal and more machining. This increases the cost of the mould. For producing ingots of longer lengths a longer static ESR mould is needed. This is not feasible in mass production. A static longer copper mould is not feasible as copper is expensive. In the continuous process a shorter mould is used. The mould may be fixed and the built up ingot is withdrawn from the bottom21. Also the ingot can be fixed with the mould moving up. The continuous operation can produce longer ingots depending upon the vertical space. This is feasible for producing bigger diameter and longer ingots. In both operations water cooled moulds are used. The ingot withdrawal rate should be proper and it should go with metal deposition rate. Figure 11a and 11b shows the set TECHNICAL FEATURE Figure 10.