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Metal Casting Technologies : March 2007
TECHNICAL FEATURE www.metals.rala.com.au 26 ELECTRODE Cast, forged or rolled electrodes of the material to be refined is used in the process. The electrode surface should be clean. The composition of the electrode should be near to the final desired ingot composition. Some elements oxidize and can cause improper chemical composition. Proper allowances should be given for the loss of oxidizing elements. Alloying elements can be added in the ESR process. The electrode can be welded to the main electrode holder. The electrode is hanged to the ESR unit and is moved up and down by a screw arrangement driven by a variable speed DC motor. When using oxidizing elements in the steel/alloy, an argon atmosphere may be used. This can reduce the oxidation of the elements. The electrode should be straight to permit entry into the mould without touching the mould wall. The presence of an oxide skin on the electrode can make it difficult to retain oxidizable elements like titanium and aluminium. Fill ratio is the ratio of electrode diameter to ingot diameter. A large fill ratio means a smaller gap between the electrode and the mould. A high fill ratio helps to promote the desirable shallow metal pool condition. Generally a fill ratio between 0.6 - 0.7 is preferred4. Too large a fill ratio may lead to arcing between the electrode and the mould, which may lead to puncturing of the mould. Figure 4 shows a cast alloy steel electrode. The cast electrode is welded to the steel electrode holder. The steel holder has a copper block attached to it. A tapered copper contact is fixed to the busbar strips. This contact fits inside the copper block. SLAG Slag is an important part of the process. The slag produces the heat for the process, protects the purified metal from atmospheric and refines the metal. By proper selection of slag composition proper chemical reactions will be there between metal and slag and the amount of undesirable impurities can be reduced. The slag should be stable at high temperature, should have reasonable electrical conductivity and its melting point should be lower than that of the metal to be refined5. Calcium Fluoride (CaF2) is the main slag constituent in the ferrous ESR process. Lime (CaO), Alumina (Al2O3), Silica (SiO2), Titania (TiO2) are also added in the slag for other requirements. Addition of Alumina (Al2O3) to the slag is common. The desulphurizing ability of CaF2 slag is increased by addition of CaO. Sulphur can be lowered by adding lime to the slag6. When using CaO based slags, care must be taken to prevent moisture pick up. Sulphur is a harmful element in steel. In quality steels low content of sulphur is desirable. Basically sulphur is removed as follows, from metal to slag7 [S]+ (O2-) = (S2-) + [O], where [ ], ( ) refer to metal and slag phases. Medovar8 et al. have shown that desulphurization is reduced when remelting under argon and suggest that sulphur can be oxidized from the slag by the reaction. (S2-) + 3/2 O2 (g) = SO2 (g) + (O2-), when remelting in air. This effect has been confirmed by Holzgruber9 and Kasin10 who reported that loss of sulphur from the slag decreases with increasing slag basicity. According to Whittaker11 et al. only 25 percent of sulphur removal from metal is retained in the slag, the balance escaping to the atmosphere. A thin layer of solidified slag skin forms around the ingot. This gives a smooth ingot surface and helps in stripping of the ingot from the mould3. Composition % Application CaF2 Al2O3 CaO 70 30 Steels and alloys without Ti or B, desulphurization 80 20 Steels and alloys without Al, Ti and B, maximum desulphurization 60 20 20 Steels and alloys without Ti or B, maximum desulphurization Figure 4.