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Metal Casting Technologies : March 2007
may be cast or wrought. This rod is called a consumable electrode. This electrode is hanged in an electrode holder which moves at a slow speed. The lower end of the electrode is continuously passed into a highly superheated liquid slag. The heat for the process is produced by a current passing between the mould base and the electrode through the slag. The slag temperature is higher than the melting point of the metal. The metal droplets melting off the electrode tip pass through the slag layer. The metal droplets travel through the slag to the bottom of the water cooled mould where they solidify. Since the molten steel has a higher density than the slag, droplets pass through the slag and are collected at the bottom of the mould as liquid which quickly solidifies, leading to progressive build up of a metal ingot. During the passage of the metal droplet through the molten slag, chemical reactions with slag occur. The slag layer is carried upwards as the ingot forms and the ingot is slowly built up in this way. A layer of solid slag skin forms around the ingot giving a smooth ingot surface and it helps in stripping the ingot from the mould3. Power is reduced at the end of the process so that pipe and central porosity can be removed3. Figure 2 shows an actual ESR process. The yellow colour indicates top molten slag layer. The flexible pipe is the fume extractor pipe. The black pipes are the water hose pipes. ESR OPERATION DETAILS The important parts in an ESR unit consist of the mould, base plate, arrangement for electrode movement, power supply, fume extractor and control equipment. The process is less widely used for stainless, carbon, and low alloy steels. If power input is not sufficient the ingot formed will have corrugated surface. Also fins can form on the ESR ingot surface due to rupture of the slag skin. MOULD Mould needs to be made of a material that has a good thermal conductivity and is sufficiently strong to withstand stresses arising due to the action of molten slag and metal and the cooling water, in addition to thermal stresses. Water cooled metallic mould is used for the ESR operation. The mould is jacketed type with an inner tube which is in physical contact with the molten metal and slag, surrounded by a water jacket, through which water is passed. It is desirable that the inner pipe of mould be composed of copper because of its relatively high heat conductivity. The moulds are water cooled to extract the heat from the process and to protect the mould. The mould material can be copper or graphite or steel. Copper mould is expensive and heavy compared to a steel mould. Mild steel mould tapered from the base upwards is suitable. Graphite moulds are less common because of carbon pick up. The shape of the mould is decided by the shape of the ingot desired and can be circular, square, rectangular, etc. Single piece or split type mould arrangements can be used. A safe gap is maintained between the mould wall and the electrode. Lot care has to be taken in split type mould arrangement as slight mismatch and air gap can cause leakage of molten metal and molten slag. For water cooling, water is circulated under pressure through a jacket or sprayed on the mould surface. The bottom forms one part of the electric supply. The base plate is usually of copper and is water cooled. Jacket for the mould liner is provided to make the structure sturdy. The jacket serves to withstand the pressure of the water flowing in the annulers. It provides mechanical support to the mould liner restricting its radial movement. Figure 3a and 3b shows two types of ESR moulds. The first type mould shows the steel inner tube along with outer jacket. Other is spray type cooling mould. Figure 2. Figure 3a. Figure 3b. METAL Casting Technologies March 2007 25