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Metal Casting Technologies : March 2007
SOME SLAGS USED IN REMELTING12 During the ESR process the slag reduces the inclusion content of the metal. This is because when the metal droplets pass through the slag, the inclusions are either dissolved by the slag or they float out and join the slag. Thus bigger inclusions are removed by this way. Inclusions that remain are smaller and are divided uniformly than in air melted steel. Ingot is homogeneous than conventional cast ingot as refining is drop by drop and there is controlled solidification. In general oxygen is removed to some extent during the process. Addition of Al2O3 increases the resistivity of the slag and it provides more heat. PROCESS STARTING Initially in the ESR process molten slag is obtained by - 1) Molten slag is made in a coreless induction furnace and introduced in the mould in the molten condition. This gives a smooth and quick start but additional furnace is required and it increases the cost. There are difficulties in feeding the molten slag in the space between electrode and mould wall because molten slag can freeze fast. 2) The slag mixture is melted inside the mould by striking an arc between the electrode and bottom plate. Once the molten slag pool is formed the electrode melting proceeds smoothly without arcing provided proper parameters have been used. In the case of top pouring of slag, care should be taken to avoid splashes to electrode or mould wall. This can cause formation of a skull which can be a problem later on13. A variation of molten slag starting introduces the slag from the bottom through the base or mould wall thus preventing formation of skull14. ELECTRICAL POWER AC or DC both arrangements can be used for the ESR process. Arrangements in ESR are - 1) Single electrode (AC and DC) and one mould. 2) Three electrodes on three phases and one mould. 3) Three electrodes on three phases in three moulds. Usually a.c used as the equipment is cheaper and as a.c helps in stirring the slag metal interface area due to electro capillary action and provides better refining of the metal by slag15. Single phase operation with one electrode and one mould is the simplest and is recommended for small melts. The process operates at currents varying from 500 A to 50000 A. The current required depends on the cross sectional area. The voltage of the order of 15 - 70 V is required across the slag. The ESR units need some form of transformer unit or generator as the power requirements are high. Plants with transformer capacities ranging from 100 KVA to 1350 KVA are in use. METAL OXIDATION Change in chemical composition of the electrode occurs due to oxidizing conditions during remelting. This can result in loss of element like Silicon (Si), Titanium (Ti) during remelting. A small loss of an element can affect the metallurgical properties of the ESR ingot. When using oxidizing elements in the steel/ alloy, an argon atmosphere may be used in the ESR process. This can reduce the oxidation of the elements. The sources of oxygen in ESR are 1) electrode 2) slag 3) oxidation of electrode 4) oxidation of the slag. For controlling oxidation the following steps can be used 1) electrode should be cleaned and painted and using an inert atmosphere 2) adding deoxidizers like aluminium to the slag, etc. SLAG METAL REACTIONS The reaction sites16 in a typical ESR are - electrode/slag, metal drop/slag, metal pool/slag, electrode/gas, slag/gas, electrode/ slag/gas, mould wall/slag. If process is done in air then the oxygen in the air would oxidize the electrode. The oxide scale which will form on the electrode will enter the slag. This will increase slag oxidation level thereby causing loss of reactive elements. The reaction between slag and atmosphere can also increase the slag oxidation level by oxidation of elements present in the slag as oxides. The three important sites are - electrode/slag, drop/slag, and metal pool/slag. ADVANTAGES ESR is a flexible process. The following are the advantages of this process. 1) Clean Metal - Clean metal is obtained as inclusions are removed. 2) Uniformity of structure and chemical composition, reduced segregation. 3) Production of a sound ingot, free from pipe, porosity and less discard. 4) Improved mechanical properties. ESR steels have better tensile and yield strengths compared to air melted steels. 5) Large sized ingots in tons can be produced. 6) Reduction of sulphur and oxygen. 7) ESR ingots can be cast in a variety of sections such as rectangular, square, round, etc. 8) Very less surface machining is required as surface is smooth. APPLICATIONS ESR based steels find applications in nuclear, aerospace and general engineering applications. a) Steels for turbine shafts, crankshafts. b) Ball and roller bearing steels. c) Refining of high speed steels to fabricate tools. www.metals.rala.com.au 28