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Metal Casting Technologies : September 2007
TECHNICAL FEATURE INTRODUCTION ith the developments in nuclear & aerospace engineering there is a need for quality steels with improved properties. Steels prepared from conventional routes are not suitable in critical applications where cleaner steels are required. The conventional casting shows inclusions, porosities, segregation and coarse grain size. Quality steels are prepared by secondary refining processes like Electroslag Remelting (ESR) & Vacuum Arc Remelting (VAR). These processes produce ingots free from non metallic inclusions, free from porosity, free from segregation and with good surface finish. In the ESR process the consumable electrode is passed into a superheated liquid slag. The heat for the process is produced by 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 melt from the electrode and pass through the molten slag and collect at the bottom of water cooled mould where they solidify. The slag temperature is in the range of 1700 - 20000 C. 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 Al2O3 increases the resistivity of the slag. The desulphurizing ability of CaF2 slag is increased by addition of CaO. Sulphur is a harmful element in steel and should be present in a very low amount. The removal of sulphur, nonmetallic inclusions and directional solidification improve the properties after ESR. The smaller inclusions which are not removed by ESR are distributed uniformly. ESR is an attractive secondary refining method compared to VAR because equipment and running costs for ESR are lower than for VAR. In this current article alloy steel will be prepared by induction melting and the cast electrodes will be ESR under two slags to compare the desulphurization. EXPERIMENTAL Mild steel scrap was air melted in a magnesia lined induction furnace (9650 Hz frequency, 25 kg capacity). Cast iron moulds (40 mm diameter & height 600 mm) were coated with zircon and dried. The moulds were preheated with oxyacetylene flame to drive off any moisture. The alloy steel was prepared by adding calculated amounts of ferro alloys (Fe-Cr, Fe-Mn, Fe-Mo & Fe-V). The melt was deoxidized with Fe-Si and aluminium. Aluminium was added in the furnace and in the ladle during tapping. To determine the chemical composition of the melt, the melt was poured in a small preheated ladle cup. This melt was poured in a sample cast iron mould to get a pencil ingot. The remaining melt was poured in a bigger preheated ladle and poured in the 40 mm diameter moulds. Figure 1 shows the melt being poured from the induction furnace into the ladle. Figure 2 shows the melt poured into cast iron moulds. The as cast electrodes have defects like porosities, segregation, & inclusions and all such defects have to be removed. Table 1 shows the chemical composition of the electrode. w By Amit M Joshi Dept. of Metallurgical Engineering & Materials Science, Indian Institute of Technology - Bombay, Mumbai 400076, India. Email: email@example.com www.metals.rala.com.au Effect of slag composition on desulphurization of steel prepared by induction melting & electroslag remelting Material C Mn Cr Si S Mo V Electrode 0.27 0.9 1.45 0.23 0.03 0.93 0.26 Table 1. Chemical composition of the electrode Figure 1. Melt being poured from the induction furnace into the ladle 68