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Metal Casting Technologies : March 2007
emand for cleaner and stronger metals in many fields of engineering is increasing. With the developments in nuclear, aviation and aerospace industries there is a need for high quality metals with improved properties. Components like turbine blades, rotor shafts for turbogenerators, aircraft engine shafts, high temperature bearings, etc work under severe temperature and stress conditions. The metals used for fabricating such parts should have superior chemical and mechanical properties. There is an increasing demand on the quality and performance of steels to meet its growing needs. Steels are produced by liquid metal route because of its cost advantage and flexibility. Steels obtained from conventional ingot are not suitable in critical applications where cleaner steels are required. Studies in metals have confirmed that sources of weakness in steels are non metallic inclusions like sulphides, oxides and dissolved gases. If care is taken in conventional steel making produce clean steel non metallic particles may enter the liquid metal from ladle lining. Also nonmetallic particles may result from products of deoxidation which precipitate during the solidification of steel in the mould. The conventional methods of steel making are not effective in producing cleaner steels. The steels made and cast conventionally by open hearth furnaces, LD converters, electric arc and induction furnaces do not meet the requirements mentioned above. The disadvantages of casting include high non metallic inclusions, porosities and segregations and coarse grain size. To produce steels which can be used in critical applications, the steels produced by conventional ways have to be refined. This refining is done by 1) melting steel in vacuum i.e. arc melting and electron beam melting and 2) by remelting the steel under a slag (Electroslag remelting, ESR process). ESR originated from the work of Hopkins in the USA in 19351. The early ESR development was done by Russian workers at the Paton Institute of Electric Welding in Kiev. Initially it was used as a method for welding thick sections of steel and later it was extended for refining steels to improve properties. VAR was developed around 1950 for industrial scale applications. The advantages of VAR were transferred to prepare good quality steels combined with vacuum induction melting. Later on it became evident that another remelting process whose principles were already known could compete with VAR: - ESR under liquid slag. Basic research was carried out in USSR, UK and Austria. In 19672 the first International Symposium on Electroslag Consumable Electrode Refining and Casting Technology was held in Pittsburgh, USA. The process developed in the USSR and its use increased in UK and Western Europe since 1960. The ESR process entered the USA inspite of high American investment in VAR. Between 1965 to 1980, the tonnage of steels produced by ESR increased rapidly. A cast ingot should be free from segregation, nonmetallic inclusions, porosity and should have a uniform chemical composition. ESR can provide a good quality ingot with the above features. The secondary refining processes such as Electroslag Remelting (ESR), Vacuum Arc Remelting (VAR) and Electron Beam Melting (EBR) are promising in producing better quality steels. ESR is attractive secondary refining method compared to VAR because equipment and running costs for ESR are lower than for VAR. Electroslag Remelting (ESR) is a process which can give improved properties to the material. Electroslag Remelting (ESR) is a process used for production of special steels and superalloys for applications where improved cleanliness, better chemical homogeneity and reduced oxygen and sulphur content are essential. THE PROCESS The principle of the process is illustrated in Figure 1. The metal to be refined is first obtained in the form of a rod which TECHNICAL FEATURE 24 www.metals.rala.com.au Advanced Casting Technology: Electroslag Remelting D Amit M Joshi Dept. of Metallurgical Engineering and Materials Science Indian Institute of Technology - Bombay, India Email: email@example.com, firstname.lastname@example.org Figure 1.