Fig. 2.22 Simplified cross-section of a coreless induction furnace.
When the charge material is molten, the interaction of the magnetic field and the electrical currents flowing in the induction coil produce a stirring action within the molten metal. This stirring action forces the molten metal to rise upwards in the center, causing the characteristic upward on the surface of the metal. The degree of stirring action is influenced by the power and frequency applied, as well as the size and shape of the coil and the density and viscosity of the molten metal. The stirring action within the bath is important, as it helps with mixing of alloys and melting of turnings as well as homogenizing of temperature throughout the furnace. Excessive stirring can increase gas pickup, lining wear, and oxidation of alloys.
The coreless induction furnace has largely replaced the crucible furnace, especially for melting high melting point alloys. The coreless induction furnace is commonly used to melt all grades of steels and irons, as well as many nonferrous alloys. The furnace is ideal for remelting and alloying because of the high degree of control over temperature and chemistry, while the induction current provides good circulation of the melt.
Channel induction furnace. The channel type induction furnace derives its name from the fact that it is constructed with a small channel of molten metal passing through the magnetic core, which has a primary winding wound around it. This channel of molten metal acts like the secondary of a short-circuited transformer, causing current flow through the metal in the channel and causing heat loss to occur by the Joule effect.
The typical arrangement of a channel furnace is shown in Fig. 2.23. The liquid metal is kept in a crucible and flows through a channel that is connected with the bottom of the crucible.
The whole furnace is typically covered by metal plates that serve both as mechanical protection and electromagnetic shielding.
Channel induction furnaces were used initially as containers for molten metal, but are now commonly used for some melting applications as well.
d) Electric Arc Furnaces
This type of furnace draws an electric arc that rapidly heats and melts the charge metal. When the molten metal is ready to pour, the electrodes are raised and the furnace is tilted to pour the molten metal into a receiving ladle.
Fig. 2.23 Simplified cross-section of a typical channel induction furnace.
Electric arc furnaces produce tremendous quantities of metal fumes; however, the furnace is normally equipped with a fume-capture system to reduce both workplace and air pollution. These furnaces are common in large ferrous foundries. Temperatures inside an electric arc furnace can rise to approximately 1900°C (3450°F). Electric arc furnaces may be categorized as direct arc or indirect arc.
Direct arc furnaces. In a direct arc furnace (Fig. 2.24) scrap steel or direct reduced iron is melted by direct contact with an electric arc. The arc is produced by striking current from a charged electrode to the metal (the neutral point), through the metal, and drawn to an oppositely charged electrode.
Fig. 2.24 Schematic illustration of direct arc furnace.
At the start of the direct arc melting process (the electrodes and roof are raised and swung to one side), a charge of steel scrap is dropped into the furnace from a clamshell bucket. The roof is sealed, the electrodes lowered, and an arc strikes the electrode. The charge is heated both by the current’s passing through the charge and by the radiant energy evolving by the arc.
The electrodes are automatically raised and lowered by a positioning system, which may use either electric winch hoists or hydraulic cylinders. The regulating system maintains an approximately constant current and power input during the melting of the charge, even though scrap may move under the electrodes while it melts. The arc contacts the scrap charge and the metal is melted. Since direct arc furnaces are sometimes used to perform refining operations, the molten metal may remain in the furnace after melting. When the melting and refining operations have been completed, the molten metal is tapped into a ladle for casting. Some of the advantages of direct arc furnaces include high melt rates, high pouring temperatures, and excellent control of melt chemistry.
Indirect arc furnaces. Indirect arc furnaces (Fig. 2.25) generally consist of a horizontal barrel-shaped steel shell lined with refractory. The arcing between two horizontally opposed carbon electrodes effects the melting of the metal. Heating is via radiation from the arc to the charge. The barrel-shaped shell is designed to rotate and reverse through approximately 180° in order to avoid excessive heating of the refractory above the melt level and to increase the melting efficiency of the unit.
Fig. 2.25 Schematic illustration of indirect arc furnace.
Indirect arc furnaces are suitable for melting a wide range of alloys but are particularly popular for the production of copper-base alloys. The units operate on a single-phase power supply, and hence the size is usually limited to relatively small units.
REVIEW QUESTIONS
2.1What are the major applications of a casting?
2.2Name the two basic categories of casting processes.
2.3Name the two basic types of silica sand.
2.4What is a pattern and how many types are there?
2.5What is the difference between a split pattern and a match plate pattern?
2.6What is a chaplet?
2.7What properties determine the quality of sand for greensand molding?
2.8What is the difference between dry sand molds and shell molds?
2.9What is the EPS process?
2.10What is the difference between investment casting and plaster mold casting?
2.11What common metals processes use die casting?
2.12What is the difference between vacuum permanent mold casting and vacuum molding?
2.13How many types of centrifugal casting are there, and what are the differences among them?
2.14What is the difference between a cupola and a crucible furnace?
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METAL CASTING: DESIGN AND MATERIALS
3.2 Casting Design Considerations