Furnaces use insulated, heated vessels powered by an energy source to melt metal. Furnace design is a complex process, and the design can be optimized based on multiple factors. Furnaces in foundries can be any size, ranging from mere ounces to hundreds of tons, and they are designed according to the types of metals that are to be melted in them. Also, furnaces are bound by the fuel available that will produce the desired temperature. For low temperature melting point alloys, such as zinc or tin, melting furnaces may reach around 850°C (1262°F). From steel, nickel-based alloys, tungsten, and other elements with higher melting points, furnaces can reach to over 3850°C (6962°F). The fuel used to allow furnaces to reach these high temperatures can be electricity, natural gas or propane, charcoal, coke, fuel oil, or wood.
Melting furnaces used in the foundry industry are of many diverse configurations. The selection of the melting unit is one of the most important decisions foundries must make, with due consideration to several important factors including:
1.The temperature required to melt the alloy.
2.The melting rate and quantity of molten metal required.
3.The economy of installation and operation.
4.Environmental and waste disposal requirements.
Some of the more commonly used melting furnaces include cupolas, direct fuel-filled furnaces, crucible furnaces, induction furnaces, and electric arc furnaces.
a) Cupolas
Some ferrous foundries use cupola furnaces. For many years the cupola was the primary method of melting used in iron foundries. The cupola furnace has several unique characteristics that are responsible for its widespread use as a melting unit for cast iron:
•The cupola is one of the only methods of melting that is continuous in its operation.
•It has high melt rates.
•It has helatively low operating costs.
•It has ease of operation.
In more recent times the use of the cupola has declined in favor of electric induction melting, which offers more precise control of melt chemistry and temperature and much lower levels of emissions.
The construction of a conventional cupola (Fig. 2.20) consists of a vertical steel shell lined with a refractory type of brick.
The charge is introduced into the furnace body by means of an opening approximately halfway up the vertical shaft. The charge consists of alternate layers of the metal to be melted, coke fuel, and limestone flux. The fuel is burned in air that is introduced through tuyeres positioned above the hearth. The hot gases generated in the lower part of the shaft ascend and preheat the descending charge.
Most cupolas are of the drop-bottom type with hinged doors under the hearth, which allows the bottom to drop away at the end of melting to aid cleaning and repairs. At the bottom front is a taping spout for the molten iron at the rear, and positioned above the taping spouts is a slag spout. The top of the stack is capped with a spark/fume arrester hood.
Fig. 2.20 Cupola furnace used in foundries.
Typical internal diameters of cupolas are 450 to 2000 mm (17.7 to 78.8 in.) diameter. A typical operation cycle for a cupola would consist of closing and propping the bottom hinged doors and preparing a hearth bottom. The bottom is usually made from low-strength molding sand and slopes towards a tapping hole. A fire is started in the hearth using lightweight timber, and coke is loaded (charged) on top of the fire and burned by increasing the air draught from the tuyeres. Once the coke bed is ignited and of the required height, alternate layers of metal, flux, and coke are added until the level reaches the charged doors. The metal charge would typically consist of pig iron, scrap steel, and domestic returns.
An air blast is introduced through the wind box and tuyeres located near the bottom of the cupola. The air reacts chemically with the carbonaceous fuel, thus producing heat of combustion. Soon after the blast is turned on, molten metal collects on the hearth bottom where it is eventually tapped out into a waiting ladle or receiver. As the metal is melted and fuel consumed, additional charges are added to maintain a level at the charging door and provide a continuous supply of molten iron.
At the end of the melting campaign charging is stopped, but the air blast is maintained until all of the metal is melted and tapped off. The air is then turned off and the bottom doors opened, allowing the residual charge material to be dumped.
b) Crucible Furnaces
A crucible is a container in which metals are melted. Foundry crucibles are usually made of graphite, with clay as a binder, and they are very durable and resist temperatures to over 1600°C (2912°F). A crucible is placed into a furnace and, after the melting, the liquid metal is taken out of the furnace and poured into the mold.
Crucible furnaces are one of the oldest and simplest types of melting unit used in the foundry. The furnaces use a refractory crucible, which contains the metal charge. If scrap metal is used as charge it must be cleansed and heated before being introduced into the furnace, as any oil or moisture could cause an explosion. The charge is heated via conduction of heat through the walls of the crucible. The heating fuel is typically coke, oil, gas, or electricity. Crucible melting is commonly used where small batches of low melting point alloy are required, such as bronze, brass, and alloys of zinc and aluminum. The modest capital outlay required for these furnaces makes them attractive to small nonferrous foundries.
Crucible furnaces (Fig. 2.21) are typically classified according to the method of removing the metal from the crucible:
Fig. 2.21 Three types of crucible furnaces: a) lift-out furnace, b) bale-out furnace, c) tilting.
Lift-out furnace. In this type, the crucible and molten metal are removed from the furnace body for direct pouring into the mold.
Bale-out furnace. In a bale-out furnace the molten metal is ladled from the crucible to the mold.
Tilting furnace. In this type of furnace, the molten metal is transferred to the mold or ladle by mechanically tilting the crucible and furnace body.
c) Induction Furnaces
The principle of induction melting is that placing a metal body in an alternating magnetic field creates eddy currents, causing losses through which the metal is heated. Skin effects concentrate these currents in the outer layers. The inductor traversed by an alternating current creates a magnetic field, which should be optimally adapted to the metal.
Varying the alternating current frequency can influence the depth of heating, but it also depends on the concentration of flux capacity, on the length of treatment, and the material’s conduction properties. Medium frequency is usually used for melting metal.
There are two main types of induction furnace: coreless and channel.
Coreless induction furnace. The coreless induction furnace is composed of a refractory container, capable of holding the molten bath, which is surrounded by a water-cooled helical coil connected to a source of alternating current. In Fig. 2.22 is shown a simplified cross-section of a coreless induction furnace.
The alternating current