Melting Practice

Module-I of Manufacturing Science-I

1.7 MELTING PRACTICE After moulding, melting is the major factor which controls the quality of the casting. There are a number of methods available for melting foundry alloys such as pit furnace, open hearth furnace, electric furnace, rotary furnace, cupola furnace, etc. The choice of furnace type is based on these four factors. • Alloy Type • Metal Quality • Production Demands • Economics Alloys have a wide spectrum of temperatures that they melt at; the list below should illustrate this point. Metal Alloy Type

Temp Range (Celsius)

Zinc

345-455

Aluminum

620-735

Magnesium

620-735

Copper

908-1180

Cast Irons

1340-1480

High Manganese Steel

1400-1455

Monel (70N, 30Cu)

1370-1540

Nickel Based Super Alloys

1430-1540

High Alloy Steels

1480-1600

High Alloy Irons

1540-1650

Carbon & Low Alloy Steel

1565-1700

Titanium

1700-1820

Zirconium

1845-1900 Table I: Melting point of common materials

Metal Quality is affected by oxidization and losses due to vaporization, which can adversely affect the chemical properties of the alloy being melted. Similarly the types of refractory used are matched to the basicity or acidity of the metal and/or its resultant (Dross or slag). Production demand can range from small batches under 30 Kg through to 100 tonne/hr continuous pour furnaces that run for days and weeks at a time uninterrupted. The economics of furnace selection relate to factors of capital depreciation, maintenance and operating labour, as well as the fuel and power consumption.

Types of Furnace Cupola It consists of a cylindrical steel shell with its interior lined with heat resisting fire bricks. It has drop doors at the bottom. After closing the door a proper sand bed is prepared. This sand bed provides necessary refractory bottom for the molten metal and the coke. Immediately above the sand bed is the metal tapping hole which is initially closed with clay till the molten metal is ready for tapping. Above the metal tapping hole normally in a position opposite to is the slag hole through which the slag generated during the melting process is tapped.

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Module-I of Manufacturing Science-I

Figure 1.7.1: Cupola • Above the slag hole is the wind box which is connected to the air blowers supplying the requisite air at a given pressure and quantity. • The lining is generally thicker in the lower portion of the cupola as the temperatures are higher compared to those in upper portion. • There is a charging door through which coke, pig iron, steel scrap and flux is charged. • The blast is blown through the tuyeres. • These tuyeres are arranged in one or more row around the periphery of cupola. • Hot gases which ascend from the bottom (combustion zone) preheat the iron in the preheating zone. • Cupolas are provided with a drop bottom door through which debris, consisting of coke, slag etc. can be discharged at the end of the melt. • At the top conical cap called the spark arrest is provided to prevent the spark emerging to outside. Operation of Cupola To operate the cupola, first, the drop doors at the bottom are closed and a sand bed with a gentle slope towards the tap hole is rammed. Then a coke bed of suitable height is prepared above the sand bottom and ignited through the tap hole or any other hole. When the coke bed is properly ignited, alternate layers of charge, flux and coke are alternatively fed into the cupola through the charging door maintaining necessary proportions and rate of charging. The charge is allowed to soak in the heat for a while, and then the air blast is turned on. Within about 5 to 10 minutes, the molten metal is collected near the tap hole. When enough molten metal is collected in the well of the cupola, the slag is drained off through the slag

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Module-I of Manufacturing Science-I hole before opening the tap hole. The molten metal is collected in the ladles and then transported to the moulds into which it is poured with a minimum time loss. The fluxes are added in the charge to remove the oxides and other impurities present in the metal. The flux most commonly used is limestone (CaCO3) in a proportion of about 2 to 4 % of the metal charge. Some of the other fluxes that may also be used are dolomite, sodium carbonate and calcium carbide. The flux is expected to react with the oxides and form compounds which have low melting point and also lighter. As a result, the molten slag tends to float on the metal pool and thus, can very easily be separated. Melting rate tonne/hour Metal : Coke

Diameter of melting zone, m

10:01

08:01

0.5 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.00

1.97 2.84 5.11 7.99 11.50 15.60 20.44 25.88 31.95

1.57 2.46 4.36 6.83 9.79 13.33 17.41 22.05 27.22

Typical charge, Kg Blast rate m3/hour

Blast Pressure KPa

Coke

Iron

1340 10.20 20 200 1940 10.50 28 284 3450 11.20 51 510 5380 11.70 80 800 7750 12.70 115 1150 10600 13.40 157 1570 13800 14.40 200 2040 17450 15.40 260 2590 21550 17.20 320 3200 Table II: Cupola operation data

Limestone

7 9 17 26 38 52 67 85 106

Bed height above tuyeres, m 1.00 1.00 1.05 1.05 1.10 1.10 1.10 1.15 1.15

Shaft height from tuyeres to charge door still, m 2.50 3.00 3.00 3.50 4.00 4.00 4.50 5.00 5.00

A variant of cupola is called hot blast cupola. In this, air supply is preheated to a temperature of 200 to 4000 C with help of the hot gases coming out of the stack or by a separate heat input. In either case the equipment gets complicated by the addition of the extra pre-heater and the circulation equipment. The main advantage gained is that the amount of heat required by the cupola gets reduced. This in turn reduces the contact of the metal with the coke and air thus reducing the carbon and sulphur pickups as well as the oxidation losses. Because of the additional equipment and extra care needed for operation, the hot blast cupolas are used only in shops that require large amounts of metal to be melt on a continuous basis. Most of the foundries operate on a batch basis. A number of sand moulds are prepared and kept ready for pouring before the molten metal is prepared. This process may take a few days to weeks depending upon the size and nature of the foundry plant. Thus it becomes necessary only to start melting may be once a week or so. Cupola has been the most widely used furnace for melting cast iron. This is because of the low cost of melting. However, less control of the final quality, and the losses involved would call for some change in the choice. Therefore liquid or gas fired furnaces and electric furnaces are becoming popular because of their better control of melting process and low melting losses. But these are more expensive compared to the solid fuel fired furnaces and therefore the higher cost is to be justified based on the better control of quality achieved in terms of the composition and temperature.

Electric Furnace For heavy steel castings, the open hearth type of furnaces with electric arc would be suitable in view of the large heat required for melting. Due to good temperature control and flexibility of operation, this furnace is widely used for melting for small to medium sized castings in ferrous as well as non-ferrous alloys. There are three types of electric furnaces on the basis of source of heating (Arc, Resistance, and Induction).

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Module-I of Manufacturing Science-I

Figure 1.7.2: Schematic representation of Arc furnace In above figure three electrodes are used that are tied to a 3 phase electrical source. The electrodes strike an arc with the metal charge. The heat of the sustained arc can be in excess of 40000 C. With such extreme heat comes the requirement of cooling with water jackets, heat exchangers and recirculation systems. Arc furnaces are of two types; Direct and In-direct types. Arc Furnaces can be configured with "Ultra-High-Powered" (UHP) transformers that can supply 600-900KVA/tonne. The furnace can have a pivoting point with a hydraulic actuator to tilt the furnace backward to skim of the dross/slag or forward to pour off the metal. The high temp capacity of this furnace lends itself better toward ferrous casting than non ferrous. The electrodes can be either graphite or carbon, and are selected to match the type of metal being melted. The In-Direct Arc furnace is similar in principle, but the arc is struck above the metal charge and is typically just one electrode. The In-Direct furnace also can be a sealed unit that operates under a reduced atmosphere for specialty metals that are sensitive to oxidization or atmospheric contamination.

Induction Furnace Induction heating is a heating method. The heating by the induction method occurs when an electrically conductive material is placed in a varying magnetic field. Induction heating is a rapid form of heating in which a current is induced directly into the part being heated. Induction heating is a non -contact form of heating. The heating system in an induction furnace includes: 1. Induction heating power supply, 2. Induction heating coil, 3. Water-cooling source, which cools the coil and several internal components inside the power supply. The induction heating power supply sends alternating current through the induction coil, which generates a magnetic field. Induction furnaces work on the principle of a transformer. An alternative electromagnetic field induces eddy currents in the metal which converts the electric energy to heat without any physical contact between the induction coil and the work piece. The furnace contains a crucible surrounded by a water cooled copper coil. The coil is called primary coil to which a high frequency current is supplied. By induction secondary currents, called eddy

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Module-I of Manufacturing Science-I currents are produced in the crucible. High temperature can be obtained by this method. Induction furnaces are of two types: cored furnace and coreless furnace. Cored furnaces are used almost exclusively as holding furnaces. In cored furnace the electromagnetic field heats the metal between two coils. Coreless furnaces heat the metal via an external primary coil.

Figure 1.7.3: Schematic representation of Induction furnace Advantages • Induction heating is a clean form of heating • High rate of melting or high melting efficiency • Alloyed steels can be melted without any loss of alloying elements • Controllable and localized heating Disadvantages • High capital cost of the equipment • High operating cost

Electric Resistance Furnace A series of Ni-Chrome elements are energized around the crucible and the radiant energy is absorbed by the Crucible and the metal. Unlike the Core/Channel Induction furnace ER type systems can take solid charge material. The resistance type heating is generally used for holding furnaces to maintain the liquid metal at a certain temperature for non-ferrous alloys such as for die casting. This has no noise, fumes, flames or potential for disaster.

Figure 1.7.4: Schematic representation of Electric Resistance furnace

Reference 1.

Manufacturing Technology by P.N.Rao , TMH page 182 to 190

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