IRON FOUNDRY BASICS
What is ironfounding?
lronfounding is fundamental to modern industry, yet it can be equally well operated on a small scale by small numbers of people in rural areas. Iron shapes or "castings" are made by pouring molten metal into m oulds made of sand. Small parts which cannot easily be shaped by the method of forging are cheaply and simply produced in this way. Cast iron is a brittle material which is strong when compressed, but relatively weak when pulled or bent. These qualities determine the uses of cast iron, which are very numerous. Cast iron is found in motor cars, tractors, ships, factories (sic), mines, houses, in the streets and roads, and almost any place one cares to look. For example, the cylinder head, water pump and exhaust manifold of a car; the grid, manhole cover and lamp post in the street; parts of a compressor, many parts of agricultural, building and textile equipment - all are made of cast iron. Every day of our lives, something made of this material is used b y most people. A decision on the level of technology - or the size and type of foundry to be set up - must depend on such factors as the capital available, raw materials supply and labour availability, the characteristics of the market and the size and type of castings required. When one has chosen the size and type of furnace, one must also make sure that there are enough moulds to take the quantity of metal melted, and that there is always enough metal to complete a pour once it is started. Sequence of operations For ironfounding there are three main levels of technology: (1) Simple operations needing low capital and low power resources, utilizing the crucible furnace. (2) More elaborate operations for longer runs, requiring more expertise and labour, capital and power, and using the small cupolette type of furnace. (3) Elaborate operations requiring high capital investment, high power resources, and expensive melting equipment. The present pamphlet covers the first two of these three po ssibilities. Information on the mechanised foundry can be easily obtained from any large foundry equipment manufacturer.
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Iron foundry basics
The Crucible Iron Furnace
The crucible furnace is the easiest method of melting iron. Figure 1 shows details of the arrangement and the different parts of a crucible furnace, using the induced or "natural" draught method. Figure 2 showing the forced draught method has, in addition, a blower, operated by a hand-wheel or motor. Figure 1: Crucible Furnace (Induced draught)
Equipment required for induced draught method A. B. C. Crucible (salamander pot) Stool Fire-bars To be purchased. Made of a special composition of clay, carbon and silicate. Suggest original purchased, replacements from used crucibles Originals of steel bar obtained from local metal merchants. Replacement bars of cast iron, which are more durable, as shown in
Iron foundry basics
D. E. F. G. H. I.
Ash pit Refractory Chimney Tongs Sliding cover Fuel
Fig. 4. Part of construction Made from local clay, which would be cheaper to obtain and be immediately available for any repair required. Constructed of local clay or used kerosene drums, the bottom sections to be lined with clay. Can be purchased along with salamander pot, or made by local blacksmith. Local refractory stone Charcoal obtained locally is recommended. Other fuels such as coal, coke, oil, etc., can be used.
Figure 2: Crucible Furnace (Forced air)
Additional equipment required for forced-air system Blower or Fan Handwheel or Motor Damper Purchased (possibly locally) together with suitable drive belts. Probably obtainable locally, handle to be added. To drive blower, Instead of operating manually with handwheel. Pulleys for motor and blower shafts required, together with suitable drive belts (V belts) Obtained and made locally.
If an increased melting capacity is required, a cupolette can be used (Figures 5, 6 and 7).
Iron foundry basics
Although the induced draught furnace is a well-proven method of melting cast iron, it is very dependent on weather conditions, and the control of melting cannot be entirely accurate. The forced draught system shown in Fig. 2 is preferable, because it allows for much more accurate control of temperature. If electric power is not available, a centrifugal fan, manually operated, can provide sufficient blast for melting. It is normal to build the furnace below ground level, and to provide a trench access for removal of ash. This arrangement has the dual advantage of a) supporting the refractory bricks, thus avoiding the need for a complicated structure, and b) providing the operator with a working platform at ground level - this reduces the physical effort required to lift the crucible. The crucible "A" made of clay graphites (salamander pots) is 180 m/m outside diameter and 360 m/m high. It sits on a stool, "8" (130 m/m high), which can be made from a used crucible. The stool is supported by fire-bars, "C", which are placed about 300 m/m from the bott om of the ash pit. These fire-bars are originally made of steel round bar, purchased from a local steel merchant, but when they need replacement, they can be cast in the foundry (cast iron bars are actually more durable). The sliding cover, "8" can be of local refractory stone. This cover should be in the closed position when the melting is in operation. The walls of the furnace, "F", are of refractory brick. The quality of bricks is directly related to their life. The furnace is of square construction, to the dimensions shown, the thickness of the walls being 230 m/m. Two spaces are left, one at the bottom of one wall for induced draught and for the clearance of ashes, and the other at the top of the same wall, for the chimney ("L") outlet. A damper is inserted in the bottom space to regulate air flow or draught. Description of Operation Wood shavings, together with some charcoal, are placed on the fire-bars around the stool and fired, before the crucible is placed on the stool by the tongs, "J". This is important because if the bottom of the crucible is cold when it is placed on the stool, the iron may not melt properly. After placing the crucible in position, a full charge of charcoal is added. The crucible is now ready to receive a charge of pig and scrap iron. It is recommended, however, that an extension or "prolong" "N" be fitted to the top of the crucible before loading. This not only helps to support the pieces of pig iron; it also helps to keep the heat in the crucible, and thus assist the melting process. This prolong can conveniently be made by knocking out, or cutting, the bottom of an old crucible, so that it fits neatly into the top of the crucible proper. Method of Loading It is not necessary to break the pig iron into small pieces, because suitable lengths can be loaded in a vertical position, providing the top ends do not project beyond the top edge of the prolong (extension). Iron recovered from scrapped machinery should be broken into small pi eces, and packed closely round the large pieces of pig. Pig and scrap iron must be carefully loaded to avoid damage to the crucible. Some loading could be done before the crucible is loaded into the furnace. Figure 3: Carrier for crucible (for pouring)
Iron foundry basics
Figure 4: Cast iron fire bar
As melting proceeds, the material in the prolong slowly sinks into the charge already melted in the crucible itself. The rapid melting of the charge should be encouraged by working the furnace nearly "flat out". There is a tendency for the hot, but not yet m elted charge in the prolong, to stick, and by poking and prodding with a long steel bar (about 10/15 m/m in diameter), the pieces will drop into the crucible. It is advisable to keep the furnace very full of charcoal, and it does not matter (indeed it is desirable) if some pieces of charcoal fall into the crucible with the iron, as this tends to reduce oxidation. At quite an early stage it is good practice to have a lid sitting on the top of the prolong; and as soon as the charge has melted down, and is contained entirely in the crucible, the prolong itself should be removed, and the sliding cover on the furnace should be closed. The charcoal must be replenished as it burns - the speed of the stoking has a significant effect on the speed of melting. Normal melting time is approximately two hours, but this may differ considerably with the type of fuel used. Approximately 56Ibs (25 kg) of iron can be melted in the crucible furnace shown in Figs. 1 or 2 at each melt. Although the object of this profile is to concentrate mainly on the melting of cast iron, it is worth mentioning that non-ferrous alloys, such as aluminium, can also be used with this system. The fact that these types of alloy become molten at a much lower temperature than cast iron does not diminish the need to take very great care in carrying out the melting operation (see Safety). Carrier for Crucible To be purchased. Recommended to be of safe construction and correct fit for crucible , can be made by local blacksmith.
The Cupolette Iron Furnace
Although the cupolette has greater melting capacity, its output could prove to be more than is required. For economy reasons, the cupolette should operate for at least an 8 hour period, after which it will require re-lininq, or repairs to the lining. Estimating an output of approximately one ton per hour, this gives 8 tons of molten metal, available in one cycle of operation. It is essential, therefore, that sufficient moulds are available to take a pour of this quantity. Any surplus molten metal can be poured into open moulds, or sand trenches, and can then be fed back into the cupolette in future melts. This surplus should be kept to a minimum, as the greater amount of metal re-melted, the greater the cost of the product. The cupolette consists of a steel shell "A" (Figs. 5, 6 & 7) in an upright position on a base-plate, which is usually supported on four steel joists or tubular columns "J". The shell is lined with good quality refractory brick "R". At the base of the steel shell are dropped doors, "S". These are hinged doors, which after the furnace has completed operation , are opened to allow the debris to be discharged. The steel shell is 1/4 (6 m/m) thick plate of riveted or welded construction . A wind belt "G" is provided at a height of 2 or 3 feet (610 to 915 m/m) above the base plate. The air is supplied to the wind belt from a fan or blower "N" and the blast is conveyed to the interior of the furnace by tuyeres (nozzles) which may be situated in, or under, the wind belt . The number of tuyeres used depends on the size of the furnace. It is usual to allow one tuver o for each 6" (150 m/m) of internal diameter. The height at which the tuyeres are set above the working bottom of the furnace depends on the capacity of molten metal required. The working bottom in the cupolette is made up with moulding and a fettling hole "0", provided at the bottom of the shell. The fettling hole is usually about 18" (450 m /m) square, and is covered by a plate held in position by a bar during the operation of the furnace. 5
Published by Practical Action on 02/02/02
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