In high-iron-tonnage operations, the cupola remains the most efficient source of continuous high volumes of iron needed to satisfy high production foundries or the multiple casting machines of centrifugal pipe producers.
This article explores successful improvement technologies in cupola equipment, including preheated air blast, recuperative hot blast systems, and duplex electric holders. It discusses the shell, intermittent or continuous tapping, tuyere and blower systems, refractory lining, water-cooled cupolas, emission-control systems, and storage and handling of the charge materials.
The article provides a discussion on the control tests for cupola, including the chill test and mechanical test. It concludes with William L. Powell, Alan P. Jorstad, Raymond W. Monroe, Mahi Sahoo, Thomas E. Keep up to date with the latest aerospace materials and technologies.
Register today for Aeromat ! Sign In or Create an Account. User Tools. Sign In. Skip Nav Destination Close mobile search navigation. ASM Handbook. Edited by. Care must be exercised to keep the stream steady from the first, and not to spill into the mold, as this may cause cold-shuts or leave shot iron in the castings. The runner basin must be kept full, for gates and runners are made with this express purpose in view, as has been stated previously. Hand and Bull Ladles. Metal must not be allowed to chill or freeze in the ladle, as this would destroy the lining when it came to removing the cold metal.
Metal left in the ladles when the mold is full, must be poured back into a larger ladle or emptied into a convenient pig bed. These latter are built in a sand bed usually near the cupola; or stout cast-iron pig troughs or chills are provided. The chills should taper well on the inside, holding about 60 pounds each. Some are arranged to swing on trunnions for convenience in dumping.
They should be smeared with a heavy oil and dusted with graphite, to prevent the metal sticking in them. It is safer to heat these pig molds as well, so that no moisture will form and cause a kick or explosion when hot metal is first poured into them. Cupola Mixture Requirements By the term cupola mixture is meant the proportioning of the various pig irons and the scrap that make up cupola charges, with the object of obtaining definite physical and chemical properties in the resulting castings.
The requirements of castings vary; and metal that would be good if run into thin stove plate, would be entirely too soft for heavy machine castings. Again, iron that might answer all requirements of a bed plate would not be strong and tough enough for steamcylinder work. The one in charge of this work, therefore, must so mix the different irons that his castings shall be soft enough to machine well if necessary, and at the same time be hard enough to stand the wear and tear of use.
Precision Essential Formerly the appearance of the fracture of a pig or of scrap was the sole guide in determining mixtures. Unquestionably the fracture of iron indicates to the experienced eye much as to its physical properties, but this method of mixing has repeatedly proved misleading.
Representative practice today recognizes chemical analysis of the various irons as most essential to the proper mixing.
Many firms now buy their pig iron and many other allied supplies by specification; and the chemical analysis of the iron must show that its various metalloids come within certain limited per cents. To understand, then, these modern methods, we must consider the subject of the chemistry of iron. Affecting Elements By the chemical definition, an element is a form of matter which cannot be decomposed, or, in other words, cannot be broken up into other forms by any means known to science.
Iron is such an element; but absolutely pure iron is of no commercial value; it is only when it is combined with impurities - or, as we must recognize them, other chemical elements - that mankind is interested in it.
In the forms of iron with which we are dealing - pig iron, and cast iron - five elements are considered as affecting their physical properties. These elements are carbon, silicon, sulphur, phosphorus, and manganese. Carbon Carbon is the most important and most abundant of all the chemical elements. It forms the principal part of many substances in daily use about us, such as coal, coke, lead pencils, graphite facings, etc. In its relation to iron, carbon is peculiar in that it occurs in iron in two forms.
One is in a chemical combination forming a hard substance with a fine grain, of which tool steel is the purest type. The other is simply a mechanical mixture forming minute facets of free carbon interposed between the crystals of the combined form. It softens cast iron, but weakens it by causing larger crystals to form. In drawing the finger across a freshly cut surface or fracture of cast iron, some of this free carbon may be rubbed off, and shows as dirt on the finger.
We shall use the term graphite in referring to this form of free carbon, and the term combined carbon in referring to the element in its combined state. Silicon Silicon, of itself, is a hardening element in cast iron, but on account of its marked influence upon carbon formations, it is usually considered a softener. During the cooling process, silicon retards the formation of combined carbon, thus increasing the formation of graphite in proportion to the increase of silicon.
At the same time, through its own influence on iron, it preserves the fine character of the grain, and so maintains the strength of the casting. In other words, within certain limits, the addition of silicon softens castings without impairing their strength. It makes iron run more fluid, and reduces shrinkage. Silicon varies in castings from 1. Sulphur Sulphur is the most injurious element in iron.
It makes castings hard, red-short, and tends to the formation of blowholes. At the melting temperature, iron absorbs sulphur from the fuel - a decided reason why foundry coke should be as free as possible from this element. Sulphur in castings should not exceed 0. Phosphorus Phosphorus tends to make iron run very fluid when melted. It is a hardener. For machine castings it should not exceed 1 per cent. Manganese Manganese strengthens, and, of itself, hardens iron.
Chemists are beginning to consider its proportions more carefully, in the belief that under certain conditions it acts as does silicon, softening the castings while retaining their strength.
It is usual to keep it below 0. Factors Of Quality The strength of a casting and the finish which it is capable of taking are largely dependent upon its having a fine even grain. We have seen that the porportions between the combined carbon, the graphite, and the silicon have decided influence upon this condition.
But the rate of cooling must also be taken into account. A thin casting cools rapidly, tends to increase the combined carbon, and, without the influence of silicon, would be hard and brittle. In a heavy casting, the metal stays liquid longer, more graphite is thrown off, and the casting is naturally softer.
Therefore light work requires a larger proportion of silicon to counteract the effect of the rapid cooling than does larger work. Chemical Analysis Modern practice makes daily analysis for the two carbons, for the silicon, and the sulphur, occasionally testing for the other elements to see that they are kept within their safe limits. Silicon, however, is used as the guide for regulating mixtures. Cupola Mixture Requirements. Continued To calculate for any result, we must first know the analysis of the irons to be used in making the charge.
We shall consider silicon as the guide. In keeping track of results, the proportion of silicon in the local scrap of an establishment can be accurately estimated. With miscellaneous machinery scrap, this is more difficult; the following, however, are safe estimates: Casting Silicon per cent Small thin scrap Large scrap ranges 2.
Method The analysis of pig iron is made from drillings taken from a fresh fracture. Between the very fine grain about the chilled sides of the pig and the very coarse grain in the center, average-sized crystals will be noticed in the fracture. It is here that the drillings for analysis should be made, as indicated in Fig.
To determine the analysis of a carload lot of pig iron, the following method is employed: Select ten pigs which will represent an average of the close, medium, and coarse-grained iron in the car. These pigs should be broken, and drillings taken from the fresh fracture. The drillings from these ten fractures are thoroughly mixed together, and about 2 ounces by weight, or a large tablespoonful by measure, is sufficient for the chemical analysis.
The result is taken as the average analysis of the carload. The smaller foundries who do not employ a chemist can get a good working analysis of their iron from the furnace from which it is bought. Or, in many cases, sample drillings. Usual Silicon and Sulphur. Calculation Of Mixture When we have the analysis of our iron, we can proceed to calculate the mixture, bearing in mind that some of the silicon will be burned out of the iron during the heat.
From 0. This loss must be deducted from the final estimate. Illustrative Examples It is proposed to make a mixture for miscellaneous machinery castings which require about 2 per cent of silicon, and we wish to use one-half scrap and three other irons, whose silicon contents are as follows: Grade or Iron Silvery No.
Scrap 2. Then we may have the following proportions of silicon, using the above irons: A No. Deduct for loss in heat Estimate d silicon in result Or, with No. One or more per cents in column B are usually decided upon before beginning calculations, and then the others are varied until the desired silicon content is obtained. With this as a guide, it is a simple matter to find the actual weight for each grade, to make up any size of charge.
For example, we wish to put 5, pounds on the bed and 3, pounds on other charges. Then, using the first mixture and the ratio between the bed and the other charges, we have: From column B Bed Other charges No.
Fuel Both anthracite coal and foundry coke are used in the cupola. Coal, owing to its density, carries a heavier load than coke, but it requires greater blast pressure and does not melt as fast as coke.
Foundry Coke Coke, for foundry use, should be what is known as "hour" coke, as free as possible from dust and cinders. Coke is made up of a sponge-like coke structure which is almost pure fixed carbon, and an open cellular structure, which makes it especially valuable as a furnace fuel because it is so readily penetrated by the blast. A representative analysis of a strong hour coke is as follows: Item Moisture Volatile matter Fixed carbon Sulphur Ash Cellular structure Coke structure Heat units per pound Specific gravity Proportion per cent 0.
Proportions Of Charge The proportions of the bed fuel, the first charge of iron, and the subsequent charges of fuel and iron vary greatly with the size and design of the cupola, the grade of fuel used, and the method of charging.
To determine the right amount of fuel for the bed, the most practical thing to do is to cut and try, especially with a new equipment. For to inch cupolas, averaging 22 inches above the tuyeres for the melting zone, with a ounce blast to start, the best way to proceed is to chalk off this distance inside the cupola before daubing up.
The distance a equals the distance from the mark inside the cupola to about 4 inches above the bottom of the charging door. When the coke is well lighted, before charging the iron, level off the bed according to this gage. The safe practice is to have the bed too high.
If the bed is too high, it is indicated by slow but hot metal; if the bed is too low, the metal is dull. After the first heat, the height may be adjusted until proper melting is obtained; then try always to work to the same height.
The weight and character of the coke charged on the bed should be carefully noted. The first charge of metal should be in the proportions of 2 pounds of metal to 1 of fuel; all others in the ratio of 10 of metal to 1 of fuel. Intermediate charges of coke should be just sufficient to preserve the upper level of the bed.
The layer is usually about 6 inches thick; its weight should be carefully taken. The action of the furnace must be carefully watched, with the object of making it melt the iron charged as rapidly as possible and of bringing it down white hot.
Also, the ratio of iron to fuel should be reduced as low as may be, without sacrificing either of these other objects. Furnaces Classification A bewildering variety of furnaces is in use in the metallurgical industries. Usually, the first iron which comes out will be too cold to pour into sand molds. During the cupola operation, molten metal may be tracked every 10 minutes depending on the melting rate and the capacity.
This is an exothermic reaction. The temperature in this zone varies from to degree Celsius. Then hot gases consisting principally of Nitrogen and carbon dioxide moved upward from the combustion zone, where the temperature is degree Celsius. The portion of the coke bed if the combustion zone is reducing zone. It is a protective zone to prevent the oxidation of the metal charge above and while dropping through it.
As the hot carbon dioxide gas moves upward through the hot coke, some of it is reduced by the following reaction. This is an endothermic reaction. The first layer of iron above the reducing zone is the melting zone where the solid iron is converted into the molten state. A significant portion of the carbon is picked up by the metal also takes place in this zone. The hot gas is passed upward from the reducing and melting zones into the preheating zone which includes all layers of charge above the melting zone up to the charging Door.
Since the layer of the charge is preheated by the outgoing gases which exist at the top of the cylindrical shell. The main application of Cupola Furnace is different types of cast iron is produced from this device like Malleable, Grey cast iron, and the copper base alloy is also manufactured by this device.
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