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Metal Casting Technologies : March 2008
METAL Casting Technologies March 2008 57 CHECKING BED PERFORMANCE Because of the high temperature of the melting zone, the cupola lining is eroded in a band several inches wide at the operating height of the coke bed. The bed performance can be checked by observing the extent of the erosion when the cupola is cold. In most cases, this severely eroded band will be 450-600 mm. (18-24 inches) above the top of the main tuyeres, and it is this height at which it is desired to carry out combustion (the combustion zone). The observant cupola operator will make regular notes of the refractory burn-out sustained. These notes serve as an excellent guide to proper bed performance, and in many cases will give indications of melting irregularities that can be eliminated before they become serious enough to affect metal quality. Checking the performance of the coke bed during actual operation is not as simple as observing the eroded refractory band mentioned, but there are several fundamental symptoms that indicate improper bed performance. These symptoms usually occur before any appreciable damage is done to the molten metal. The alert cupola operator will notice and learn these symptoms and peculiarities of cupola melting in time and can make the necessary adjustments when necessary. Probably the most apparent symptom of irregular bed performance is a sudden change in spout metal temperature without apparent cause. In most cases, this sudden change is due to a low coke bed. The first indication will be a sudden rise in temperature, followed within a few minutes by a gradual but definite drop in metal temperature. This drop in temperature will continue until the air input and the existing bed height are put in balance again. The slag color is another factor that can be used to advantage in checking bed performance. Normal acid cupola slags are a grayish-green color when the bed height and the air input are in balance. If the slag gradually changes from the normal grayish-green color to a dull black color, the bed has decreased below its normal operating height and the necessary steps should be taken to reestablish the proper air input to bed height ratio. Should the slag become very light green or cream color, it is an indication that too much flux is being used. In addition, excessive loss of silicon and manganese, together with low carbon pick-up and sudden changes in melting rate are almost sure signs of an improper bed performance. Chill tests taken regularly will be helpful in recognizing these changes. In any of the foregoing events, the precautionary steps should be taken as soon as possible in an effort to eliminate erratic bed performance which may cause cold or oxidized metal. In each case of faulty bed performance the symptoms are similar and their remedy is simple for the alert, well-trained operator. The addition or subtraction of coke in the coke charge is one method of correction, but the more immediate correction is to adjust the air input, as narrated above. RECORDS ESSENTIAL FOR MONITORING PROPER BED PERFORMANCE The practice of maintaining a regular record of cupola runs of the foregoing factors that are pertinent to bed performance is considered time well spent. It is this record that keeps the well-trained operator alerted for signs of trouble, and aid him greatly in his effort to produce good quality iron in every run. NORMAL DATA FOR DETERMINING CUPOLA SIZE NEEDED The following table will help in choosing the size and number of cupolas needed to meet planned production. A cupola is normally operated only about 6-7 hours, on the average, per day. Although everyday operation is possible, it is more convenient and perceptive to have two cupolas alternating, if everyday operation is desired. One last word: If the metallurgical integrity of the castings is a relevant factor, duplexing with an induction furnace could be done. The cupola, because of its higher efficiency, would be the melting unit and the metal could be transferred to an induction furnace for adjustment of composition and superheating the induction furnace is the best for this job. ● REFERENCES: 1. American Foundrymen's Society, The Cupola and Its Operation, Chicago: American Foundrymen's Society 2. Heine, Richard W.; Loper, Carl, R., Jr.; and Rosenthal, Philip C. Principles of Metal Casting. New York: McGraw-Hill Book Company, Inc 3. Bautista, John H. D. Practical Quality Control in Foundries. Balmain, Australia: Rala Publications, Metal Casting Technologies Magazine