There is probably as much reason for changes in the plan of melting iron as in moulding jobbing work. Melters will sometimes get nervous at being ordered to charge up their cupola in as many different ways as there are days in the week. A foreman that understands his business very seldom lays out a system, or a table of charges, for his melter to follow day after day in a regular jobbing - shop. The foreman may have various reasons for wanting his melter to make all these changes. To - day he may want the cupola to melt extra fast during the first of the heat, and slowly after some heavy casting is poured, in order to have melted iron to feed with. To - morrow, seeing that some moulder will not get ready in season, this order may be reversed. As he does not want to keep his men late when it can be avoided, he orders the cupola charged, so that the men having small work can be pouring off while the large casting is being got ready. This casting, that perhaps weighs 5 tons, may not be thick in any of its parts, so as to require much feeding, and the bottom can be dropped soon after it is poured.
In this way, the only moulders kept late are the ones that were going to keep the whole shop's crew behind, which, for a shop that pays overtime, would be expensive, and in any case is not pleasant for the men.
On some days the shop - floor may be covered with a class of work that is better for being poured with dull iron, and the next day the work may be such as to require very hot iron. Again, there will be heavy and light castings, requiring entirely different grades of iron; and to complicate matters, the foreman, if an observing man, will see that the brand of iron is not of the same grade as the last car - load.
Following are two of the many plans that may be adopted to meet different conditions. One is for melting special grades of iron, and the other to retain the bed in a cupola after melting a heat for a break - down job, or for a piece of casting that is wanted in a hurry.
The cupola man may receive instruction not to drop the bottom, but to prepare it to melt iron again in the course of 3 or 4 hours. The way to do this is as follows: - Leave the blast on until you are sure all the iron in the cupola is melted, and, instead of dropping the bottom, knock out the front breast, and with a bent hook pull out all the clinkering coke or coal and iron cinder that can be felt or seen. Then fill up the breast - hole with loose sand, and every 5 or 10 minutes take away the sand and pull out again whatever clinkers or iron cinders will have formed, repeating the operation for the first 1/2 hour or so, until you are sure that all the droppings of iron and clinkers are pulled out. After this, every 1/2 hour will be sufficiently often to clean the bottom out. The stopping up of the breast every time the clinkers are cleared out is done to prevent the fuel from burning away, and also to keep the clinkers and droppings of iron from being chilled with the air.
After the cupola is well cleaned out, there should be some fuel shovelled in, so as to freshen up and keep the fire in good burning condition. When the moulders have their mould or moulds about ready, make up the • breast as usual, and shovel in the fuel for a bed, the same height as for a regular heat. After it gets to burning, charge up the iron wanted, put on the blast, and you will soon have your cupola melting iron again. The first 2 or 3 cwt. of iron is generally dull, and sometimes will have to be poured into a pig - bed. After this, the iron will come hot enough for ordinary casting.
The question of how large a heat a cupola run in this way would melt could tot be better answered than, by the following: - A casting, the weight of which was about 2500 lb., was poured about 11 o'clock in the morning. The iron was all blown down, the breast knocked out, and the cupola treated as above described, until the time for the regular afternoon heats, which were never less than 12 tons. The blast was again put on, and after the first few hundred pounds, the iron was as good and as hot as usual. The time that the cupola was held from one heat to another was about 4 hours. The size of this cupola was a 5 - ft. shell.
To prevent the mixing of different grades' of iron when melted at one heat, has been the cause of a deal of thought and many experiments with foundrymen. Some foundry, owners make a practice of melting only one grade of iron at a time. If they have a roll to cast, they will only charge up the iron weighed off for it. The blast will then be put on, and all the iron in the cupola melted and tapped out. The blast is then stopped and the bed renewed with coke. Another grade of iron is then charged up and all melted down. As many as 3 distinct blow - outs have been made during the same heat. The first was about 7000 lb. for a roll; the second, about 2000 lb. for soft work, and the third was common iron to finish off a heat of about 8 tons. The size of the cupola that this was done in was about a 4 ft. 6 in. shell. The objection to this style of melting is, that there is a little more coke used, and it takes 1/2 to 1 hour longer to run a heat off.
It seems almost an impossibility to run a straight heat, when there are 2 or 3 different grades of iron to melt, without having them mix more or less, and the less the weights of the different grades to be melted the more will they be liable to mix. For example: Charge an ordinary cupola, with a regular feed of a special grade of iron, with the usual amount of fuel on top, and so on, charging with distinct grades of iron. As the grades of iron melt, pour some castings, the weight of which should not be less than 50 lb. On the following day, melt the special grades of iron by themselves, and pour some castings, and then compare the runners and gates, and you will see that there is a difference.
It is generally known that hard iron will melt sooner than soft iron, and most foundrymen, when making a casting of hard iron, have the hard iron charged first, to make sure of having the casting of good sound iron and of the grade wanted. If they have soft iron to run, it is generally charged on the top of the hard iron. This plan cannot always be approved of, as there are always more or less particles of any grades or charges of iron left remaining among the fuel and on the bottom and sides of a cupola, and which will affect 2 or 3 other charges.
A plan that works well, when there are 2 charges of soft iron wanted, is to melt the hard iron first; then, instead of putting the soft iron directly on the top of the hard iron, charge about 2 charges of common iron - either soft or hard. On top of these the soft iron will be charged. After all the hard iron is down, then the common is tapped out, until, by the number of ladles carried off, it is apparently all melted. At this point, the soft castings are poured according to the degree of softness wanted. The softest casting wanted, if there have been 3 charges of soft iron, should be taken from what is thought to be the middle or second charge.
In some cases, where only a small amount of very soft iron is wanted, you may charge up the soft iron on the top of the bed, which should be burning well, and should not have in as much fuel by 4 in. to 6 in. as for ordinary heats. This iron will be put in 1/2 to 1 hour before any other charge, and when all is ready to have the rest of the charges put in, make the first charge of fuel (that which is placed between the first and second charges of iron) a large one - as much larger than usual as the bed was left low. By this means, the large charge of fuel takes a longer time to get hot enough to melt the upper charge of iron, and when the first charge of iron is melted, the second, or large charge of fuel, will come down and raise the bed up to the proper height to run the balance of the heat off. By this plan, hard iron has been charged on the top of soft iron.
When not taking out the soft iron too closely to the amount charged up, the castings have been as soft as if the hard iron had never been charged up. It is in having only small quantities of different grades of iron to melt, that there is serious trouble with their mixing together. With large quantities, there is more chance of having castings the grade wanted; but even then the melter must use judgment in seeing that the iron is charged as it should be, and the foreman should be watchful, so as to know that the iron is taken away from the cupola as the grades melt or come down. There is nothing about a foundry that requires such changes in its management as a jobbing foundry cupola. (T. D. West.)