Charcoal iron is now being used only where the source of supply is close, and its cost is but a dollar or two over the best Mal leable* or °Bessemer Malleable,* as coke irons made specially for the malleable foundry are called (the last named being a Bessemer iron with the phosphorus a little higher than is allowed for steel). It is best to use these classes of °malleable* instead of the so-called pig irons, for they are made with an extra amount of coke, and are much less oxidized than the irons blown under poor furnace con ditions.
The mixtures used in the malleable foundry are as follows: Where the cupola is used for malleable castings in addition to making pots for annealing, the regular mixture as used for the air furnace or open hearth furnace must be reduced slightly in silicon, as the cupola burns out less of this element. Hence about 0.25 per cent is to be added to the amount of silicon required in the casting. In the case of the furnaces, about 0.30 per cent to 0.35 per cent must be added. For pots it is advisable to use good pig irons and to utilize salamanders and the large scrap pieces that are unsafe to put into the furnace. The silicon in the pots as cast should be about 0.60 per cent and hence about 0.85 per cent should go into the cupola.
The mixtures for air furnace and the open hearth furnace are about the same. Possibly the air furnace should have a little more sili con, as it is under the influence of the gases from combustion longer. There is also necessary an occasional use of ferrosilicon, especially in the open hearth, as through acci dent the metal may be badly burnt and the addition of ferrosilicon brings about the proper composition, though the metal should not be put into castings, but cast into pigs, to be fed subsequently into the regular mixtures in small quantities at a time. About 250 to 500 pounds ferrosilicon is usually all that is necessary for thispurpose. The actual amount can be calcu lated at the time from the supposed loss of silicon in the bath.
Malleable castings are made in the cupola, the air furnace, the open hearth furnace, the electric furnace and in the crucible. The last named process is now only practised in Europe, being too expensive in this country, though turning out a most excellent product. The cupola process makes the poorest castings, as the metal is in contact with the fuel in this method. Hence the absorption of sulphur and oxidizing influences which are partly avoided in other processes. The peculiar constitution
of the metal as cast makes it necessary to anneal it at a temperature some 200 degrees F. higher than ordinary air-furnace iron, which means a greater expense for wear and tear on the ovens and annealing pots. This class of castings is therefore only used for the cheaper grades of malleable casting, such as pipe fit tings and hardware castings, where great strength is not essential, and enough ductility is had to satisfy the demands made on the work. The selling price is also about half a cent a pound less than the high grade metal.
The bulk of the malleable castings made in this country comes from the air furnace. This is an excellent melting process, can be manipu lated easily, is not too expensive, and will prob ably continue to be the method used in most of our malleable works. The air furnace, as used for malleable purposes is somewhat dif ferent than that used for making rolls and gun castings. It is illustrated herewith.
The entire roof can be taken off in sections, called °bungs,* so that the sand bottom can be made, and the charge put in. Where more than one heat is made without remaking the bottom, only a few bungs are lifted away after the first heat and the furnace is charged quickly, so as to keep it hot as long as possible. Fir ing is done at one end, and the charge when ready is tapped at one side, or both if two spouts are provided. It is very important to get a heat out quickly, as the metal is continu ously oxidizing while in the furnace, after it has reached the proper composition. A 10-ton heat, when poured into small castings often takes three-quarters of an hour to tap, and hence the first iron and the very last may be two different things. Hence the dividing up of the work to get the class of metal best suited to the castings to be made. The amount of coal used to melt in a well-constructed air furnace is four pounds iron to one pound coal. In the case of the cupola, while ordinary gray iron practice requires one pound coal to every eight pounds iron, in malleable work, it takes one pound coke to only four pounds iron, or just the same as good air furnace practice. It is but just to say, however, that in many foundries of the country the air furnaces are so poorly constructed, kept in repair and operated that oftentimes one pound coal melts only two pounds iron.