The number of furnaces of this type far exceeds all others. In the issue of the Foundry Magazine for February, 1910, the number of the different types of furnaces engaged in the production of malleable in America was given as follows: air furnaces, 3G9; cupolas, 42; open-hearth furnaces, 21.
Figs. 158 and 159 show two general types of air furnace. In the furnace represented in Fig. 159 the bath is immediately behind the bridge, while that shown in Fig. 158 has its bath at the end, remote from the bridge.
Fuel is placed on the gate through the charging door D, which is of cast iron lined with fire brick. The hearth is where the metal is placed when charged. E is the stack through which the gases finally escape. B is the bath where the molten metal collects and at its lowest point the tapping hole is located. The metal is charged by removing a part of the roof as shown at 0, a section of which is shown in Fig. 160. Peepholes are shown at PPP, which are for observation.
The iron is charged on the hearth so as to leave openings between the pieces, and the molten metal should be skimmed from time to time so that it may receive the direct action of the burning gases passing over it.
Fig. 161 shows a type of air furnace complete. Fig. 162 is a view of the opposite side with a melter in the act of firing. Fig. 163 shows the slag hole, and the slag just skimmed off.
The air furnace requires greater skill to operate than does the cupola, and the fuel ratio is higher, but, if of good quality and properly fired, is not excessive and should average about 3 of metal to 1 of fuel.
It is said the open-hearth installation represents the highest type of melting yet devised, but the high first cost, combined with frequent and heavy repairs and skill required to operate, confine its use to the largest plants, and as long as the trade is satisfied with the quality of the product of the more easily operated air furnace, it is quite doubtful if the open hearth will be generally adopted. The description of the open-hearth furnace which has been given in Steel-Casting Practice may be referred to.
That the reader may clearly understand the basic principles involved, it is necessary first to discuss the various materials entering into the mixture. These include pig iron, sprues, faulty castings, annealed scrap, steel scrap, and ferro-alloys.
As all the materials have a bearing on the finished casting and should only be used as they affect this, it is at this time well to give an analysis for good malleable castings, which is as follows:
Proportion (per cent)
0.75 to 1.25
There is considerable latitude allowed due to the class of work to be produced. Makers of heavy castings exclusively may specify their silicon from 0.75 to 1.50 per cent, while for very light work silicon from 1.25 to 2.00 per cent may be the rule. Total carbon should never run below 2.75 per cent in the hard, i.e., before the annealing. Manganese should average about 0.40, not much lower, and never above 0.60. Sulphur should always be as low as possible, 0.07 per cent being the high limit in the casting, hence the necessity for choosing irons for the mixture which will insure this. Phos-phorus below 0.225 per cent is desired.
Formerly it was thought that only charcoal pig was suitable for malleable practice, but today, owing to improved blast-furnace practice, coke irons which are known to the trade as coke malleable give first-rate results and are used to a considerable extent, yet it is seldom that the writer has heard of a mixture that did not include one or more of the well-known brands of charcoal iron known to the trade as Mabel, Briar Hill, Hinckley, and Ella. It is generally conceded that a mixture containing several brands of iron, or at least two or three grades of the same brand, produces better castings.
The next problem which presents itself is the disposition of gates, sprues, and discards, which are daily accumulated in the works, and the per cent of which varies with the class of work produced, running as high as 60 per cent in the lightest work or not exceeding 25 per cent for very heavy castings. From this it will be seen that it is quite likely to be a varying element. The practice of taking the silicon content for every heat before castings go into the annealing operation gives opportunity for fairly exact calculations. Corrections may be made in the mixtures as may be found necessary, otherwise, should there be an accumulation of such material, it would soon become an unknown quantity and so be a source of annoyance as well as of loss to the management.
The annealed scrap offers more difficulties owing to the effect which even small amounts have on the quality of the product. Quite frequently there is no attempt to use this in the regular mixture but rather to utilize it in the production of annealing pots of which more will be said later.
The use of steel scrap in malleable mixtures is proving beneficial although the amount is at present limited to something like 10 per cent. This material should not be charged with the regular mixture, but should be introduced into the bath of molten metal so that it may be quickly covered by the protecting slag, for the steel must not be allowed to burn as this would seriously injure the quality of the castings.
It is also possible to use small amounts of gray-iron scrap, though it is seldom necessary to do so, and usually 5 per cent is the limit.