In choosing metals for the heat it first is necessary to ascertain the silicon content desired in the castings, and then to select such amounts of the different brands of pig iron at hand as are required to bring about the desired result, first making careful calculation of the amount of scrap to be cared for and its effect on the mixture as regards its silicon content. The previous section on Melting may be referred to regarding gray-iron mixture.
In conclusion, it would be well for the reader to clearly understand that it requires greater skill to produce malleables, and for this reason a well-equipped laboratory and a good metallurgist are almost necessities where high quality is the order of the day.
The furnace having been duly charged for the day's melt with the desired mixture of metal, we will now consider the casting operation which differs only from gray-iron practice in that the metal contains a larger per cent of carbon in the combined form, melts at a lower temperature, and also cools more quickly in the ladle, and for this reason must be handled more rapidly. The hand ladles are of somewhat smaller capacity, usually about 25 pounds, as compared with 40 to 60 pounds in gray-iron practice.
This lighter burden allows the molder greater freedom in his movements. It is common practice to see the molder catching-in to the stream of molten metal and running to his floor that the molds may receive the benefit of the hottest metal possible. While this is possible in malleable-iron work, with the larger ladles of gray-iron practice, this would not only be exhausting, but highly dangerous as well.
It must be remembered that the majority of work is light and that the floor space required for the setting-up of sufficient molds to pour, say, a 30-ton heat, is considerable, and even with the furnace located as it should be in the center of the works, there must be long carries. These are in part overcome by the installation of some overhead trolley system using 500-pound ladles; and where the nature of the work permits - i.e., it is not too light - this method is of course preferable.
As there is great danger of the castings developing cracks if exposed to the air while still red hot, it is far better practice to let them remain in the sand until they are at least black hot when they may be safely shaken out: the exception being some special work, as brake wheels, for example, in which there are developed great casting strains. In this case it is found best to shake out while still red hot and quickly place them in a so-called reheating oven which is already fired very hot. This furnace, after being fully charged with the day's output of this special work, is closed and the fire allowed to die out over night, when the castings will be found to be partly annealed, though not in a malleable sense. This treatment is found of considerable value in certain classes of work which otherwise might be hard to save.
The castings having become cool so that they may be readily handled, a rough separation of castings and gates should be undertaken and the cores removed as far as possible, after which the castings should be inspected and all missruns or otherwise defective castings removed. The discards along with the gates and sprues should then be tumbled to remove the sand scale before being returned for remelting, otherwise it would form an excessive slag and require a larger amount of fuel to remelt.
After this rough sorting the castings are ready for the hard-rolling room which contains a series of tumbling barrels, all of which should be equipped with an exhaust system, or else the dust created in this department will become the bane of the plant. Fig. 164 shows an installation of modern exhaust tumbling barrels. The castings should be rolled only long enough for the removal of the sand scale, usually accomplished in from 20 to 30 minutes. Some classes of work may be so delicate that it would be impossible to clean them in this manner without too great a loss from breakage, and in such cases the acid bath may be resorted to, or perhaps the sand blast which is fast superseding other methods of cleaning all classes of castings. Fig. 105 shows a New Haven sand-blast tumbling barrel which is well adapted to this work.
After this hard-rolling process the castings are removed to the trimming room where the gates and fins remaining may be easily removed with a hammer, thus saving much time in the grinding room after the annealing.
We now arrive at one of the most important departments of the plant. No matter how carefully all previous operations may have been conducted, all will have been in vain should any neglect creep into this part of the practice, for it is here that the very nature of the casting is changed from its hard and brittle state showing a white fracture, to the so-called black-heart malleable, the fracture showing a steely rim with a black velvety core.
This change is brought about by gradually bringing the temperature in the annealing ovens up to about 1600 degrees Fahrenheit, and by maintaining that temperature from 3 to 4 days, after which the fire is allowed to slowly die down; this whole process requires from 8 to 10 days. There have been attempts made to shorten this annealing process, usually by increasing the temperature some 200 degrees, thereby reducing the total period of the process to about 6 days; but this has been done at the expense of quality, and the best results are obtained by the former method.
After inspection in the trimming room, the castings are brought to the annealing room which has its floor space divided into two parts - the packing floor, and the ovens. There are usually two rows of ovens, one on each side of the building, and the clear space between is used for packing the pots and also for dumping them after the annealing. The floor of this is made of 1-inch iron plates.
The annealing boxes, or taggers, may be either square or round, or perhaps more generally oblong, and are first cast 1 inch thick. These pots are piled three or four high - the first one is placed on an iron stool - all joints being luted up with mud made by adding water to the burnt sand from the rolling room with perhaps the addition of a little fire clay. The scale used for packing is cinder squeezed from muck balls where wrought iron is made. It was formerly the practice to spread this scale upon the floor and sprinkle it daily with a solution of sal ammoniac to rust it, but later-day practice has proven this unnecessary.
As the castings come into the annealing room the operator places a pot on the stool, shovels in some scale, carefully placing in a layer of castings in such manner that none comes in contact with the sides of the pot or no two castings touch each other, then more scale and more castings are added until the pot is filled, as shown in Fig. 166. On this another pot is placed and duly packed, this to be continued until the pots are piled as high as desired, after which all joints are sealed with mud making the whole pile more or les; air-tight. After this has been accomplished, the pots are placed in the oven either by a hand or by a power charging machine, as illustrated in Fig. 167.
The annealing oven is quite simple. The principle involved is the introduction of heat from some convenient point and its distribution in a uniform manner, and the introduction of as little air as possible. The combustion space should be no larger than necessary, the draft regulation perfect, and the bottom of the oven underlaid by a series of flues which allow the gases to circulate before escaping into the stack, so there may be as little loss of heat as possible. Oil, gas, or coal may be used for fuel as best adapted to the locality.
Fig. 108 shows the interior of an oven, while Fig. l69 shows the boxes in place. Fig.170 is a side view of an oven showing the firing doors and the ash pits. Having placed the full number of boxes in the oven, the front is closed and the ovens are fired. As before stated this operation requires from 6 to 10 days from the time the fire is started until the oven has cooled sufficiently to allow the removal of the boxes.
As the boxes arc withdrawn from the oven and are taken to the floor of the annealing room, they are suspended from an overhead trolley, or by a crane, and the castings are removed by striking the boxes several sharp blows with a medium-weight sledge hammer, the castings and scale falling upon the floor. The castings are now picked from the scale and it will be noted that there is some tendency for the scale to adhere to them. This may be removed by the ordinary rolling barrels, after which any gates or fins remaining should be ground off. The amount of labor required for this operation depends upon how carefully the gates were moved while castings were in the hard. After a final inspection, the castings should be ready for shipment. Figs. 171 and 172 show the castings being sorted out and ready for shipment in the shipping room.
The use of the pyrometer in connection with the annealing furnace is almost obligatory. Fig. 173 shows a standard type of recording pyrometer. The pyrometer equipment is often placed in the office of the head executive of a local plant; it is possible for him to plug into any two of his battery of annealing furnaces at any time during the day. Also, in the morning, there is recorded a true record of temperatures for the night before. As no operator knows whether his furnace is under observation or not, this system has the tendency to keep the men at all times alert.
The amount of finish given the castings varies with local conditions and class of castings produced. There are some classes of work where, by the use of leather scraps in the soft-rolling room, the work is so carefully cleaned and polished that the castings may be tinned or nickeled and sometimes gold- or silver-plated, making very beautiful work in which great strength is combined with cheapness of production.