The term malleable - iron castings means an iron that has been cast into any desired shape, and then malleableized by removing the carbon by a process of annealing, which consists in burning off the whole or a part of the carbon combined with the iron from which the castings were made. In the manufacture of malleable - iron castings, the first object is to get the proper kind of pig - iron, for all iron is not suitable for making malleable iron by the process of annealing. From the states in which carbon exists in cast - iron, this has been classified into 3 principal subdivisions. The first is " grey " metal, or " No. 1 foundry pig," in which the carbon is not combined with the iron, but is in the graphitic state, and may be seen in large flakes when the iron is broken. These flakes are sometimes called " tissue" and "black - lead." The second division is "mottled" cast - iron. In this the carbon is partly combined with iron and partly in the graphitic state, which gives the iron a spotted or mottled appearance. The third division is " white " cast - iron.
In this the carbon is combined with the iron, and is unseen.
Grey or No. 1 foundry iron is best for ordinary foundry castings, because it contains the most carbon, and is softer and wi11 remain fluid longer than either the mottled or white irons, yet it is not best for malleable castings, for the carbon in it is not combined with the iron, and in converting the castings into malleable iron the carbon is extracted from the iron without melting the castings, and if this class of iron is used the castings will be full of small holes after they have been malleableized, and will not have the required strength.
The iron that will make the best malleable castings is white cast - iron, for in this the carbon is completely combined with the iron, and when it is abstracted from it by the annealing process, it leaves a perfectly sound and smooth casting. But in using this iron for malleable castings another trouble arises. The iron contains so little carbon that it will not retain its fluidity - long enough to be run into light castings, and almost all malleable castings are very light; so that this class of iron cannot be used.
As the grey or No. 1 foundry iron contains too much carbon, and the white iron too little carbon, the best iron for malleable castings must be the mottled iron, which is between the two extremes. This iron is always used for malleable - iron castings, and none but the very best brands of cold - blast charcoal mottled iron will produce a good malleable casting.
Iron for malleable castings may be melted in a cupola or in either of the reverberatory furnaces. But the iron melted in a reverberatory furnace always produces by far the best castings; for the iron is not melted in contact with the fuel, as in the cupola, and it is not deteriorated by the impurities contained in the fuel. There is also the advantage that, should the iron contain too much carbon, part of it may be removed by the oxidizing action of the flame.
As most malleable castings are very small, they are generally moulded in snap - flasks, with greensand, from metallic patterns or match plates. The castings, before they are annealed, are as hard and brittle as glass, and they must be handled with care to prevent breaking. These castings are put into a tumbler or rattle barrel, where they are cleaned of all adhering sand, and become polished by mutual friction; to anneal them properly, it is very essential that thay should be thoroughly cleaned. The cleaned castings intended for conversion into malleable iron, are next packed into iron boxes, with alternate layers of fine iron scales from rolling - mills. The boxes are then closed at the top by a mixture of sand and clay, and all the cracks are carefully luted, to prevent the admission of air. The boxes are next put into the annealing oven, where they are subjected to a white heat, not sufficiently hot, however, to melt the boxes. They are kept at this heat for a week or more, and then allowed to cool off gradually. After the castings have been properly annealed, they are covered with a film of oxide of different colours, and resemble in appearance that kind of Champlain iron ore called peacock ore. These various colours of the oxide are a sign of good malleables.
This adherent oxide is removed from the casting by another passage through the rattle barrel, and the process of malleable - iron making is finished.
Powdered iron ore is sometimes used in place of iron scales, but it is not so good, for it contains more or less silica and earth, which, at the temperature of the annealing oven, will fuse and form a slag or cinder, and prevent the oxidizing action on the castings. For this reason, scales are to be preferred, and care should always be taken to keep them as free from earthy matter as possible. In every " heat " or annealing operation, the scales part with some of their oxidizing properties, and before they are again used they must be pickled and reoxidized. This is done by wetting them with a solution of sal - ammoniac and water, and mixing and drying them until they are thoroughly rusted, when they are again ready for use. The annealing boxes were formerly made of soft iron, but at present they are mostly made of hard iron - the same as the castings are made of. The hard iron boxes become annealed the same as the castings, and will last longer than the soft iron boxes. These boxes are generally made about 20 in. long by 14 in. wide and 14 in. deep. They are set one on top of another in the annealing oven, but never more than two high. The lower one has a bottom cast in it, but the top one has no bottom, and is merely a frame set on the lower box.
These boxes only last a few heats, and the small boxes are said to last longer than the larger ones.
There are several different kinds of annealing ovens in use, and some very important improvements have been made in their construction in the last few years. The best in use at the present time is one with a fire on each side of it, and so arranged that the flame from the fuel does not enter the oven or strike the boxes. This oven is not allowed to cool off, but is kept hot all the time, and at one end there is a door, through which the annealing boxes are removed while at a white heat, and are replaced by cold ones. The door is then closed, and the boxes heated to the required degree. This kind of oven is most economical in use, for it requires less fuel than any other, and is not injured by expansion and contraction in cooling and reheating, as the other ovens are. When annealing the castings in the oven, care should be taken to not have the temperature of the oven too high, nor the heat too prolonged, or the castings may be burned and hardened after they have been softened. After the castings have been thoroughly decarbonized by annealing in the oven, they are virtually a commercially pure iron, and are the same as wrought - iron without fibre, and fibre may be imparted to them by rolling or hammering.
Yet these castings without fibre are sometimes equal to the best wrought - iron for strength, and may be bent double when cold without breaking them. (Iron Age.)
The process is conveniently applicable only to small castings, although pieces of considerable size are sometimes thus treated. Handles, latches, and other similar articles, cheap harness, mountings, ploughshares, iron handles for tools, wheels and pinions, and many small parts of machinery are made of malleable cast iron, or as steel castings. For such pieces, charcoal cast iron of the best quality should be selected, in order to ensure the greatest possible purity in the malleable product. The castings are made in the usual way, and are then embedded in oxide of iron - in the form, usually, of hematite ore - or in peroxide of manganese, and exposed to the temperature of a full red heat for a sufficient length of time to ensure the nearly complete removal of the carbon. The process with large pieces requires many days. If the iron is carefully selected, and the decarbonization is thoroughly performed, the castings are nearly as strong and sometimes hardly less malleable than fairly good wrought iron, and they can be worked like that metal. They will not weld, however. The pig - iron should be very free from sulphur and phosphorus.
The best makers have usually melted the metal in crucibles having a capacity of 50 to 75 lb., keeping it carefully covered to exclude cinder and other foreign matter. The furnace is similar to that of the brass foundry, 2 to 2 1/2 ft. square, and the fire is kept up by natural draught. The temperature is determined with sufficient accuracy for the practical purposes of the iron - founder by withdrawing a portion on an iron bar. If hot enough, the drop burns on exposure to the air. If right, the metal is poured quickly. The "cementation," or de - carbonization, is conducted in cast - iron boxes, in which the articles, if small, are packed in alternate layers of the decarbonizing material. As a maximum, about 800 or 1000 lb. of castings are treated at once. The largest pieces require the longest time. The fire is quickly raised to the maximum temperature, but at the close of the process the furnace is cooled very slowly. The operation requires 3 to 5 days with ordinary small castings, and may take 2 weeks for large pieces.
This process was invented in 1759. Decarbonization is often performed, in the production of steel castings, by a process of dilution accompanied with possibly some "dissociation." By the preceding method the carbon takes oxygen from the surrounding oxides, and passes off as carbon monoxide (carbonic oxide); in the process now referred to the carbon of the cast iron is shared between the latter and the wrought iron mixed with it in the melting - pot, and a small portion may possibly pass off oxidized. The latter method has been practised to some extent for a century. Selected cast iron and good wrought iron are melted down together in a crucible, and cast in moulds like cast iron. The metal thus produced contains a per-centage of carbon, which is determined by the proportions of cast and wrought iron in the mixture. The amount is so small, frequently, that the castings can be forged like wrought iron. (Prof. Thurston, Materials of Engineering.)