LST-IRON. It is said that cast-iron was first made in Sussex as early as the year 1350, but it was not until the end of the Tudor period that the practice of iron founding was introduced into England from the continent of Europe where it had already been considerably developed. Al though cast-iron and pig-iron have the same chemical composition, the expression cast-iron is generally applied to pig-iron which has been remelted in a crucible, or furnace, and cast into moulds.
Classification.—Cold- and hot-blast irons (see PIG-IRON) are divided, according to their content in phosphorus, into two main groups: (a) Hematite and (b) Basic irons. The former contain only small percentages of this element, about .07%, while in the latter the phosphorus may reach as much as 3%. Each of these main groups is again subdivided into at least six grades, Nos. I, 2, 3, foundry, forge, and white irons. In passing along the series from "No. 1" to "White" the percentages of silicon and manganese gradually decrease, while those of combined carbon and sulphur increase. Hence it is possible, by mixing irons from different groups, to obtain cast-iron of almost any desired compo sition, due allowance being made for the changes which occur dur ing remelting. Iron may be classified also according to (a) the method of manufacture; (b) the purpose for which it is in tended; and (c) its composition. It was formerly graded by break ing the pig and examining the fracture, but this method has been largely superseded by chemical analysis.
(a) Method of Manufacture: I. Coke Pig is smelted with coke ; always with hot blast.
2. Charcoal Pig is smelted with charcoal, by hot or cold blast.
(b) Purpose for Which Intended: 3. Bessemer Pig.
4. Basic Open Hearth Pig.
5. Malleable Pig, for malleable cast-iron castings.
6. Foundry Pig.
7. Forge Pig is an inferior grade used for puddling and for some classes of foundry work.
(c) Chemical Composition: 8. Silicon Pig or High Silicon Pig.
9. Low Phosphorus Pig.
Io. Special Low Phosphorus Pig.
II. Ferro-Alloys and Special Cast Irons (e.g., ferro manganese) .
When pig-iron is melted under conditions which permit of the oxidation of the silicon, as in the cupola and reverberatory fur nace, an iron which presents a greyish fracture when broken, and is quite soft and easy to machine, will on continued remelting ultimately become white and intensely hard. Conversely, by melt ing white iron with increasing quantities of siliceous pig-iron it is possible to convert the white into grey iron. These differences in appearance arise from changes in the condition of the carbon. Cast-iron contains upwards of 3.5% carbon, which in very grey iron is almost wholly in the graphitic state; this condition arises from the presence of large amounts of silicon, an element which has the property of decomposing carbide of iron
into its constituents, iron and carbon. As the silicon is oxidized by re melting, more and more of the carbon combines with the iron, until ultimately, in white iron, carbon in the form of graphite is entirely absent. Manganese also is oxidized during remelting, while sulphur on the other hand is absorbed from the fuel.
The following are examples of the grades in common use : For thin ornamental work, such as grates, stoves, hollow ware, and other light castings, where in order to fill the mould great fluidity is required, a mixture of irons containing from 2.5% to 2.8% silicon and from 1.3% to 1.5% phosphorus is in common use ; but in medium-sized castings and general work, where somewhat greater strength is required, iron containing a greater percentage of combined carbon and a reduced silicon con tent is necessary; and as strength rather than extreme fluidity is the main consideration, the phosphorus also may be reduced. For use in this connection irons containing silicon from 2% to 2.25% and phosphorus from I% to 1.3% are suitable. Where still greater strength is required, as in machinery castings, phos phorus and silicon are still further reduced, and a mixture of irons yielding an average content in silicon and phosphorus re spectively of I • I 5 % to 1.2% and 0.4% to o.6% is employed. In cases where castings have to resist wear, and have at the same time to machine with a highly polished surface, a very close grained iron is necessary, this condition being provided by an ad mixture of reficled and cold-blast irons with an average silicon content of from I% to 1.3%, and about 0.45% to 0.70% phos phorus. So far, only castings of the same physical properties throughout have been considered ; for some classes of work, how ever, such as rolls used in the sheet metal industries and some types of car wheels, it is necessary to have an intensely hard sur face and a soft interior in one and the same casting. This is ob tained by taking advantage of the fact that given a suitable grade of iron, it is possible by rapid cooling to retain the carbon in the outside portion of the casting in the combined condition. This process is known as "chill casting" and the parts of the mould where the metal is to be hardened are made of iron instead of sand, so that, when the molten metal comes in contact with them heat is so rapidly extracted from it that the separation of graphite is prevented, and the carbon retained in the combined condition; while towards the centre of the casting, where the rate of cooling is not so rapid, decomposition of the cementite can take place, and in consequence the iron in this region is quite soft. In this class of work the selection of the pig-iron is of the greatest importance, slight differences in composition having very marked influences on the depth to which the hardening effect penetrates. Iron for chill casting should be low in phosphorus, and silicon also should not be too high, or the carbon will not remain in combination. About 1.5% of silicon is probably best, but opinion is not unani mous on this point.
Ordinary cast-iron is melted readily and easily cast into any desired shape, but it has the disadvantage that castings so made are relatively weak and brittle. It is possi ble, however, by making use of the facility with which carbide of iron can be split up into its constituents, to make castings which are stronger than ordinary cast-iron ones, and yet possess a considerable degree of ductility, almost approaching wrought iron in this respect, though unlike forgings of the latter metal they cannot be welded, except by the "bronze rod" process, and at a temperature below the melting point of iron. Castings made in this way are called "malleable cast-iron castings," or shortly "malleable castings." The process has been in use for many years, having been described by Reaumur in 1722. For the production of cast ings by this method a refined hematite pig-iron is used containing from 2.5% to 3.0% carbon and from 0.5% to i.o% silicon (the lower figure for large, and the higher one for small castings), and having as little as possible of sulphur, phosphorus, and manganese. Sulphur is supposed to promote the formation of blow-holes, but it has been found that 0.25% may be present without ill effect. Silicon and manganese prevent the castings being converted; while an excess of phosphorus causes brittleness in the finished casting. The castings, when removed from the moulds, are extremely brit tle, on account of the nature of the pig-iron used, and when broken present a white fracture ; they are packed in boxes containing red hematite ore and then annealed for a period of four or five days at a temperature of 800° to 900° C. After this treatment they present a grey fracture and can be readily dressed with a file or chisel; they will also admit of a considerable amount of defor mation without fracture. The process described is the one fol lowed in Europe. In America, where the decomposition of the car bide of iron, not the direct oxidation of the carbon, is regarded as the chief essential in the process of conversion, the annealing of the castings is carried out in a slightly different manner. They are packed in boxes containing sand or clay with or without admixture of oxide, and annealed at a temperature rather lower than that used in European practice, viz., 700° to 800° C. By either method a malleable casting is obtained, but the products of each when broken present a very different appearance and are designated "white heart" and "black heart" castings respectively, terms which explain themselves.
Since about 185o the use of cast-iron for structural purposes has been gradually superseded by steel, but so many other applica tions have been found for it, that it is still in considerable demand, as it is the cheapest form of commercial iron. Its tensile strength, though considerably less than that of steel, is ample for many pur poses, while in compression its strength is greater than that of any other known metal.
See E. L. Rhead The Principles and Practice of Irowfounding (Man chester, 191o) ; T. Turner The Metallurgy of Iron (5th ed. 1918) ; W. H. Hatfield Cast-iron in the Light of Recent Research (and ed. 1918) ; H. A. Schwartz American Malleable Cast Iron; Richard Moldenke The Principles of Iron Founding. (T. BA.)
