to be advantageous for many purposes, as it gives greater fluidity, and at the same time increases the strength of the metal. But with larger porportions of phosphorus the phosphide remains fluid while the rest of the metal solidifies, and being deposited throughout the mass, it causes the casting to be brittle, and to fracture easily from shock. So characteristic is this effect of phosphorus that in practice it is found possible to tell approximately the proportion which is present in a sample of pig iron by the readiness with which a pig of a given grade will fracture when thrown upon the pig breaker. The ready fusibility of the phosphide eutectic causes phosphoric iron to be unsuitable for application where the metal has to resist high temperatures, as in the case of ingot moulds, fire-bars, etc., for which purposes hematite iron should be employed. For ordinary strong castings of good quality about 0.5 per cent. of phosphorus gives excellent results, while for the general run of foundry practice, where fluidity and soundness are of more importance than strength, from 1 to 1.5 per cent. phosphorus may be allowed; but beyond this higher limit the further addition of phosphorus causes such marked brittleness as to allow of the metal being used only for very inferior purposes. The presence of phosphorus appears to have little or no effect on the proportion of combined carbon, but it reduces the shrinkage, its action in this respect being direct, and not chiefly through its action on carbon, as is the case with silicon.1

Manganese.

Iron can be purchased commercially containing any proportion from 0.1 to about 90 per cent. of manganese. The richer alloys are known as ferro-manganese. Intermediate irons containing from about 5 to 20 per cent. of manganese are called spiegels. With metal so rich in manganese the iron-founder is seldom concerned, though for special purposes the addition of a small quantity of ferro-manganese, as а softener, is occasionally recommended, particularly with sulphurous irons. The presence of manganese tends to increase the proportion of combined carbon, and hence increases the hardness and shrinkage, but decreases the fluidity of the metal. Mr Keep calculated that an addition of 1 per cent. of manganese leads to an increase of 40 per cent. in the hardness of the metal. Strong irons usually contain a certain proportion of 1 See Keep, Cast Iron, p. 79.

manganese, but it is doubtful if it is ever beneficial to introduce more than 1 per cent. in ordinary foundry practice.

We have now considered those elements which are invariably present in greater or less proportion in cast iron. Before leaving the subject, a few words may not be out of place in reference to a few other elements which occur less frequently.

Aluminum.—It was anticipated at one time that an extensive application of aluminum would be met with in the iron-foundry, as the influence of this metal is very similar to that which is exerted by silicon, except that aluminum is more energetic in the conversion of combined carbon into graphite.1 The use of aluminum for foundry purposes was very carefully studied by Mr Keep, who concluded that for ordinary foundry use silicon was, on the whole, to be preferred. The disadvantages in the use of aluminum are that the cost is increased, while metal which contains aluminum is less fluid than ordinary cast iron, and appears to have a skin upon its surface which prevents it filling the crevices of the moulds. This skin is also apt to break away in the form of scum, which forms irregular patches on the surface of the casting. The amount of aluminum required is so small that the presence of rather more oxygen than usual in the metal may result in little or no aluminum remaining in the casting, and hence the product is apt to be very irregular. At the same time it must be acknowledged that when just the right proportion of aluminum can be introduced the product is often exceptionally good.

Arsenic is frequently present in small quantities in cast iron. Its action resembles that of phosphorus, but the quantity is usually not sufficient to cause the iron-founder any anxiety.

Copper is not an uncommon constituent, but the amounts which are usually present cause no trouble to the iron-founder. In small proportions it is usually rather beneficial than otherwise. The same can be said of nickel in quantities not exceeding about one per cent.

1 See Melland and Waldron, "The Influence of Aluminium on the Carbon in Cast Iron," Jour. I. and S. Inst., 1900, vol. ii. p. 244.

Titanium is not an unusual constituent of cast iron, though the amount present is seldom over 1 per cent. It imparts a characteristic velvety mottled appearance to the fractured surface of grey cast iron, which is seen in fig. 17. This sample contained 0.28 per cent. of titanium. In the proportions in which titanium is usually present it does not appear to injuriously affect the physical or mechanical properties of cast iron, or to alter the proportion of graphite, or the power to chill. On the other hand, Bahlson states that a small

quantity of titanium, when present in cast iron, leads to increased bending and tensile strength.1

Composition of Strong Cast Iron.

Having thus briefly discussed the influence of each of the elements with which the iron-founder has to deal, it may be well now to consider what may be regarded as a typical cast iron for general foundry purposes. Here it should be mentioned that no particular kind or grade of cast iron is equally good for every purpose. There

1 Iron and Steel Inst. Jour., 1902, ii. p. 550.

is in fact no "good" or "bad" cast iron, as metal which is entirely unsuitable for one purpose may be exactly what is required for another. As a typical example of good cast iron we may take the material which is employed by the engineer for best qualities of work, where strength, solidity, fluidity, and good working qualities are required. Such a metal, with relatively small alterations of its silicon and other constituents, can be made to answer for any other purpose in the foundry. Examination of a large number of results of chemical and mechanical tests, including those of the American Commission on Metal for Cannon in 1856; the Cast Iron Experiments conducted at Woolwich in 1858; of my own experiments; and of a number of tests recorded by other observers,1 show that with cast iron the maximum tensile strength is obtained with about 94 per cent. of metallic iron, and with the other elements in approximately the following proportions :—

Starting with this typical composition, the founder can vary his mixture according to the work which may be in hand, having in view the requisite strength, fluidity, price, or other special conditions which may be necessary. In cases where the metal is required to chill, the proportion of silicon usually varies from about 6 to 1 per cent., the amount depending upon the depth of chill needed and the size of the casting; a lower silicon being naturally employed with larger mases of metal. If a fluid or thin iron is wanted the silicon may be increased to about 3 or 3.5 per cent. and the phosphorus to 1.5. For such material a lower manganese is usually to be recommended, and the metal should be cast at a rather higher temperature than usual. For the production of castings with very fine surfaces, fluid metal with high graphitic carbon and relatively high in silicon and phosphorus is employed, and it is important that the metal should be poured at the right temperature. Smooth finely dressed patterns, which are often of polished metal, are used, and the moulds should be made with sand of good uniform quality, and should

1 For particulars, see Metallurgy of Iron, p. 238.

be well vented. In some cases considerable stress is laid upon the character of the "facing" applied to the surface of the mould, though excellent results are obtained in other foundries in which no special facing is applied.

Composition of American Foundry Iron.

As there is often a distinct difference in the composition of American and British cast iron of the same grade, owing to the former being somewhat lower in sulphur and silicon, it may be well here to include a series of typical specifications for the chemical composition of American foundry irons, as given in the recently issued and valuable Report of the British Iron Trade Commission of 1902 (page 236).

Special Hard Iron (Close Grained).

Silicon must be between 1.2 and 1.6 per cent. (below 1.2 the metal will be too hard to machine; above 16 it is liable to be porous unless much scrap be used).

Sulphur must not exceed 0.095 per cent., and any casting showing on analysis 0·115 or more of sulphur will be the cause for rejectment of the entire mix. (Above 0.115 per cent., sulphur produces high shrinkage, shortness and "brittle hard" iron. Exceptionally, however, as for frictional wear in brake shoes, etc., the sulphur may run up to 0.15 per cent.)

Phosphorus should be kept below 0.7 per cent., unless specified for special thin castings. (High phosphorus gives brittle castings under impact.)

Manganese should not be above 0.7 per cent., except in special chilled work.

Medium Iron.

Silicon to be between 1.4 and 2 per cent. (Silicon at 1.5 gives the best wearing result for gears.)

Sulphur must not exceed 0.085 per cent., and any casting showing on analysis 0.095 per cent. or more of sulphur will be the cause for rejectment of the entire mix. (Sulphur preferred at 0.075 to 0.08 per cent.)

Phosphorus should be kept below 0.7 per cent., except in special

work.

Manganese should be below 0·7 unless otherwise specified.