Ore Treatment

In regard to dissolved gases, while non-ferrous metals do not undergo a "boiling" process in which gas is generated during refin ing and melting, as in steel, they yet absorb considerable quantities of gas from the atmosphere of the furnace and from other sources. In some cases this leads to difficulty in securing soundness whether in ingots or in shaped castings. Copper is an example of this kind, but the phenomenon is encountered t3 a marked extent also in aluminium and its alloys and in certain kinds of bronze. Two methods have been recently evolved for freeing non-ferrous metals from dissolved gas. The most positive of these is that of "pre solidification." It consists in allowing the molten metal or alloy to solidify very slowly, preferably in the crucible and in the fur nace where it has been melted. During this slow solidification the crystallizing metal expels the gas, which has time to escape to the surface. As soon as solidification is complete the metal is rapidly re-melted and cast.

Another method of gas-expulsion consists in bubbling through the molten metal a more or less inert gas which, by local reduction of the partial pressure of the dissolved gas, leads to the removal of the latter. This method is successful when nitrogen is bubbled through molten aluminium alloys. Nitrogen, however, is itself slightly soluble in many metals, so that this process is not univer sally applicable. In some alloys, on the other hand, one of the constituent metals is itself volatile and escaping bubbles of its vapour serve to expel other gases in the same manner as bubbles of an inert gas. This is, perhaps, the reason why brass—with its large content of volatile zinc—readily yields sound castings.

The production of shaped castings entails processes and prop lems widely different from those of ingot production, but it is im possible to discuss them even in outline here. They vary according to the nature of the metal to be cast. Some reference has already been made to cast iron. Steel offers much greater difficulty in regard to casting. The tendency of the metal to liberate gas during solidification is one of these : another is the high temperature of molten steel which makes severe demands on the materials used as moulds. Castings are made in two types of mould—sand moulds and "permanent" or "chill-moulds." The former have to be specially prepared—sometimes by means of machines—from pat terns for each casting. The "moulding sand" of which they are made must be carefully adapted to the needs of the metal to be cast. For steel it must be highly refractory so that it may not melt and "burn" on to the steel. Sand for moulding, however, must be sufficiently plastic to form moulds of adequate strength and co hesion, while the mould must remain sufficiently porous to allow the rapid escape of air, steam and other gases.

The use of metal or "permanent" moulds offers many advantages where the process is applicable. The constantly repeated cost of sand moulding is eliminated, but against this must be set the high first cost of the metal mould or "die." The latter is usually only justified where the same casting is required in great numbers. For many metals there is also a distinct advantage in regard to struc ture and strength which results from the relatively rapid solidifi cation which takes place in a metal or "chill" mould. For steel it

is not easy to use metal moulds but they have in recent years found application in regard to certain kinds of cast iron. For the non-ferrous metals chill casting is extensively practised, especially in the case of some aluminium alloys. For the lower-melting non ferrous metals the process known as "die-casting" has found exten sive application on account of the accuracy of dimensions which can be attained. Die-castings can be used for many purposes with out subsequent machining and are widely employed in the cheaper kinds of light machinery, domestic and office appliances, etc.

Die-casting consists essentially in producing castings in metal "dies" or moulds, but as a rule the process is carried out by forcing the molten metal into the mould rapidly under pressure. This is done in machines which also open the die and eject the casting, the operation being rapidly repeated. The widest use of this method is made in castings of low-melting alloys of zinc, tin and lead. It has recently been extended to many aluminium alloys and even to certain types of brass and bronze. The difficulties of the process, however, increase rapidly as the melting point of the alloy rises.

Reverting to the ingots in which a great part of the world's metal production is cast, we have to consider briefly the principal processes by which the finished products are produced. In these the metal is forced to assume new shapes by the application of large mechanical forces, which may be applied to the material while it is either hot or cold.

Mechanical Working.

The purpose of subjecting metal to the mechanical working is not merely that of bringing it into a de sired shape. Mechanical work affects the structure and properties of most metals and alloys in a markedly favourable manner. When metal solidifies from fusion, especially if it does so in large masses and therefore slowly, it forms a crystalline structure which is relatively coarse—sometimes very coarse indeed. While it is true that, whatever the treatment to which metal may be subjected, it remains essentially crystalline, yet the size or scale of that struc ture is highly important as affecting physical behaviour. Mechan ical working always breaks down the original "cast" structure and replaces it by a much finer and more satisfactory structure. In some metals, of which steel is the most important example, it is possible to produce a "refined" structure, comparable in scale with that of wrought metal, by heat treatment alone, but this is only practicable for pieces of moderate size because larger masses cannot be cooled rapidly enough to bring about the desired changes. In addition, mechanical working, particularly in the case of steel, serves to close up, and perhaps to bring about welding of cavities existing in the cast material. It also brings about a re distribution of the non-metallic impurities. These become elon gated in the direction in which the metal is caused to flow under mechanical treatment and in some cases confer upon the material something which has the appearance of grain or fibre. There is, however, no true fibre in any metal, although "fibrous" fractures are frequently obtained and are often rightly regarded as indica tions of good quality.