Ore Treatment

The first step in bringing metal into the desired finished state consists in causing the liquid metal to fill a mould, where it solidi fies as a solid casting which has the shape of the interior hollow of the mould. Although superficially simple, this process offers many difficulties. These are greatest where the metal melts at a very high temperature and in those cases where the liquid metal absorbs large volumes of gas which are liberated during solidifica tion.

A large proportion of the liquid metal produced throughout the world is required for future mechanical working, such as rolling or forging and is therefore cast into moulds of very simple shape.

The most striking example is the production of steel ingots, which range in size from a small rectangular block weighing a few pounds to huge tapered octagonal masses weighing over a hundred tons. Except in the case of crucible-melted steel, the liquid metal is first run from the refining furnace into a large "ladle"—generally a steel box open at the top and provided with a refractory lining.

These ladles are sometimes capable of holding considerably more than ioo tons of liquid steel and are moved about the furnace building by powerful overhead cranes. For filling they are brought in front of the furnace, from which the liquid steel is allowed to escape through a tap-hole at the base. From this hole the liquid steel runs along a gutter or "lander" lined with refractories and falls into the ladle as a cascade from which great showers of sparks arise. A photograph of this striking spectacle, taken at the proper moment, makes a picture at once impressive and inspiring. From the ladle the liquid steel is afterwaids allowed to flow into the cast-iron ingot moulds.

The solidification process is comparatively simple in a pure metal free from gas, but as steel is a complex alloy of iron with carbon and other elements, and contains dissolved gas as well as oxides, sulphides and silicates in suspension, the process becomes complex. In spite of an immense amount of laborious investiga tion the details of the solidification of steel are neither fully under stood nor entirely under control. The principal factors which play a part in the process of ingot solidification are briefly as follows : (a) Contraction.—The steel after solidification occupies a smaller volume than in the liquid state. Since the layer of metal next to the wall of the mould solidifies first, and successive layers of solid metal are formed on the solid crust thus initiated, the last liquid portions are left at or near the centre of the ingot. A

funnel-shaped contraction cavity or "pipe" is formed which may extend well down the centre of the ingot. This "pipe" must, for most purposes, be eliminated and a large part of the upper portion of the ingot must be cut off and discarded. This can be partly avoided by using containers, usually of fire-clay and known as "sinkhead" or "hot-tops" placed on top of the open end of the mould. The molten steel is allowed to fill these as well as the mould proper, and little or no solidification takes place in these "heads." They consequently provide a reservoir of liquid steel from which the contraction in the ingot proper can be made good. This method confines the pipe more or less to the "feeder head" of the ingot, and the amount which has to be cropped is much reduced.

(b) Gas

Liberation.—Liquid steel, like most liquid metals, is capable of dissolving large quantities of gas, especially hydrogen. Most of this gas is driven out of solution when the liquid metal crystallizes and then partly escapes to the surface. A considerable amount, however, fails to escape unaided and forms cavities or bubbles in the solidified metal. It is contended that in steel which is to be rolled or forged such gas cavities are harmless because their surfaces, when subsequently pressed together at a high tem perature during rolling or forging, unite by welding. Such welding, however, does not always occur; a great many of these cavities can never weld, because their walls become coated with impurities, such as silicates and sulphides, gathered from the adjacent steel during the pasty stage of solidification. These can be traced as long lines of "non-metallic enclosures" in the finished steel. The formation of gas cavities or "blow holes" in steel can be to a great extent regulated by the manner in which the oxidising processes in the steel furnace are conducted. If the reactions between oxide of iron and carbon are allowed to complete themselves so that the agitation or "boiling" of the steel in the furnace ceases, the pro duct is a "dead melted" steel in which gas cavities will be compara tively few. Such steel, however, shows marked "piping" as described above. Even if the process is stopped at a point where there is still some oxide of iron present in the bath, the steel can be "killed" by the addition of a relatively small quantity of alum inium. Steel killed in this way "pipes" like "dead melted" metal.