Ore Treatment

If, on the contrary, the steel is brought into the ingot mould while still actively generating gas—so-called "wild" steel—this pro cess continues to some extent during solidification. A good deal of the gas there liberated is trapped in the solidifying steel which, consequently, contains a large number of "blow holes." The formation of these holes counteracts the effects of the normal con traction of the steel, and the steel does not appear to shrink while solidifying—it may, indeed, appear to rise in the mould. Such steel is called "rising" or "rimming" steel. It is also—perhaps more frankly—described as "unsound" steel. It has, from the point of view of mass-production, the advantage that as there is no definite "pipe" almost the whole of the ingot can be rolled, thus affording a larger yield of finished steel.

(c) Segregation.

As steel is a complex substance and the rate of solidification of any but the smallest ingot is necessarily slow the constituents have time to undergo separation or "segre gation" which results in non-uniformity of chemical composition in different parts of the ingot. This subject cannot be adequately discussed in a general article but it is important to mention it because segregation plays a very important part in affecting the quality of large steel ingots. It is, of course, a natural process which, under conditions of slow undisturbed cooling in large masses, cannot be prevented except, possibly to some extent, by a careful choice of the composition of the steel itself. At the same time it has to be regarded to some extent as imposing a limitation on the size of the ingot, and consequently of the forgings or plates, which it is desirable to use where high and uniform quality is essential.

(d) Contraction Stresses.

Since the outer portions of an ingot necessarily solidify while the inner portions are still liquid, the latter—on subsequent solidification and contraction—exert a powerful inward pull on the outer portions. Masses of large size are therefore apt to be under severe internal stress and may, sometimes, crack spontaneously or when placed in a re-heating furnace. The formation of such cracks or "clinks" can be largely avoided by correct design of the ingot moulds. Part of the diffi culty is also avoided by the practice—adopted also with a view to economy—of drawing the ingots from the moulds while the steel is still very hot, so that the interior may still be partially liquid, and placing them in so-called "soaking pits"—a kind of pit furnace in which the temperature throughout the ingot has the opportunity to become equalized and the steel is, moreover, kept at a tempera ture suitable for forging or rough rolling.

The production of ingots or slabs of the non-ferrous metals and alloys, while subject to the same general laws and phenomena as steel, differs in regard to the conditions which have to be met. Non-ferrous ingots are never very large so that many of the prob lems which arise from large size, very slow cooling and extensive segregation, do not enter into the corresponding non-ferrous prob lem. On the other hand the non-ferrous ingot has frequently to meet special conditions. The quality of the surface of the cast slab is often important. In steel, so long as there are no cracks or foldings, smoothness of the surface is not essential. Brass, on the contrary, has frequently to be rolled out into a finished product with a smooth surface, so that the nature of the ingot or slab surface is very important. In the same way, blow-holes lying just under the ingot skin, which are often regarded as normal in steel, are not permissible in non-ferrous metal which is to be rolled into strip or sheet, since these cavities tend to open out during the annealing process following cold-rolling, and to cause defects known as "spill" and "blister." Much more attention is therefore paid in non-ferrous ingot casting to the details of mould surface, dressing, etc., and also to the method of filling the moulds. In steel practice this is very frequently done by allowing the fluid metal to run into the mould from an opening in the bottom of a large ladle. In non-ferrous practice, where the masses to be han dled are much smaller, the molten metal is often poured direct from the crucible, or even from the furnace, pouring being done "over the lip" of the crucible and into the open top of the mould. For certain special purposes a process has recently been developed in France (the Durville process) in which the molten metal is first poured into a ladle or receptacle which is connected by a channel or short "runner" with a number of parallel small ingot or bar moulds. Ladle, runner and moulds are mounted together on trun nions about which the whole assembly is slowly rotated so that the ladle rises and the moulds are lowered. The metal then flows stead ily from ladle to moulds. The resulting bars have remarkably per fect surfaces, but the process is readily applicable only to copper alloys containing a small amount of aluminium. Many other pro cesses for filling ingot moulds have been developed, some working by gravity with or without the interposition of funnels or separators of various kinds, others by pressure applied to the liquid metal that forces it into the mould.