The author introduced his new mode of treating ingots at the Darlington Steel and Iron Company's Works, in Darlington, early in June this year, and they are now blooming the whole of their make, about 125 tons a shift, or about 300 ingots every twelve hours, by such means.
The machinery at Darlington is not adapted for rolling off in one heat; nevertheless they have rolled off direct from the ingot treated in the "soaking pits" a considerable number of double-head rails; and the experience so gained proves conclusively that with proper machinery there will be no difficulty in doing so regularly. The quality of the rails so rolled off has been everything that could be desired; and as many of the defects in rails originate in the heating furnace, the author ventures to predict that even in this respect the new process will stand the test.
Many eminently practical men have witnessed the operation at Darlington, and they one and all have expressed their great surprise at the result, and at the simple and original means by which it is accomplished.
The process is in course of adoption in several works, both in England and abroad, and the author hopes that by the time this paper is being read, there may be some who will from personal experience be able to testify to the practicability and economy of the process, which is carried out in the manner now to be described.
A number of upright pits (the number, say, of the ingots in a cast) are built in a mass of brickwork sunk in the ground below the level of the floor, such pits in cross-section being made slightly larger than that of the ingot, just enough to allow for any fins at the bottom, and somewhat deeper than the longest ingot likely to be used. In practice the cross section of the pit is made about 3 in. larger than the large end of the ingot, and the top of the ingot may be anything from 6 in. to 18 in. below the top of the pit. These pits are commanded by an ingot crane, by preference so placed in relation to the blooming mill that the crane also commands the live rollers of the mill.
Each pit is covered with a separate lid at the floor level, and after having been well dried and brought to a red heat by the insertion of hot ingots, they are ready for operation.
As soon as the ingots are stripped (and they should be stripped as early as practicable), they are transferred one by one, and placed separately by means of the crane into these previously heated pits (which the author calls "soaking pits") and forthwith covered over with the lid, which practically excludes the air. In these pits, thus covered, the ingots are allowed to stand and soak; that is, the excessive molten heat of the interior, and any additional heat rendered sensible during complete solidification, but which was latent at the time of placing the ingots into the pit, becomes uniformly distributed, or nearly so, throughout the metallic mass. No, or comparatively little, heat being able to escape, as the ingot is surrounded by brick walls as hot as itself, it follows that the surface heat of the ingot is greatly increased; and after the space of from twenty to thirty minutes, according to circumstances, the ingot is lifted out of the pit apparently much hotter than it went in, and is now swung round to the rolls, by means of the crane, in a perfect state of heat for rolling, with this additional advantage to the mill over an ingot heated in an ordinary furnace from a comparatively cold, that it is always certain to be at least as hot in the center as it is on the surface.
Fig. 2
Every ingot, when cast, contains within itself a considerably larger store of heat than is necessary for the rolling operation. Some of this heat is, of course, lost by passing into the mould, some is lost by radiation before the ingot enters into the soaking pit, and some is lost after it enters, by being conducted away by the brickwork; but in the ordinary course of working, when there is no undue loss of time in transferring the ingots, after allowing for this loss, there remains a surplus, which goes into the brickwork of the soaking pits, so that this surplus of heat from successive ingots tends continually to keep the pits at the intense heat of the ingot itself. Thus, occasionally it happens that inadvertently an ingot is delayed so long on its way to the pit as to arrive there somewhat short of heat, its temperature will be raised by heat from the walls of the pit itself; the refractory mass wherein the pit is formed, in fact, acting as an accumulator of heat, giving and taking heat as required to carry on the operation in a continuous and practical manner.