INGOTS Gases in Liquid Metal.—Early attempts to make steel in England by Bessemer's process were failures because the metal when poured into ingot moulds boiled over and the remaining portion when solid would be full of blow-holes, like
Swisscheese. Robert Mushet noted that the only apparent difference between sound ingots and the spongy ones was that the former were made of high-manganese pig iron and a little manganese remained in the steel; consequently he proposed adding some of the metal after the blow. Thus originated the practice already mentioned of adding spiegel or ferro-manganese to heats of steel made by any of the Bessemer or open-hearth modifications. Superheated liquid steel absorbs gases from the atmosphere or oxygen from the slag. This gas is only slowly evolved as the steel cools off, but it boils out vigorously just at the freezing point. In the crucible and acid open-hearth process much of this oxygen is eliminated by holding the melted metal quietly in the furnace until reactions between the siliceous linings and the metal allow the latter to absorb some silicon which in turn is very effective in removing dissolved or combined oxygen existing in the steel, and causing it to solidify quietly without effervescing, which imparts superior physical properties. This time-consuming reac tion is avoided when producing cheaper steels in great ton nages by adding the degasifyers to the steel in the ladle, or even as it is poured into the ingot. Melted spiegel-eisen is used for medium carbon Bessemer steels. Since spiegel ordinarily contains 20% manganese and 5% carbon, it will be observed that suf ficient manganese to quiet the steel will also introduce consider able carbon. Should this be too much for the desired grade of steel, ferro-manganese is used instead. Since a relatively small amount of this is required small pieces are placed in the ladle, and melted by the inrushing steel ; if the metal on casting still seems too effervescent, a few aluminium shot are thrown into each ingot. The principal effect of each of these metals is to rob harmful iron oxide existing in the steel of its oxygen, thereby transferring it to MnO,
or
as the case may be. These oxides are highly insoluble in metal, and tend to accumulate into slag particles and float; but there is little opportunity for elimination when the reactions occur in the ladle or ingot mould. Entrapment of submicroscopic inclusions is not an unmitigated evil ; under proper control the effect is to restrict crystallization of the steel during solidification and in later heat treatment, and such fine grained steel has superior toughness and most other physical properties (except depth of hardening).
It is usual practice to transfer steel from converter or open-hearth to ladle somewhat too hot to pour; also to use a ladle of correct size so most of the slag overflows. Ladles, therefore, contain from 5 to 200 tons of steel, depending on the furnace served (see fig. 4). They are strong steel pots lined with fire brick, and equipped with trunn ions and shackles for crane transport. The hot metal is kept in the ladle sometimes as much as half an hour, to en able all trapped slag particles to float. Then the ladle is carried to the ingot moulds, and the metal teemed through a hole in the bottom. When one mould is filled, the metal stream is interrupted by lowering the stopper until the ladle may be moved to an empty mould. Splashing against the sides of the moulds must be avoided, for such splashings are the most gen eral cause of surface defects on the finished articles.
Steel castings are ordinarily teemed in the same way; furnaces in steel foundries are usually very small as compared with furnaces serving rolling mills. 97%
of all the steel made is cast into ingots.
Well-killed steel contracts in volume when it changes from liquid to solid ; consequently a mould poured level full will have a top cavity, or pipe, when solid. Good practice so proportions the chilling effect of the mould, the temperature of the steel and the rate of pouring that the ingot solidifies from the bottom up. A hot-top in the form of a firebrick collar, set on top of the mould, as shown, is a most useful device for keeping the top portion of the metal fluid until the last, and in condition to feed and fill the shrinkage cavity below (see fig. 4). A certain segregation or concentration of alloying elements also occurs toward the pipe; consequently all the defective top must be cropped as indicated and remelted as scrap. Segregation and piping is also minimized by casting ingots big-end-up and by slow pouring of rather cool metal. The latter is well ac complished by having several ingot-moulds on one stool, teeming into the central one, and allowing metal to rise in the others through connecting passages in the common base. Enough gas is left in most low carbon tonnage steels so a slight effervescence occurs in the mould, just enough to counteract the piping ten dency. When properly made and cast, such ingots will have a flat top, a smooth solid surface, but a number of deepseated small blow holes. During subsequent rolling these are squeezed shut and the metal welds together more or less effectively. Metal of this sort is called "rimming" steel; the dense surface of the metallic ingot is an ideal start for thin sheet, which is almost all surface.
Size of ingots depends upon the use to which the metal is to be put. Ingots of alloy steel and high grade tool steel sometimes weigh no more than ioo lb. Forging ingots are from two to five times the thickness of the biggest part of the resulting forging; those for rails, plates and structural shapes are adjusted to the size and capacity of rolling mill, and the cross section of the desired product ; for rolling mills they are generally square in cross section, with rounded corners, and about three times as high as thick. A common size is 2x2x6 'ft. weighing about 6 tons. Forging ingots are frequently fluted hexagons or octagons in cross section. The moulds are heavy walled iron castings, made of low-phosphorus low-sulphur pig iron, such as is suitable for the acid Bessemer process, sprayed with lime wash or other coating. When the metal in the ingot has well solidified, the train of moulds is moved to a strip per, an overhead crane with a vertical plunger which can be forced down by suitable gearing on top of the ingot, holding it down against the stool. A pair of stirrups then loop over the ears on either side of the mould, and lift; if the inner surface of the mould is in good condition and smooth, no difficulty is had in thus removing the mould, and it is then placed on an empty car alongside. At this time the surface of the ingot is at a full red heat, yet the interior may still be white hot. To equalize this heat the ingot is placed in a soaking pit, a square fire brick chamber large enough to contain two to six ingots standing on end ; an arched cover rolls aside for entrance. Aux iliary heat is supplied as necessary by a gas-air flame, operating through regenerative chequer-work, ports and auxiliary control as described for open-hearth furnaces. Pit capacity for at least Ioo ingots is required to maintain a steady supply of properly heated metal to a modern rolling mill.