Steel Castings
The chief desideratum in melting steel in an open-hearth furnace when the metal is intended for castings is to melt as hot as fuel will make it and keep oxidization as low as possible in the bath, by admitting only just enough air to insure thorough combustion. The charge usually consists of pig iron containing about 8 per cent of manganese. Spiegeleisen containing perhaps 14 per cent is sometimes used, along with enough of other pig free from manganese to procure that percentage in the resultant mixture. The quality of steel required determines subsequent operations for softening, refining, etc., which is accomplished by additions to the bath of such materials as will favor the change desired. Various kinds of scrap, blooms, etc., are introduced in a semi-molten condition, so as to prevent any sudden reduction of temperature in the bath. Tests are made by dipping and pouring a small ingot which while red-hot is hammered down to a thin plate, and by its resistance to flexure, etc., indicates the condition of the metal and its fitness for casting. In some steel-foundries this knowledge is obtained by cooling the test-piece in water and breaking it on an anvil. When the metal is found to be sufficiently soft and pure, the final ingredients for converting it into steel, consisting generally of a specially manufactured pig iron containing manganese and silicon (silico-spiegel), is introduced, along with more or less ferro-manganese. The whole is then stirred well with a rabble, and if found correct is ready for tapping.
Many small castings are sold for steel that have been simply cast from good white pig iron very low in phosphorus and silicon, and afterwards annealed in hematite ore or smithy scales, after the manner of malleable-iron castings. See MALLEABLE-IRON CASTINGS.
An opinion that gains favor rapidly is that the Bessemer process of manufacturing steel will be ultimately recognized as superior to the open-hearth. Even now, owing to the fact that hot metal of any desired mixture may be obtained from the converter more frequently throughout the day than is possible in the open-hearth, which is limited to about three heats a day, many firms are working a small Bessemer converter for the lighter class of castings.
As steel castings must be poured from the bottom of the ladle by means of a stopper, just as ingots are filled (see INGOTS), there is practically no difficulty in delivering the metal clean, as the slag is all held on the surface above; but, owing to the liability of leakage when the stopper is damaged, it requires considerable dexterity to fill a number of small moulds successfully by this means. The larger ones are simple enough. If all the runner system cannot be contained within the flask, and the mould must necessarily be rammed in the pit, runner-cores made specially from very refractory materials are set against the gate apertures and continued one upon another up to the surface, where suitable arrangements are made for receiving the stream from the bottom of the ladle. By this means the molten steel is prevented from encountering the non-refractory materials composing the pit-sand and runner as well as casting is contained in a dry-sand mould. Hot as it may appear when melted, steel is by no means as fluid as cast iron, for which reason the runners and gates must always be made proportionately larger; but, like cast iron, it is always easier on the mould when the metal enters from the bottom. Of course it is necessary in very large castings, whether cast vertical or slanting, that additional runners be placed near the top also.
Risers on steel castings should be markedly heavy in proportion, and preference should be given to a position that will favor an equal distribution of the liquid pressure exerted; yet the highest heavy portions of the casting are properly chosen as a rule. An important feature in these heavy risers is to give them taper sufficient to favor an easy withdrawal from the hard sand when contraction commences; otherwise they are liable to draw the casting apart.
To discover a facing that would successfully resist the intense heat of molten steel and produce a smooth casting like cast iron has been the aim of very many who have engaged in this business. Very naturally such substances as were commonly employed for furnace construction, melting-crucibles, etc., were among the first employed. Pulverized fire-brick, with some clay and a wash over with brick-dust and water, was one of the earliest facings used. Moulds made from sands or other mixtures that are stiffened with flour are apt to crumble away at the slightest touch if the heat applied to dry them has been sufficient to burn the flour; and this is why molasses, which makes equally as good a bond if intimately ground into the sand, is now preferred. A ferric clay found in Switzerland, containing oxide of iron 40 per cent, graphite 2 per cent, is employed as a facing in some foundries. It dries exceedingly hard, will take about one third silica sand, and makes good moulds and cores and moulds for light work. It may be mixed with water for loam. The chief objection to its use is its extreme hardness, which compels the use of softer material wherever contraction is likely to be intercepted. In some parts coke, old fire-bricks, crucibles, the artificial graphite from gas-retorts, and many substances of a like nature, constitute the chief materials used for mixing with the sand employed for facing.
One of the best facings for ordinary steel castings is simple silica sand (the purer the better) and molasses, brought to the proper consistency by grinding well together.
To obtain a core that will meet every requirement in very large castings is still an unsolved problem; for if a mixture be made with the softer material, to favor easy extraction, it yields to heat and pressure, and invariably forms a mixed mass of steel and sand, while the harder and more refractory material bakes almost solid. Ordinarily the silica sand mixed with coal-tar or molasses is the best. The artificial hardness imparted by these ingredients is dissipated by the intense heat, the sand is again friable, and it falls out. Shrinkage in steel amounts to more than double that of good cast iron--from 3/16 to 5/16 in a foot. This, in conjunction with the more rapid cooling compared with cast iron, necessitates a plentiful use of ashes, or their equivalent, in both cores and moulds, at such places as would be likely to interfere with a free and uninterrupted contraction of the whole. If liberty cannot be obtained by this provision, then the hard mould must be weakened by digging away the sand in the immediate vicinity of whatever is being obstructed, or perhaps lifting the casting out of the flask entirely and covering it with sand or hot ashes. Sometimes this is done, and the casting, instead of being covered, is at once consigned to the annealing oven. Castings that are well proportioned seldom betray the slightest sign of cracking when the latter method is adopted.
In extreme cases, where joining portions are liable to be separated through a more than ordinarily rapid contraction, the parts apt to yield may be thrust forward by means of a set-screw in the side of the flask, acting upon a plate previously set in the mould for that purpose.
Various means are employed to hold straggling parts of steel castings to the main body. There being little or no fibre to the metal, they snap off short at the least provocation; every sharp angle in the mould is a source of danger on this account. It is customary, therefore, to connect each of these weak extending parts to the body of the casting by a bracket or brackets, which are subsequently removed; angles of every description are eradicated by strict attention to filleting, or rounding every corner.
Cast-iron chills also serve a good purpose on these castings, for, besides imparting a smooth surface, without chill, wherever they are placed, they at once absorb the heat, congeal the surface, and thus in a very simple manner prevent contraction fractures in many instances.
Fins cut at certain parts will sometimes induce almost immediately congelation, and prevent rupture by the added strength of the comparatively cold metal, which extends its strengthening influence, more or less, into the casting by reason of the prematurely local shrinkage created. Not unfrequently parts that are dissimilar in magnitude are drawn apart by the antecedent shrinkage of the thinner body of metal. This too may often be prevented by attaching a thin connecting web or webs, extending some distance beyond the point of junction, either way; the web being thinner, sets more rapidly than either of the other divisions, and holds them together by reason of a prior contraction.