Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/372

356 IKON To obtain just the right grain under any given circum stances requires an amount of special knowledge and experience of a peculiar kind only to be gained in the foundry itself, the mixture of brands that answer well for a certain kind of casting not being necessarily at all suit able for one of different size ; different mixtures, moreover, are apt to differ more or less in the amount of contraction taking place in solidifying and cooling, so that a somewhat different allowance for shrinkage must be made in different cases ; as a rule the moulds are made about 1 per cent, larger in each direction than the casting is intended to measure (one-eighth inch to the foot). 1 For complex forms, a number of different pieces are required to make up the whole mould, each piece being a metal box or " flask" containing sand mixed with charcoal powder, loam, or similar materials somewhat varying in their nature according to the char acter of the casting, moistened so as just to cohere together and enable the outline of the casting to be defined by the surface of the mixture. The " pattern " or model being placed in a suitable posi tion, the flasks are separately moulded to the variously shaped parts, and then set up (being bolted or otherwise fixed together) so as to enclose a space shaped precisely to the pattern ; the molten metal being then allowed to run into this space, by tapping the cupola and letting the fluid iron pass along a runner or gutter on the sand bed floor of the casting house, gradually fills it up, the scorise floating on the top ; the air which previously filled the mould passes away through orifices left for the purpose, along with steam and gases formed by the action of the hot metal on the materials of the mould. To ensure the casting being free from scoria?, and to give sufficient hydrostatic pressure to enable the impression to be sharp, the liquid metal is allowed to rise to some little height above the top of the casting by making the highest part of the cavity of the mould to be some inches or more below the surface of the floor, so that the metal fills up the tubular hollow above the mould thus formed, making aprojection ("gate " or "git") above the top of the casting ; this is ultimately detached by a hammer or chisel, as are also the ridges formed where the different flasks meet, and any similar protuberances at places where a little of the sand has fallen away from the surface of the mould, thus increasing the dimensions of the cavity at such places. More simple forms are cast in moulds prepared with a smaller number of flasks, two often sufficing, or for some purposes one only; machinery is employed for moulding such objects as gas pipes, railway chairs, &c. , required in large qiiantities. For certain pur poses metal moulds are used, or combinations of metal and sand moulds; owing to the greater conducting power, the iron in contact with the metallic parts of the mould is rapidly solidified ; a pecu liar hardness is thus communicated to the casting, which is then termed a " chilled " casting. For the best qualities of chills certain particular classes of pig are requisite ; thus for the cast iron car wheels used largely in America the brands of pig preferred are cer tain kinds of cold blast charcoal brown haematite or specular iron ; iron smelted from the pure magnetites of Lake Champlain does not chill in the required way so as to produce an outer film of white iron constituting the " tread " of the wheel, passing into a mottled iron with a soft grey inner centre, thus combining the maximum of strength with a hard wearing face. The peculiarity of some of the American cast iron in this respect enables machinery of certain kinds to be constructed in part of that kind of metal with a strength difficult if not impracticable to obtain with the same weight of metal from most British brands. In order to produce the chilling effect a cast iron ring is imbedded in the sand moxild so as to em brace the circumference of the wheel to be cast ; the metal is con sequently rapidly solidified in contact with the iron ring, the " chill," or portion solidified as white iron, penetrating inwards some 75 inch. It is found that confining the chill to the middle portion of the outer circumference only, and not communicating it to the entire flange and the opposite outer portion of the external surface, gives greater strength without diminishing the resistance to wear; this is effected by narrowing the cast iron chilling ring. Casting under Pressure. During the solidification of iron, and especially of steel, after running into moulds, bubbles of gas are often extruded, causing the substances to become vesicular and honeycombed, especially at the upper portion ; the gas thus evolved from Bessemer metal was found by Bessemer and Henry to be almost entirely 1 Wrightson has recently made some interesting observations on the variation in the volume occupied by a given mass of molten cast iron during solidification and subsequent cooling; see Journal I. and S. Inst., 1879, 418, and 1880. carbon oxide; 2 on casting in a mould from which the air was rapidly pumped out, ebullition was set up, owing tu the rapid evolution of gas, just as ordinary spring water apparently boils under the exhausted receiver of an air pump. Three methods of overcoming this practical diffi culty of vesicular structure being set tip have been used. The first, or "dead melting," applied to cast crucible steel, is simply to allow the crucibles to stand for some time in the furnace with the molten metal in them before casting ; in this way the iron oxide disseminated through the mass is acted upon by the carbon, and this source of gases eliminated ; according to Bessemer, silicon is also introduced (by the action of the metal on the pot-material), the presence of which retards or stops altogether the gas evolution (see 33). The second method is the application of this principle in a more direct way by adding silicious pig, preferably as silicious spiegeleisen (silico-manganeisen), to the fused metal ; this process has been long used on the Continent in one form or another by steel-makers who have attained a high reputation for their cast steel pro ducts, e.g., in Krupp s works, at Terre Noire, &c. The third method is one which prevents the formation of bubbles of gas by mechanical means, the fluid metal being- subjected to powerful compression during its solidification. In 1856 a patent for this was taken out by Bessemer, the mould being closed by a strong cover and hydraulic power employed to force inwards a stout wrought iron plunger ; Whitworth s system of casting "compressed steel " is a more perfect development of this notion. Other methods of arriving at the same end have been also suggested, the pressure being derived either from the admission of high pressure steam on the top of the ingot mould, or by the gene ration of gases by the heat of the metal introduced on some chemi cal composition in the upper part of the closed mould ; thus by employing a mixture of nitre and coal dust, the mould being closed by a screw plug and strongly bound round to strengthen it, a high pressure can be exerted, regulated by varying the amount of mix ture used in the first instance. A description of the steam process as employed at the Edgar Thomson Works, Pittsburg, is given by H. R. Jones in the Journal I. and S. Inst., 1879, 477 ; it appears to be simple and inexpensive as compared with the Whitworth hydraulic arrangements, but generates far less pressure, 3 Whitworth steel being compressed by forces up to 6 to 9 tons per square inch ; increasing the pressure up to 20 tons produces little or no further effect, but, ceeteris paribus, the ductility of the compressed metal increases with the pressure used when below some 6 tons per square inch. According to Euverte, no practically valuable results were obtained at Terre Noire nor at St Etienne by the application of intense pressure to the open-hearth steels there made, indicating probably that the removal of gas bubbles effected by Whitworth s operation in crucible steel is accomplished chemically in soft so-called steels of low carbonization made with silico-manganeisen. The modus opcraiidi of pressure in consolidating steel during casting appears from Bessemer s observations as to the more copious evolution of gas on diminishing pressure to be simply the appli cation of the well-known law of increased solubility of gas in any given medium with increased pressure; under high pressure the fluid metal retains the gas dissolved just as soda water does the carbon dioxide whilst it remains in bottle; but, just as effervescence is pro duced in this latter case as soon as the pressure is relieved, so in the case of steel would gas bubbles appear under the ordinary pressure which would not have been developed under high pressure. It is, however, somewhat difficult to understand why steel should retain hydrogen and carbon oxide dissolved when at a high tem perature and perfectly fluid, and should extrude it on cooling some what and when just on the point of solidifying ; but the pheno menon is not an isolated one, fused silver behaving in precisely the same way towards oxygen, and thus giving rise to the well-known " spitting" of silver during solidification. The function of silicon in preventing the extrusion of gas may be ascribed either to its com municating the physical peculiarity to the steel of dissolving as much 2 Miiller has recently found hydrogen to be the chief constituent of the gases contained in the bubbles found in ordinary solid cast steels, along with nitrogen, and much smaller quantities of carbon oxide than those found by Henry. 8 Recent experiments made in England with Bessemer ingots have indicated that no appreciable diminution in honeycombing is brought about by the use of steam at only 40 or 50 Ib pressure per square inch.