Page:The American Cyclopædia (1879) Volume IX.djvu/384

 370 IRON difference in the two varieties. A more cor- rect statement of the fact would perhaps be, that the color 'of the pig metal, or in other words the amount of graphite separated, is due, other things being equal, to the time or rate of cooling. White iron caused to solidify very slowly becomes gray ; and gray iron cooled quickly becomes white. The cause of this dif- ference is conceivable on one supposition only, viz. : that there is a limited range of tempera- ture, probably near the point of solidification of the metal, within which the separation of the carbon from the iron takes place, and that the amount of carbon separated in any given in- stance is proportional to the time consumed by the cooling pig iron in passing through this range of temperature. It is evident tljat the time required for the metal to cool a given number of degrees, near its point of solidi- fication, must depend partly upon the tempera- ture of the surrounding moulds. The hotter the metal leaving the furnace, the more will it have heated the moulds as it approaches solidi- fication, and consequently the slower will have become the rate of cooling, the longer will be the period during which carbon can separate, and the larger will be the amount of graphi- tic carbon in the final product. Carbon, as already remarked, increases the fusibility of iron. In chemical combination it renders iron brittle, the brittleness decreasing in proportion as the carbon separates as graphite. Silicon is nearly always present in pig iron. White iron is occasionally almost free from it, but the darker sorts may contain as much as 8 per cent. ; J to 3 per cent, is usual. The con- ditions favoring the production of a highly siliconized pig iron are slow working, a high temperature in the furnace, and a cinder rich in silica. Silicon, like carbon, renders iron more fusible. The temperature of solidifica- tion of pig iron rich in silicon is therefore relatively low ; and this fact, combined with the high temperature of production, affords ample opportunity for the carbon to separate as graphite. We consequently find such pig iron always highly graphitic, and very difficult to chill, or convert into white iron by sudden cooling. In many articles made of cast iron, such as rolls, car wheels, &c., it is desired to combine toughness of structure with a hard wearing surface. This is effected by casting the object in a suitable mould of iron, so that the molten iron shall be suddenly solidified on the outside, and rendered white to a moderate depth, while the mass of the casting remains gray. The casting is subsequently annealed, to relieve the tension caused by the unequal cooling. The irons most suitable for this pur- pose are produced with charcoal and with a cold or but moderately heated blast, and are of exceptional purity. Any considerable amount of silicon prevents the iron from chilling. Sili- con renders iron brittle and weak. When pres- ent in very large quantity it makes the iron worthless both for castings and for conversion into wrought iron. It plays an important part in the pig iron employed for making Bessemer steel, supplying by its oxidation the greater part of the heat required to retain the metal in a molten condition. The amount of silicon in I Bessemer pig iron varies from 1 to 3 per cent. ' Silvery or glazy pig iron, occasionally produced when the furnace is working very hot with an excess of fuel, is white, but has none of the properties of white iron properly so called, and is weak and worthless for all purposes. It has not been thoroughly investigated. An analysis (No. 19) given above shows it to contain over 5 per cent, of silicon. Sulphur is present in many ores of iron and in almost all mineral coals. The hotter the furnace and the more basic the cinder, the more sulphur will be re- moved in the cinder. Where the opposite con- ditions exist a large part of the sulphur in the charge will be found in the pig iron. The in- fluence it exerts on pig iron has not been de- termined with precision. According to Eg- gertz, 0'4 per cent, of sulphur renders pig iron stronger and more mottled. Swedish cannon iron contains from 0'07 to 0-1 per cent, of sul- phur. It is the general impression among iron founders that sulphur renders pig iron harder, whiter, and more infusible ; but experimental proof is wanting on this point. Phosphorus is almost always present in cast iron. Few iron ores or limestones are absolutely free from phosphorus ; and almost the entire amount of this element present in the charge is absorbed by the iron, which it renders thinly liquid when fused, and crystalline and hard when solid. Such iron is well adapted to form orna- mental and intricate castings, since it fills the mould well and brings out the fine outlines with sharpness. Less than - 5 per cent, of phosphorus does not materially affect the physi- cal properties of pig iron ; and more than 5 - 5 per cent, renders it too weak and brittle to be used. The following table shows the amount of sulphur and phosphorus in some well known brands of English pig : BRANDS. Phosphorus, jer cent. Sulphur, per cent. Wbitebaven (hematite) 0-144 0-056 South Wales 0-478 0-098 South Staffordshire 0-480 0-061 0-540 0-082 Scotland 0-780 0-288 0-S6S 0-045 1-070 0-040 1-148 0-267 1-820 0-08S In the conversion of pig into wrought iron by puddling, the phosphorus and sulphur are to a considerable extent eliminated. In the conversion into Bessemer steel, however, these elements remain with the iron, and therefore only the purest pig irons can be used in this process. The maximum amount of phosphorus (the most deleterious element) that Bessemer pig iron may contain is O'l per cent. Manga- nese resembles iron in many of its chemical