CASTINGS MADE IN METALLIC MOULDS 139 CHAPTER VI Castings made in Metallic Moulds Castings made in metallic moulds are embraced in three groups: (a) chill casting, (£) die-casting, (c) casting in permanent moulds. These have nothing in common beyond the fact that cast iron forms the whole or a portion of the moulds. The conditions which control the pouring of liquid metal into moulds of porous sand and those of iron are so different that the foundries using metallic moulds are entirely separated from the sand foundries. Chill Casting.—The fact is familiar that the effect of pouring liquid metal in contact with a cold metallic surface is to harden—"chill"—the por- tion that comes in immediate proximity with it. This is utilized in portions •'•$ yjiiJLii "'•"•'•'••• ']l'}&>&*$\'i?'*': Fig. 47.—Chill Mould for Trolley Wheel of numerous castings that are subjected to severe wear, as the treads of trolly wheels of all kinds (figs. 47 and 48), in the rolls (fig. 49) for the iron and steel works, for plough points, for mining stamps, stone breakers, balls and rollers used in crushing and grinding mills, the bores of some wheel boxes, &c. In all these cases the mould is of a composite character, being composed of metal over the areas that have to be chilled, and of sand elsewhere. Composition of the Metal to Chill.—The grey iron used for ordinary cast- ings, in which the carbon is nearly all in the graphitic condition, will not chill beyond a surface hardness of the thickness of stout paper. This is of no value for service. An average thickness is generally required of from -J in. to | in., extended in massive articles to i in. To produce this, it is necessary to select a highly mottled iron, in other words, one in which a considerable proportion of the carbon is in the combined condition, and the total carbon Content high. And as silicon tends to throw out carbon in solution into the graphitic state, the proportion of this element must be kept low. Sulphur and phosphorus should be higher than for grey iron castings, since they intensify the chilling effect. Manganese, below one per cent, is beneficial.