140 FOUNDRY WORK It is desirable to take test bars when making mixtures from new brands of pig or selected scrap. Thorough melting is essential, and more coke will have to be used than for the more fluid grey irons. The precautions to be observed in venting, gating, and pouring for sand moulds are required here. The Design of Moulds to Chill.—Only that portion of the mould which corresponds with the area to be chilled is of cast iron, the remainder being Z3£s&±yKtfr—i h\ i""r ' k'vt ' "i\ V«" V v'411 Fig. 48.—Chill Mould for Roller Fig. 49.—Chill Mould for Roll rammed in green or in dry sand. Success mainly depends on the mass of metal in the chill. It must be large, in order to enable it to carry off the heat from the casting poured, with sufficient rapidity to produce the necessary- depth of chill. If this action were delayed too long, what would happen is, that the cementite would have time to break up into iron and graphite, thus: Fe8C = jFe + C. Cementite or iron carbide, Fe3C, is unstable when cooled slowly. The walls of a chill therefore range from 4 in. to 8 in. in thickness, depending on its diameter. The risk attendant on thick walls is that of fracture, since the inner zones, expanding most, are tied by the outer.