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Full text of "Modern Mechanical Engineering Vol-I"

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

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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.