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104                       METALLURGY   OF   CAST   IRON.
speed is sufficient to fill the runner as desirable, a little knoll, as at A, Fig. 24, is generally formed in the c i iron runner,'' as shown. This causes a sufficient body of metal to collect and keep itself fluid until the flow is increased enough to overflow the knoll, by which time the chances are the flow will have increased to such a decree as to send a fair stream
down the iron runner. The iron in first flowing down the runner carries more or less slush of iron and dirt in the front of its stream. This will often pile up so as to require to be broken by means of a wooden pole in the hands of a man, as seen in Fig. 34. As soon as the metal has reached and filled the lower bed, a " cut-es pigs united together, and often much thicker in depth than usual. These are called " jump cores,'* and are formed by reason of the body of sand in the mold separating the pigs, being raised or pressed to one side by the action of too quick a flow, poor sand, or a little "boil." It has been no uncommon occurrence for metal to come so fast down the iron runner that it could not be controlled, and by reason of covering over a large area, cause a whole tap to go under the drop, or, worse still, require dynamite to break it up sufficiently small to be charged into the furnace, along with the ore, or sold for scrap metal to be re-melted in air furnaces or big cupolas.