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244                       METALLURGY   OF   CAST   IRON.
heats Nos. 7 and 9, we find that manganese in the ladle is very effective in softening the iron, or very sensitive in producing radical changes.
The effect of manganese on the strength of cast iron has a tendency, as a rule, to make iron stronger. In adding manganese to molten metal, the iron should never be dull, but as hot as practicable, in order that all the manganese may be melted in such a manner that a homogeneous mixture may result. Where iron is dull, a fracture may often show little bright spots or grains of manganese alloy that did not melt and mix properly with the iron. In such cases more harm is done than good. A study of the tests shows that the best results for strength were dependent upon certain percentages of increase. Anything above or below this was injurious. The increase of manganese in the molten metal ranged from 25 to 60 per cent. The effect of adding manganese to molten metal on the other elements shows an increase in the silicon and decrease in the sulphur, with phosphorus remaining fairly constant. With the manganese in the cupola, the silicon, sulphur, and phosphorus are decreased. The complete Table 36 of analyses affords one excellent material for study and information on. these points.
One peculiarity noticed, in making these tests, was seen in the high manganese of tests Nos. 2 and 6 causing the sand to peel most freely from the castings and leaving a skin covered with flakes of graphite, whereas, with the same iron free from the ferro-manganese mixture the sand stuck strongly to the casting. All the bars poured with the iron .having manganese added in the cupola showed this effect to a greater or less degree. No doubt this is the cause of some castingsos. 4, 5, and 6, However, when we get to low silicon irons, as to.n. in ladle	1,772 Ibs.	.100"	-326 "	I.IOO	.242	3