Metallurgy Of Cast Iron

CHAPTER XL.
MIXTURES FOR LIGHT MACHINERY AND STOVE PLATE CASTINGS.
flixtures for light machinery, sewing machines, stove plate, hollow ware, and hardware, etc., castings call for very soft grades of iron. In making such castings it is rarely wise to use any other iron than pig and shop scrap. As a rule there is much more shop scrap obtained from making light work castings than from heavy ones. In light work the shop scrap generally ranges from 25 to 40 per cent, of the weight necessary to be charged for a heat. As melting iron hardens it, there must of necessity be sufficient silicon added every heat to restore the scrap to the mixture's original softness. For this reason light work shops generally find that their own shop scrap is all they can wisely use.
The percentage of silicon in light work mixtures, as they stand ready for charging — which includes an average of the silicon in the pig and shop scrap — may range from 3.00 to 3.80. This would give a silicon in the castings resulting from the mixture of such pig and shop scrap of from 2.70 to 3.50, according to the grade of softness desired in the castings. When the silicon exceeds 3.75 in castings the body or surface may be often found harder than with lower silicon. This is much affected by the percentages of total car-I, page 247.