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METALLURGY OF CAST IRON.
to be machined. In castings not requiring a finish, such a mixture may be used in castings as thin as ^ of an inch and still be soft enough to permit being chipped in the cleaning.
The general run of castings ranging from }4 to 4 inches in thickness, that require to be sufficiently soft to be machined and possess similar strength per square inch, may often range in analysis of mixtures as seen in the approximate Table 56. It is understood that these analyses include pig iron and scrap mixed, or pig alone, as either mixture would stand ready for charging. It is not to be expected that the sulphur, manganese, phosphorus, and total carbon can be obtained in keeping with the increase of silicon shown. However, should the sulphur or manganese be increased from that shown in the Table, the silicon should be increased in such a proportion as to maintain a hardness similar to that obtainable by the analyses shown.
Should the total carbon be higher than shown for the larger thickness then the silicon would require to be proportionately lower to maintain similar strengths or hardness. It is to be remembered that as a rule the total carbon comes highest in low silicon irons, which is the reverse of the order shown for carbon in Table 56, see chapter XXXIII, page 247.
TABLE 56.—APPROXIMATE ANALYSES OF COKE IRON MIXTURES.
Thickness of Casting. Silicon. Sulphur. Manganese. Phosphorus. Total Carbon.
W 2-75 .02 •30 .70 3.75 to 4.00
i» 2.50 .02 .30 •<>5 3.50 to 3.75
i%" 2.25 .02 .40 .60 3.25 to 3.50
2" 2.0O •°3 .40 -55 3.00 to 3.25
-zW 1-75 •°3 .50 -50 2.75 to 3.00
3" 1.50 .03 •50 -45 2.50 to 3.00
zW 1.25 .04 .60 .40 2.50 to 3.00
4" I.OO .04 .70 •35 2.50 to 3.00