282 METALLURGY OF CAST IRON.
bon, sulphur, phosphorus, and manganese in the iron. The more total carbon the less silicon required, on account of carbon softening* iron, as can be seen by a study of Chapter XXXIII. The following Table 57 gives an approximate idea of the highest silicon contents it is generally wise to have in soft or light castings, in combination with the total carbon; the.other elements, sulphur, manganese, and phosphorus being fairly constant at the respective percentages considered best for making soft castings:
Silicon ........ ....... Total Carbon ........ 3-75 3.00 3-70 3-25 3-65 3-50 3.60 3-75 3-55 4.00 3.50 4-25
The percentage of sulphur, manganese, and phosphorus generally found in light castings is, as a rule, .06 to .08 sulphur, .40 to i.oo manganese, arid .50 to 1.25 phosphorus. It will be readily understood, from a study of Chapters XXIX. to XXXII., that an increase of sulphur and manganese hardens iron, while phosphorus increases fluidity and brittleness, and that for thin or light castings requiring very fluid metal high phosphorus is necessary. As iron for light castings must generally be soft, care should be taken not to let the sulphur and manganese exceed the above amounts in castings. To obtain these percentages in castings it will, of course, be necessary to have less sulphur and higher manganese in the mixtures before being charged, as is explained in Chapter XLV.
The same regular analyses in different mixtures of irons may not give like softness in castings. This may be due to the quality described on pages 161 and 261, or to some brands of iron possessing more of a.50 to 3.00