A FEW OF THE CAST IRONS ════════════════════════╤════════╤═════════╤════════╤═══════╤══════════ │Silicon,│Graphite,│Combined│ Total │ │Per Cent│Per Cent │Carbon, │Carbon,│ │ │ │Per Cent│ Per │ │ │ │ │ Cent │ ────────────────────────┼────────┼─────────┼────────┼───────┼────────── No. 14. White Cast Iron│ .70│ .10│ 2.65│ 2.75│Very Hard No. 15. Annealed │ .70│ 2.70│ .05│ 2.75│Machinable Malleable Iron │ │ │ │ │ No. 16. Cast Iron for │ 1.00│ 1.00│ 2.00│ 3.00│Very Hard Chilled Castings │ │ │ │ │ No. 17. Semi-steel │ 1.75│ 2.80│ .40│ 3.20│Machinable No. 18. Gray Cast Iron │ 2.00│ 3.10│ .30│ 3.40│Machinable No. 19. Soft Gray Cast │ 2.50│ 3.30│ .15│ 3.45│Machinable Iron │ │ │ │ │ ────────────────────────┴────────┴─────────┴────────┴───────┴────────── Now, the above must be understood as being _typical_ compositions only. There are, of course, irons of all intermediate compositions, also, and while the total, graphitic and combined carbons, typically, are about as indicated, there may be wide variation. To illustrate what a variety of chemical and physical properties may be produced, let us assume that the total carbon in a certain cast iron is 3.25 per cent. If this carbon is all in the _chemically combined_ form (i. e., combined with the iron to form the very hard compound which is known to the metallographist as “cementite”) the fracture will be white and the alloy extremely hard. If none of this carbon is combined, but all is in the form of graphite flakes throughout the alloy, the fracture will be “gray” and the alloy soft and machinable. It is possible to produce either of these two conditions or practically any intermediate stage; i.e., we can almost at will split up the 3.25 per cent of carbon into varying percentages of graphitic and combined carbon—the total always equaling 3.25 per cent. [Illustration: No. 30. VERY SOFT CAST IRON. NOTE LARGE GRAPHITE FLAKES ] [Illustration: No. 31. MEDIUM HARD CAST IRON The “combined carbon” is in the roundish, dark parts. It is the “combined carbon” that increases the strength of cast iron and steel. ]