CHAPTER X MIXING IRON THE mixing of iron for the cupola is done either by fracture or by chemical analysis. Mixing by Fracture The fracture of the freshly broken pig is taken as the index of its composition. A dark gray color, with coarse open crystalline grain indicates a soft iron, and, as a rule, one capable of carrying a large percentage of scrap. As the color becomes lighter and the grain closer, hardness increases and less scrap can be used. Very hard irons are mottled or white and are used for special work. A broken pig may present a dark fracture with open grain, but with a fine white streak showing at the outer edges of the fracture. Such an iron will make hard castings, owing to the presence of too much manganese. Blow holes and worm holes indicate sulphur or ferrous oxides. Iron showing these with frequency should be used carefully. Segregations, much lighter in appearance than the rest of the fracture, frequently appear. These indicate higher percentages of carbon, sulphur or manganese at those particular spots and the iron should be used with care. Mixing by fracture is uncertain and is liable to produce irregular and unsatisfactory results. The foundryman must always proceed cautiously and can only arrive at the results desired by careful trial. The following mixtures are taken from West's "Foundry Practice." Locomotive Cylinders 2600 pounds car wheel scrap. 600 pounds soft pig. Marine and Stationary Cylinders 50 per cent No. I charcoal. 50 per cent good machinery scrap. 33 per cent car wheel scrap. 33 per cent good machinery scrap. Rolling Mill Rolls 50 per cent car wheel scrap. Small Chilled Rolls 1300 pounds old car wheels. 100 pounds No. 1 charcoal. 300 pounds steel rail butts. Kettles to Stand Red Heat 1300 pounds No. 1 charcoal pig. 700 pounds good machinery scrap. Chilled Castings to Stand Friction (no strain) 200 pounds white iron. 200 pounds plow points. 100 pounds car wheel scrap. Ordinary Castings 33 per cent No. 1 soft pig. 67 per cent scrap. Thin Pulleys 66 per cent No. 1 soft pig. 34 per cent scrap. Sash Weight 67 per cent scrap tin. 33 per cent stove scrap. The advent of the chemist into the foundry offers means to avoid many of the uncertainties coming from the selection of irons by fracture, and the more advanced foundrymen are now mixing their irons by analysis. Mixing Iron by Analysis This method of mixing iron is by no means entirely removed from uncertainties. The chemist is not yet able to insure the production, from irons of known chemical composition, of castings of definite physical characteristics. Analysis should be supplemented by physical tests. Again, while the foundryman may have correct analysis of his pig iron, if scrap is used to any extent, especially foreign scrap, he must approximate the elements contained therein. The statements made on page 307 offer some little assistance, but, in general, reliance must be placed on experience in this respect. Where the scrap comes entirely from previous casts, one can readily arrive at its constituents and much uncertainty is removed. The qualities necessary for different grades of castings may be summarized as follows: 1. Hollow Ware, Stove Plate, Sanitary Ware. Require fluidity, softness; must be high in silicon and phosphorus; low in combined carbon. 2. Light Machinery Castings. — Require fluidity, softness, strength and absence of shrinkage. Must be high in total carbon and manganese; low in sulphur and contain less silicon and phosphorus than grade No. 1. 3. Heavy Machinery Castings. - Require softness, strength and low shrinkage. Should be lower in silicon, phosphorus and graphitic carbon than No. 2. Higher in combined carbon and manganese; low in sulphur. 4. Castings requiring great strength should be low in silicon, graphitic carbon, sulphur and phosphorus. Combined carbon should be about .50 per cent; manganese .8 per cent to 1.0 per cent. 5. Car Wheels and Chilled Castings. Require low silicon, phosphorus, graphitic carbon and sulphur. High combined carbon and manganese. 6. Chilled Rolls. - Require low silicon, graphitic carbon and phosphorus. High combined carbon. The following table is abstracted from "Proceedings of the American Foundrymen's Association," Vol. X, Part II, which contains the results of a long series of tests made by their committee to standardize test bars. The mixtures are not given as being recommended by the committee for the several purposes, but simply to indicate the practice of some of the larger American foundries. At a later period Prof. J. J. Porter, at the request of the American Foundrymen's Association, undertook the investigation of the compositions used for various classes of castings, with a view to formulating standard mixtures. His report embraces every variety of work and contains tabulated analyses of several hundreds of mixtures in use. The averages of the mixtures in each class of work, together with those suggested by Prof. Porter, are subjoined. |