When we say that cast iron is not produced through a refining operation, it must not be inferred that no change in composition occurs during the remelting. There is some change, notably a loss through oxidation from the air blast of a little of the silicon and manganese. Aside from this there usually will be absorption of enough, or sometimes more than enough, carbon from the coke used in melting to make up for the carbon which is oxidized. Usually some sulphur also is taken up from the fuel. There is, however, no such actual or intended alteration of composition through burning out of the metalloids as is necessary for the production of wrought iron and steel.
But from this we must not assume that the manufacture of cast iron for chilled rolls, car wheels, machine parts, valves and fittings, etc., is an easy proposition. As we will soon see, accurate regulation is required of metal for proper depths of “chill” for rolls, car wheels and castings which must have high resistance to wear. Too, the metal for valves and fittings and other more or less complicated castings for high steam, air, ammonia, water, etc., must be uniform, of close grain, strong, yet soft enough to machine easily at the extremely high speeds which modern efficient tools and methods demand. The production of the best metal for such work requires the use of properly selected materials, judicious mixing, and clever operation of the cupola furnace, that the molten metal delivered to the foundry for the pouring of the molds may be hot and fluid and of the right composition for the particular work in hand.
Sampling Cars of High Silicon Pig Iron
It is always interesting and instructive to follow the materials through their course from the “raw” state to finished products, and, therefore, we are going to take you on a little trip from the receiving yard of a firm making cast iron goods where we see the cars of pig iron just in from the blast furnace and where the materials are sampled and held pending analysis, to the laboratory where the samples are analyzed, then to the storage bins where the materials are unloaded, and, later, with the weighed charges, to the cupolas which convert them into molten cast iron of the proper composition and quality for high grade castings.
Sampling Other Pig Irons
Pig irons of lower silicon content cannot be broken easily with a sledge but usually are thrown from a height across an iron block.
Twenty years ago it looked as if the iron foundry would be one of the last strongholds of “rule-of-thumb” to give way to scientific methods. It does not look so to-day, though there are many foundries which yet buy and use their pig iron on the basis of fracture; i.e., the foundryman guesses by judging of the color and closeness of grain and other characteristics of fresh fractures of the pig irons how suitable they are for his purpose and in what proportion to mix them. A skillful man can get fair results in this way only so long as he uses the small number of brands of pig iron with which he is perfectly familiar, and even then there must be but little fluctuation in composition of the irons used and he must be allowed considerable latitude in the quality of the iron which he produces.
Success by this method is even more difficult now than it was ten years ago, for the advent of many new blast furnaces and their greater variety of products have made this rule-of-thumb mixing a much more uncertain matter than it formerly was. Machine-made pigs, which are so generally on the market now, give fractures which tell little regarding their compositions.
