From the preceding chapters we now know pretty well the place which cast iron occupies in the iron family. In the chapters which have succeeded the one in which we discussed the blast furnace and pig iron, every one of the products except crucible steel has been produced through some “refining” operation which greatly changed the composition, structure and properties of the product. Cast iron is not the result of a refining operation in this sense of the word. It is produced through simple mixing of pig irons of various compositions, usually with some admixture of iron castings of similar composition which have outlived their usefulness in the industrial world and have been returned as scrap to be remelted.

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.

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.

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.

While some foundries still attempt to accomplish this difficult and sometimes impossible feat, the majority are now applying more scientific methods to their manufacture of cast iron.

Though the eye cannot tell surely from the fracture the composition or quality of the iron which is used in making up the charges, chemical analysis does definitely give this information. Therefore, every car of pig iron purchased by this firm is sampled and analyzed, the composition of all other materials used in its mixtures is determined, and, irrespective of fracture, which may or may not tell the truth regarding their composition, the raw materials are charged with respect only to their actual content of the metalloids. The resulting molten iron each day is analyzed to confirm the correctness of the mixture and to furnish analysis of the “sprues” which next day are to be used as a part of the day’s charge. Physical test bars, too, are cast each hour or so, and the tensile, transverse strengths, hardness, shrinkage, etc. are accurately determined in testing machines and recorded. In this way absolutely nothing is left to chance or to guess work, and, as you may surmise, any slight deviation from the composition desired is shown at once and the mixture immediately changed to the extent necessary to bring the iron back to normal. It is surprising within what narrow limits of variation compositions and physical properties can be held, with furnace operations continually under such surveillance.

As the basis for its cast iron, many thousands of tons of pig iron are each year used direct from the blast furnaces. The raw materials come in railroad cars or by boat. The inspector who represents the metallurgical department enters each car and inspects the materials, taking from each a representative sample for analysis. In the case of pig iron this will be from four to eight half pigs, it having been found by experience that these represent very well the contents of the car. So each car of material is held without unloading until it has been determined by inspection and analysis that it is fully up to the specifications upon which the iron was purchased.

Arriving at the laboratory, the half pigs from each car are drilled, equal amounts of the drillings being taken and mixed in an envelope which bears the name of the brand of iron, the number of the car, the date, etc. The sample pigs from each car are treated in this same way, each car being treated individually.

The envelopes containing the drillings then go to the chemists. Frequently samples from fifteen or twenty cars of pig iron, with as many other samples of various derivation, are being analyzed at the same time for the four or six different constituents which it is necessary for the metallurgists to know and control in order that a highly satisfactory product may result. Though a hundred different determinations may be in progress at the same time, spelling “chaos” in the mind of one not entirely familiar with the details of the work, it will be interesting to single out and explain briefly how the samples are analyzed.

In such analytical work everything is based upon weight; i.e., constituents are determined and reported in percentages by weight. In chemical laboratory work everywhere the metric system is used, the cumbersome English system of weights and measures being practically impossible. Thus, the metric system is the international scientific standard. The unit taken is the gram, which is equivalent to ¹⁄₄₅₃ part of an avoirdupois pound. One gram of pig iron drillings is such an amount as could be held on an ordinary ten-cent piece.

Working with such small amounts of the sample, exactness and skill are extremely necessary. The balances used are necessarily very delicate—just as delicate as were the scales upon which the jeweler weighed your diamonds—you remember, of course. On these balances we can weigh an inch-long mark made by an ordinary lead pencil.

As the results of the analysis have to be known inside of three or four hours that the cars may be quickly unloaded in order to avoid demurrage, which is the penalty for holding cars longer than the allowable time, separate portions of each sample are weighed out for determination of the silicon, manganese, sulphur, phosphorus, graphitic carbon, and combined carbon. These are necessary in order to determine that the iron is up to the quality specified in the purchase contract and also to provide for its most efficient use in the manufacture of iron castings.

The exactly weighed portions are put into clean, numbered beakers, which are small pieces of high grade glassware that will stand sudden changes of heat and cold. Some of these portions are dissolved in nitric acid, some in hydrochloric acid, others in combinations of acids. In each case the drillings go into solution in the acids, and after various treatments of boiling, evaporating, filtering, etc., well known to those of the chemical profession, the desired results are obtained. In some cases it is by actually weighing a constituent which has been filtered out and burned to ash of a constant known composition, in others it is by comparison of color with standards of known composition, and sometimes it is by other means.

In all of this analytical work the chemist must take care to lose not one drop of the solution or one grain of the ash from the burned “precipitate,” as the “filtered out” constituent is called.

The pig iron is always bought upon guarantee that it will contain a certain percentage of silicon—the element which in cast iron is known as a “softener.” But this is not the only thing necessary in the iron that is purchased. It must also show proper specified quantities of manganese, phosphorus and carbon, which also are very desirable elements in iron castings, and as little of that undesirable element, sulphur, as possible. Therefore they pay in proportion to the content of silicon, manganese, phosphorus, and carbon—and penalize the seller for sulphur.

The laboratory holds copies of the contracts upon which these materials were bought. If, upon comparison, the analysis obtained complies with the terms of the contract, an O. K. unloading slip is made out and the receiving department is given directions into what raw-material bin in the receiver building it shall be unloaded. If not fully up to the standard called for in the contract, the purchasing department is notified and the car is either rejected or accepted upon some proper terms of adjustment if it can be used without detriment to the product in which it is to be utilized.

Cars of coke, limestone, fluorspar, etc., are inspected, analyzed and treated in the same way, so that nothing is left to guess work. The compositions as determined by the laboratory serve not only as the basis for acceptance or rejection, but the analyses of accepted materials are forwarded at once to the metallurgists, who from them figure the mixtures to be used in the cupolas.

Having great stocks of analyzed raw materials in the labeled bins in the receiver building, the metallurgists who supervise the mixing and melting of the iron determine by mathematical calculation just what irons and how much of each must be taken to give molten iron of the best composition and properties for the castings.

The total iron materials charged must have a definite amount of silicon, of manganese, of phosphorus and of carbon. For a 4,000–pound charge for soft cast iron, for instance, the total silicon in the materials which make up the charge must be somewhere near 118 pounds, the manganese and the phosphorus about 30 pounds each. The usual losses of these materials through oxidation are known, of course, and sufficient excess has been allowed that the desired final composition will result.

On several scales which are regularly inspected and kept carefully adjusted, the weighers weigh out the prescribed quantities of the raw materials. “Buggies” holding nearly one ton each are loaded in turn with coke, with proper amounts of pig iron, cast iron scrap, sprues from the foundry castings of the preceding day, and a proper weight of limestone flux. Each charge of two tons of iron requires four buggies for its transportation from the raw-material bins and scales to the cupolas.

The old-time way was for laborers to dump the charges into the cupola and spread the materials by hand, but in modern foundries better ways have been provided. Here a charging machine operated by compressed air lifts into the furnace, one by one, the buggies of coke, and of the other materials, whence, after the dumping of their contents, the buggies are returned to the receiver building to be filled again.

Thus many charges per hour pass through the yawning charging doors of the cupolas, being dumped in fast enough to maintain the level approximately even with the bottom of the charging door.

In starting, a wood fire has been made on the sand bottom of the cupola. This is covered with coke in such a way and in such amount that, when ready for charging of the metal, a column of glowing coke extends to a distance of one foot or two above the tuyères. Upon this “bed” in alternating layers are piled the weighed amounts of pig iron, sprue, scrap and limestone as described above. Following each charge is a layer of coke sufficient in amount to replace the “bed” coke which is burned away in melting the iron charge, thus maintaining the top of the bed of coke throughout the day at approximately the same height.

Ever since 7 o’clock A.M., when the twelve to sixteen ounces of blast pressure was put on, the charges have been descending gradually from the charging door. Encountering the intense heat in the “melting zone” at the top of the bed of coke a little above the tuyères, the iron melts and trickles down through the three to five-foot bed of glowing coke on to the sand cupola bottom or hearth where it accumulates. The tapper, with his iron bar and “bod stick” with its little ball of moist fire clay, alternately opens and plugs the tap hole at the bottom of the furnace as occasion requires, but throughout the day of ten or more hours there is almost constantly a full stream of iron flowing from the spout. The big “bull ladle” which receives it, in turn gives it up to smaller or “shank” ladles, in which it is conveyed along trolleys to still smaller ladles from which it is poured into the sand molds to form the castings.

As in the blast furnace, limestone is added as flux to make liquid and dispose of sand, dirt, scale, etc., which are detrimental. The liquid slag formed from union of limestone with these impurities floats upon the molten iron in the cupola hearth, as it is less than half as heavy as the iron itself. It flows almost continuously from a higher hole called the “slag hole,” in the rear of the furnace and just beneath the tuyères. The slag has little value except as material for filling purposes, etc. So-called “slag wool” can be made by blowing air through it. Sometimes the blast from the cupola blows it in such a way that this pure white “wool” is formed and blows out of the slag hole of the cupola. About Christmas time some of the workmen take quantities of it home for decoration and for fireproof whiskers for “Santa Claus.”

These operations go on continuously throughout the day, each cupola making the particular grade of cast iron or “semi-steel” which is best adapted to the particular castings to be poured, size, shape and purpose of

the castings being the three main determining factors.

When nearing the end of the day, charging ceases, the charging door is closed and the last charge, gradually descending, melts and flows into the bull ladle about an hour later. As soon as the bull ladle is emptied it is run out of the way, the cupola is drained of all iron and considerable slag through the tap hole, and the bottom doors of the cupola are dropped by pulling from under them the “props” which have held them in position. The great mass of bed coke follows with a great burst of heat, light and flame. This is quickly subdued with a stream of water and removed. When cool, the slag and accumulations are chipped from the cupola lining, burned areas are patched with bricks and stiff, fire-resisting mud, the bottom doors are raised and fastened in position, and the eight-inch sand bottom is packed in ready for the next day’s run. After building a fire and getting a good “bed” of glowing coke, the cupola is ready again for the charging of iron.