Unlike the modern Bessemer, open-hearth, and other steel products which are reworked, i.e., rolled, forged, etc., cast iron is a comparatively old alloy dating back over several centuries. It cannot be rolled, forged, or otherwise reshaped, so its final form must be given to it at once by pouring or “casting” the molten metal into a mold. Its castings serve exceedingly well in the hundreds of places for which they are adapted. They are comparatively cheap, can be readily duplicated in small or large quantities, and those from the softer grades of cast iron may be machined easily.

These cast iron alloys have only from one-third to one-half the strength of steel or wrought iron and are, comparatively speaking, very brittle. Where resistance to severe shock must be withstood they should not be used. Also, some varieties have a “habit” of growing larger upon repeated heating and cooling. This “permanent growth” is particularly noticeable when the alternate heating and cooling is at red heat or over. Pieces of cast iron have been made to gain 15 per cent in linear dimensions, and it is quite common knowledge among machinists that a piece of cast iron which is slightly too small can be permanently expanded by heat.

Nevertheless, the cast irons have large and legitimate fields in which they are very serviceable. From most of their important present uses they are not likely soon to be displaced.

Cast irons in considerable variety of compositions and physical properties are available, as was indicated by alloys Nos. 14 to 19 which were given in the table on page 83, part of which is here reproduced. In alloys 3 to 13 the carbon exerts the great influence on the physical properties, and this is true also of the cast irons. But all of the latter have a total carbon content of more than 2½ per cent, and, under certain conditions, some of the carbon assumes a different form from that which we encountered in the steels. This modified form is “graphite,” well known to us as a flaky, black, greasy-feeling material, which is soft and very fragile. Graphite in the iron alloy naturally weakens it and, as it is itself such a good lubricant, it makes cast iron machine easily if sufficient amount is present.

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.

The “precipitation” of graphite which is necessary for softness is brought about mainly through the influence of silicon, which we before termed the “softener.” Other conditions being equal, the higher the silicon (if not above 4 per cent), the higher will be the graphite and the lower the combined carbon; and vice versa, the lower the silicon the lower will be the graphite and the higher the combined carbon. It is mainly due to the “combined carbon” which is left after precipitation of the graphite that the alloy has greater strength, hardness, and closer grain. So, just as the steels are stronger and harder as the carbon increases (in steel all the carbon is combined), so, other conditions being equal, the strengths and hardnesses of the cast irons, within usual limits, increase as the combined carbon increases.

Just here it is interesting to remember that from the standpoint of metallography cast irons are simply steels in which there is what we might call an impurity or an adulterant, graphite crystals. It will be seen at once that could these graphite crystals be removed from the cast irons shown in photomicrographs No. 74, No. 92d, No. 30 and No. 31, we would have alloys quite similar in appearance to the steels shown in photomicrographs No. 3b and No. 22c which appeared on pages 77 and 78.^[7]

So the softer cast irons which are used for valves and fittings, machine parts, radiators, hollowware, etc., have high silicon. Parts that do not have to be machined can be of “harder” iron; i. e., made of iron having lower silicon content.

Manipulation of the silicon content is not the only method by which the hardness of cast iron can be influenced. Graphite can “precipitate” (i.e., separate throughout the casting) only if sufficient time is given it to do so. That is, the cooling of the casting after pouring must be sufficiently slow. In a sand mold the iron remains molten for a time, and after solidification it cools slowly enough that the greater portion of the carbon separates as graphite. Therefore, castings of proper composition made in sand molds are soft and machinable.

If the iron is poured into a mold the surfaces of which are made of iron, the molten metal upon entering becomes solid almost as soon as it takes the form of the mold, and it cools with great rapidity. Under such conditions the carbon of the alloy is denied the time necessary to change into the graphitic form and the casting has a white fracture and is so hard that it cannot be machined.

There are many purposes for which the alloy should have extreme hardness and the great resistance to wear which accompanies such hardness. The wearing faces of gears, brake shoes, rolls, and car wheels, for instance, must be hard. For such products, white cast iron, the extremely hard condition of the alloy, just referred to, is utilized. Such castings are usually produced with a white cast iron face, but with a gray iron interior, gray iron being less brittle and less likely to break under shock or strain. A car wheel, for instance, has approximately an inch in depth of white iron on the surface which lies next to the rail on which it runs.

Such are known as “chilled castings.” Molds for them are usually made of sand, with pieces of iron (called “chills”) imbedded where white iron is to be produced. The molten iron next to the sand surfaces cools in the usual way and is gray and soft, while that which lies next to the “chill” is white and extremely hard. The “depth” of the chilled layer can be increased or diminished according to the thickness of the iron “chill” used, its temperature, and by the composition and temperature of the molten cast iron with which the mold is poured.

The sulphur and total carbon of the molten cast iron also have considerable influence on the depth of “chill.”

There is a cast iron alloy which is familiarly known as “semi-steel.” It is simply a high grade and stronger “gray” iron and must be classed as a cast iron, as our table on page 180 shows. While it could undoubtedly be made from materials which are commonly used for cast iron, it is practically always produced by charging with these a certain amount of steel scrap to bring about the lower silicon, phosphorus and total carbon desired.

Because steel has a higher tensile strength than has cast iron, many have inferred that it was the steel addition which made semi-steel stronger than the ordinary soft cast iron alloy. The rather unfortunate name, “semi-steel,” apparently was given because of the steel used and the intermediate strength which the resulting product possessed.

However, during the melting down of the charge the steel scrap becomes molten and its constituents merge with those of the other iron materials charged. We get out of the cupola, then, a mixture which, disregarding the losses and gains due to the air of the blast, the fuel, etc., is an average of the materials charged. We, therefore, no longer have any steel, but a cast iron which has a somewhat lower silicon, phosphorus and carbon than the softer cast irons. The greater strength of the alloy is due to its composition and only indirectly to the fact that steel was used in its production. The physical properties of the steel charged have been entirely obliterated in the melting process.

This view that semi-steel only indirectly gets its increase in strength from the steel charged is confirmed by its structural appearance under the microscope, as was shown in numbers 74 and 92d which were given on page 79, and the photomicrographs given here, and by its extreme brittleness under hammer blows. Under such shock it is but little more resistant than cast iron.

This weakness under “shock” was shown by tests from which the table which follows was compiled. Bars one inch square and thirteen inches long laid on supports exactly twelve inches apart, were struck at the center by a twenty-five pound weight. It took seven blows to break the cast iron bar, the semi-steel bar required eleven, while cast steel withstood ninety-two blows. Even this does not adequately express the great resistance of the cast steel (another alloy not yet discussed), for the height of the “drop” was being increased one inch with every blow, and the cast steel bar, on account of its bending, had to be regularly turned. The total foot-pounds exerted by the blows are given in the table which follows:

Semi-steel is a very close-grained alloy of ten or twelve thousand pounds per square inch greater strength than cast iron. It is a most satisfactory material for medium and larger sized castings for which cast iron formerly was used.

Did we say that cast iron was very brittle? So it is, comparatively speaking. But just as the chemist will tell you that there are no substances which are absolutely insoluble, just so does cast iron appear to be extremely brittle only when compared with the iron alloys of considerably less brittleness.

The three pictures of the hooped and twisted casting illustrate how unwise it might be to speak with absoluteness. A few years ago the casting illustrated on pages 190 and 192 was brought to this country by a visitor to Europe, with an expression of regret that cast iron produced in this country did not have such qualities of elasticity as had cast iron made abroad. Whereupon, without any change whatever in his iron mixture or cupola practice, the superintendent of a well-known foundry made castings which were exact duplicates of that submitted. The three photographs shown further on (Figs. A, B, C) were of one of these castings. The ability to bend without breaking is, of course, largely due to the shape.

As a matter of fact, such castings were not at all new in this country, having been furnished by American foundries for electrical work for many years. Cast iron springs, piston rings, and many other articles of cast iron are regularly made, which show such elastic quality.

We have said considerable about “castings.” In general we know what castings are, but in the minds of some there may be a little uncertainty as to the manner in which they are produced.

There are few lines of human endeavor which require greater judgment and skill than does the making of molds for castings. Sound judgment based on long experience, knowledge of conditions under which the work immediately in hand must be done, observation, and accurate, deductive reasoning as to the causes of failure are absolutely necessary for success.

In general, molding may be said to be done in “pit” or on the “floor” for large work, on the “bench” for smaller work, or by “machine.” Pit, floor, and bench molding are applicable for production of castings of all sizes and descriptions and this general type which we might term “hand molding,” is the form that has been practiced longest. Molding machines are more or less recent inventions which have enabled certain standard shapes and sizes of castings which are in sufficient demand to be produced in great numbers by unskilled workmen and therefore at less cost than is possible by the older hand method.

Each design for casting may be said to demand individual treatment, and the molder must select that method out of the many which alone, perhaps, can be successful. The subject is such a broad one that little will be here attempted further than to give by description and illustration the predominant points of the making of molds and castings. A simple, typical case of bench molding will be taken, that the relation of pattern, mold, core and casting may be clear.

The molding sands used are usually natural sands which contain greater or lesser amounts of clay, which, when moist, acts as a “binder” of the grains of sand. When used without drying, the mold is said to be a “green sand” mold; if dried, a “dry” or “baked” mold, as the case may be. The majority are “green sand” molds.

For the usual casting of which only a few or several duplicates are wanted, the “split” pattern is generally the most convenient.

The two halves of the mold, the “cope” (top) and the “drag” (bottom), are separately made in the two parts of the “flask” or molding box by “ramming” properly selected and “tempered” (moistened, mixed, and sieved) sand over the halves of the pattern. Of these, the drag is made first over the lower half of the separable pattern placed flat side down upon a bottom board. After “ramming,” i.e., packing the sand, just hard enough but not too hard, this half mold is reversed and the top half of the pattern placed upon the lower half, now at the upper face of the drag and flat side up. A little “parting powder” or fine, dry sand is sprinkled over the fresh surface of the half mold so that the upper half, next to be made, will not stick to the lower half, but can be lifted off at the proper time.

The cope half of the two-part “flask” is now put on, filled and rammed with sand as was the drag. Any extra sand is scraped off with a straight edge and at the proper place a hole is cut with the “sprue cutter” straight down through the cope to the “parting.” More commonly, perhaps, this “sprue” hole is made by withdrawing a “sprue” stick (of wood) about which sand had been packed during the making of the cope. It is through this hole that the molten metal will be poured into the mold.

Lifting the cope or top half, it is turned upside down, and, after cutting in the drag the “runner” or “gate” connecting the “sprue” hole with the casting, the halves of the pattern are carefully drawn that the sand may not be disturbed. Now in the cavity left in the drag, to make the hole in the casting, is hung the baked “core” of sand, held together by flour or rosin or a “drying” oil. The cope is carefully replaced upon the drag, thus “closing” the mold.

As will be noted from the drawings, there is left between the core and the mold a space all around, which will be filled by the metal of the casting when poured. Therefore the surface of the core shapes the inside, and the mold itself the outside and ends of the casting.

The molten metal, entering through the vertical “sprue” hole, flows along the “runner” and into the mold through the more or less constricted entrance called the “gate.” The gases formed during pouring and the air with which the mold was filled are driven out through the porous bodies of sand of the mold and core. Had the mold been rammed too hard the gases could not escape through the sand and an imperfect casting would result.

The poured mold is allowed to stand until the metal has solidified and cooled sufficiently, when the casting is “shaken out.” The sand is returned to the molder to be used again. The sprue and runner are broken from the casting, which, after cleaning by “tumbling” with others in a revolving mill, or otherwise, goes to the machining and assembling shops.

Some form of the above general method is everywhere used for the production of all kinds of castings, except for those which can be made by machine at a lower cost for molds.

This kind of molding, which we have termed “hand work,” requires expert molders and is too slow and expensive for the hundreds of standard shapes and sizes of castings which are in great and constant demand. The latter are made on cleverly devised molding machines working with compressed air or by hand power applied through a lever. The pattern is attached to the machine, set and very accurately adjusted by a skilled mechanic. Thereafter the sand is rammed, the runner formed and the pattern drawn by the machine itself, all of these very critical movements being therefore rapidly and unerringly duplicatable any desired number of times by unskilled labor, which has but to put on the parts of the “flasks,” feed in the sand, set the cores, close and remove the mold, and begin the next.

Sometimes there is but one, but for the smaller sizes there are often ten or twenty, and, occasionally, as many as two hundred pieces or castings in a single mold.