We have seen how primitive man hunted and fought with no implements and weapons better than clubs, bows and arrows, and stone hatchets, and how his wife cracked and ground the corn between flat rocks or in mortars of stone. In the succeeding “Bronze Age” we found ornaments, idols and tools being made of copper or the copper alloy, bronze. It was only after the next great advance that we found man utilizing iron for his purposes of civilization. This metal, which with us is so common, was in those days very expensive, so much so that it could be used only for purposes of war and as the gifts of kings.

But the world was traveling fast and it was not long before the iron-carbon alloy, steel, was produced. Even so, many hundreds of years elapsed before the present wonderful age was ushered in through the great inventions of Henry Bessemer and the Siemens brothers. And while fine steels for swords and tools have had an incalculably great influence upon the development of the human race, it was the mammoth production of Bessemer and open-hearth steel which permitted its general use as a material for construction of ships, bridges, buildings, and for railroads, that made this the “Age of Steel.”

Speaking in terms of the power house, it is also the “Age of Cast Steel.” Twenty-five years ago the manufacturer and power house man were quite content with their “saturated” steam temperatures and pressures. With cast iron valves and fittings their plants were well equipped.

But the world did not stand still. It became known that by heating the steam out of contact with the water in the boiler it lost the moisture which it carried and became dry and then could be charged with as much additional heat as it was desired to give to it. This “superheated” steam, of course, would do more work and it had also certain other advantages which the old-fashioned “saturated” steam had not.

But while cast iron fittings gave satisfactory service up to temperatures, say, around 450° F., they faltered when forced to work under the new conditions which meant decidedly higher temperatures and pressures. And, too, the repeated heatings and coolings which were often necessary, disclosed a disadvantage previously unknown—a so-called “permanent growth” of the cast iron which was attended by loss of strength, and altogether it was soon found out that when superheated steam was to be used, higher types of materials were advisable than those which had been used under old conditions.

Superheated steam has rapidly come into general use. Some of the new locomotives and most of the modern power plants are now built for as much as 200° superheat, i.e., a total temperature of approximately 600° Fahrenheit.

Valves and fittings of cast steel not only are the articles “de luxe” for such service but they have come to be considered the necessary articles and their advantages have only fairly begun to be appreciated.

Though our most august scientific societies are proposing and debating upon systems of classification which shall include and satisfactorily define all of our ferrous metals, a satisfactory one has not yet been evolved, and, considering the intricacy of our ferrous metallurgy and the discoveries which are being made almost daily, the outlook for a strictly logical classification is not yet flattering.

With “Cast Steel” our metallurgical nomenclature is again faulty. Before what we now call the “steel casting” was known, crucible steel was poured into ingots, “forged” into tools just as it now is and often went under the name “Cast Steel” to distinguish it from the contemporaneous material, wrought iron. So to-day we buy many tools and implements which bear the name cast steel, which we know to have been forged in bringing them into their final shape.

But it is not these which we mean by the term, cast steel, but rather those steel products which get their final form by being “cast” from a fluid condition into a mold. These are what are rapidly coming to be understood when the term “cast steel” is used.

Satisfactory metal for steel castings may be made in any of three or four types of furnaces, but, as was suggested before, the making of molds for castings is a fine art, as is the preparation of the metal which is to go into them. Further, the making of that special class of castings which are to withstand water, steam or air pressure is a very different thing from the making of steel castings for other purposes, and this is too often forgotten.

For the former are necessary particularly close-grained castings, free from flaws or spongy spots. Under the great pressures applied such defects would certainly allow leakage.

Whatever the method of production of steel for castings the metal is poured into molds to receive its final shape. Because of the intensely high heat of the steel only sands of great refractoriness (resistance to heat) can be used as material for the mold. White silica sand is such a material and is generally used, mixed with enough clay and molasses-water to give it “bond.” While molds for some steel castings are made in “green” (i.e., undried or unbaked) sand, baked molds are preferred for fine finish and surest results. After the making of the molds in the usual way they are sprayed with very finely powdered white sand or quartz mixed with a little molasses-water. They are then thoroughly dried in an oven.

Cast steel shrinks during cooling even more than malleable iron and the pattern and mold must be made to allow for this. Upon the freezing of the surfaces of the casting with consequent attainment of rigidity, the interiors, which freeze last, may have cavities unless means for avoiding them is provided. For this purpose heavier pieces, which later can be cut off, are cast upon such parts of the casting as tend to have “shrink holes.” These may be likened to receptacles filled with fluid metal, which being larger than the parts of the castings which they “feed,” hold excess metal in fluid condition until the casting itself has become solid throughout. Such are usually called “risers” or, in Europe, “lost heads,” and the molten metal in them flows down into the interior of the casting and fills the shrink holes which are forming. Not only must the risers be large enough that the metal in them is the last to solidify but they must be built high enough above the casting that sufficient pressure is exerted on the steel entering the shrinking parts to make its entry sure.

Baked molds, of course, are comparatively rigid. As the risers which stand on top of the flanges and other high parts of castings aid in resisting the natural shortening of long castings during and after “setting” of the metal, there is great liability that the still red-hot casting will crack somewhere along its length. It is therefore necessary to loosen with bars the sand of the mold as soon as the metal of the casting has set, particularly between the risers, and to break out the sand of the core inside, around which the shrinking metal might crack were the sand left in its hard packed condition.

After the casting is shaken out from the mold, it is cleaned and the risers cut off either by sawing or with the more modern oxy-acetylene torch flame.

Steel castings should be annealed in order to “refine,” i.e., make finer the grain of metal and to equalize “strains” which are set up in the castings during cooling. Coarse grain and internal strains tend to make the castings brittle. No such extended annealing, however, is necessary as is the case with malleable cast iron, for no divorcing of carbon from the iron with separation of free carbon is possible. The castings are carefully heated to a temperature of about 1600° or 1700° Fahrenheit and allowed to cool slowly.

After annealing, they are cleaned and excrescences removed by chipping, after which the castings are tapped, drilled or otherwise machined according to the purposes for which they are intended.

While more costly in manufacture and installation than are those of cast iron, valves, fittings and other cast steel products are, so far as we now know, practically permanent. Their notable shock resisting quality is well shown in the following table which is reprinted from page 188.

It is to be noted that while malleable cast iron far surpasses “semi-steel” in this property, though their tensile strengths are ordinarily somewhere near the same, cast steel, in turn, offers more than six times the resistance of the malleable iron to shock and has nearly double its tensile strength. It is this great strength and resistance to shock, heat and pressure, with freedom from “permanent growth” under alternate heatings and coolings that make cast steel such a valuable material for the many purposes for which castings are to-day employed. Millions of steel castings annually find varied application.

In modern power houses and other commercial steam and hydraulic installations particularly, steel castings have come to be the materials usually specified and approximately the only ones which satisfactorily serve under the severe conditions of to-day.

Undoubtedly the first steel castings were poured from crucible steel, though we must remember that the crucible is a melting and not a refining furnace. This was only natural. In the crucible the metal can be made very hot and fluid, and if of proper composition and properly “killed” crucible steel makes very fine castings. Crucible steel castings, however, are not in as fortunate a position as are other products of this high grade material. Tool steels ordinarily bring high enough price that there remains a profit to the manufacturer though his manufacturing cost is necessarily high. In the steel casting line, however, there is much keener competition and crucible steel has had considerable difficulty in maintaining its place. It seems to be a matter of price alone.

Open-hearth steel is very largely used for steel castings, more than two-thirds of all made in this country being of this material. About one half of these are poured from basic open-hearth metal, and the other half from acid metal. It is generally considered that the product of the acid-lined furnace is a little freer from over-oxidation.

Open-hearth steel cannot generally be as hot and fluid as are the steels made in other types of furnaces. For this reason as well as because of the larger size of the usual open-hearth furnace, small castings are not generally poured from this steel. It is for steel castings of considerable size and where there are sufficient orders to warrant a steady and large output that the open-hearth has its place. True, smaller open-hearths are now built, some of them of only two or three tons capacity, but, in general, the standard open-hearth for steel castings is of fifteen tons or more capacity and of the style of the open-hearth furnaces which were described in Chapter IX.

In their proper sphere they are highly satisfactory, but they are “inelastic” in that they must be run continuously day and night and should not be allowed to cool until extensive repairs are imperative.

It was mentioned that in the open-hearth process the furnace is always hotter than the metal which it contains and that the heat which can be put into the steel is limited by the ability of the refractories of which the roof and side walls are made to withstand melting. In the Bessemer process the metal is hotter than the furnace because the heat is generated by combustion of certain of the metalloids contained in the metal itself. As metal for castings must be very hot and fluid the Bessemer process is very satisfactory for the making of steel for castings.

It has, also, the advantage of “elasticity.” The supply of metal is practically continuous and one furnace can make from one to eighteen or even more heats on day turn only and be shut down for the night turn or longer and then started again without such loss as would result from the shutting down of an open-hearth furnace with regenerators.

For the making of metal for steel castings, very small-sized Bessemer converters are used which make from one to three tons of metal per blow. Some converters of as little as one-half ton capacity are being used. While some are of the “bottom-blown” type already described, the majority are what are called “surface-blown” or “side-blown.” In these, from four to eight round tuyères, about one and one-half inches in diameter each, pierce the brick or ganister lining just above the surface of the bath. They slope downward a little toward the bath so that when the converter is tipped to its upright or blowing position the air blast will strike the adjacent edge of the metal and blow across its surface. This three or four pounds per square inch of air blast keeps the metal in circulation, meanwhile burning out its silicon, manganese, and carbon, just as it does in the larger bottom-blown converters. Surface-blown give hotter metal than do bottom-blown converters and very fine steel castings are made from their metal. For these converters, which are practically all acid-lined (i.e., with silica or clay brick or ganister), metal low in phosphorus and sulphur is regularly drawn from a cupola specially run for the purpose.

The remaining recognized type of furnace for steel for castings is the comparatively new electric furnace.

Commercial melting of metals by the electric current has been sought for half a century. In 1879 the first furnaces of promise were patented by Sir William Siemens, one of the Siemens brothers who became so well known through their great work with the open-hearth furnace, the gas producer and many other things metallurgical. While Siemens melted as much as twenty-two pounds of iron per hour in his furnace, the cost of the electric current at that time was so high as to be practically prohibitive for the manufacture of steel in competition with the open-hearth, Bessemer and crucible processes.

Little of great moment in the electric furnace line developed during the nineteen years which followed. Then, in 1898, Stassano in Rome, Italy, constructed a furnace in which three carbons gave an electric arc above the surface of the bath. About the same time, Heroult, a Frenchman, was developing the electric furnace which to-day has become so well known in this country, and which bears his name. Other well known furnaces of the arc type are the Gronwall-Dixon, the Snyder, the Girod and the Rennerfelt.

In general, electric furnaces have more or less round steel shells with shallow brick, magnesite or sand-lined hearths, and sidewalls and removable roofs of brick. Heat, of course, is furnished electrically. In most of them long carbon electrodes are lowered through holes in the roof until the lower ends strike an arc with the metal on the hearth. The number of carbons may be from one to four or more depending upon the style and size of the furnace and the manner in which electrical connections are made. All of the furnaces mentioned have been used for the production of steel for castings and the Heroult and Girod are in use in larger sizes for electric steel for rails and miscellaneous products. The steel is first cast into ingot molds and is later rolled down into bars, rods, etc.

All of the above use carbon electrodes and are known as “arc” furnaces. There is a distinctly different type of furnace which, also, is in use in commercial sizes. This is the “induction” furnace. In this, what is known to electricians as a secondary current is “induced” in the bath itself and heats the metal. Of this type the Kjellin and the Rochling-Rodenhauser are the best known in this country. While they are in use in the larger sizes for production of steel for ingots, these two furnaces do not seem to have been used to any extent for metal for steel castings.

The details of construction of the furnaces which are used for metal for castings are more or less different, but they are not of particular interest to us. The working of all is similar and a general description should suffice.

Whether starting with furnace cold or hot, materials in molten or in the more usual “cold” form are charged on the shallow hearth of the furnace. The charging doors are closed, the current is turned on and the carbon electrodes are lowered until an arc is struck between the upper electrodes and the metal on the hearth, which in some way is made to connect with the negative electrodes. In one or two of the types mentioned the arc plays between the carbons, all of which are above the bath.

At first there are great fluctuations in the current intensity because of the uneven surface presented by scrap steel on the hearth. In a short time, however, the current steadies. The intense heat of the arcs soon brings cold steel to a molten condition.

Occasional attention from the attendant is necessary to see that the melting is even and that any outlying pieces of steel are pushed to the center where they must melt.

In the basic-lined furnaces lime is usually charged with the cold steel. With the iron oxide which is added from time to time this forms a highly oxidizing slag, which, after it takes the phosphorus from the metal, is skimmed off.

As you will remember, the other processes stop at this point, little further refining being possible. In the electric furnace, however, the sulphur, also, can be reduced to almost any desired amount by use of a further addition of lime, and greater heat. Not only can the sulphur be reduced to very small percentages but the over-oxidized bath can be brought to neutral condition and the green or black slag made white with return of its manganese and iron to the bath. This is accomplished by addition of small amounts of powdered coke or coal. The whole process is under very accurate control.

With a practically white slag, which is the signal that the deoxidation of the bath is complete, and the sulphur reduced, the steel is ready to pour provided it is hot enough. Tests of this are usually made either by pouring a little of the steel from a small ladle and observing its fluidity or by observing the quickness with which the end of an iron bar is melted off when plunged into the metal in the furnace.

Of all the metallurgical steel furnaces, the electric furnace is the most susceptible of accurate control. With the heat applied directly to the metal in the cleanest way possible, i. e., without the admission of coal ash or gas or air of the blast, the atmosphere in the tightly closed electric furnace can be made “oxidizing,” “neutral,” or “reducing” at will. The metal can be held in the furnace and additions made, samples taken, and the operations conducted with regulation and certainty.

This newly devised metallurgical apparatus is coming to be largely used in the production of tool steels. While it has not displaced the crucible method for the production of steels of the very highest qualities, it has proceeded far enough in this direction in the very limited number of years since its introduction, that it is certain that the crucible, even for tool steels, is to have a keen competitor. Tool steels in considerable variety are to-day being quite satisfactorily made in the electric furnace and it is not at all unlikely that steels of the very highest grade will shortly be produced by this method.