CAST IRON, WROUGHT IRON, AND STEEL

IT USED to be that wars were fought for gold, but nowadays the possession of rich iron mines is enough to arouse the cupidity of neighboring nations less favored by nature. In fact, even though an ounce of gold is worth twice as much as a ton of iron, the value of the iron we dig out of the earth each year is far greater than that of gold. When that rough ore is converted into iron and steel and then into thousands of useful articles, its value mounts so high that it cannot be estimated. The qualities of iron and its alloys are so excellent and so varied under different treatment that this metal may truly be said to form the foundation of all our mechanical progress. On the one hand it spans our wide rivers, carries the burden of heavy freight trains, or, in the form of armor plate, resists the terrific impact of high-powered shells; on the other hand the same metal, spun into a hair spring, governs the ship’s chronometer, or, in the compass, points a trembling finger to guide the navigator on his course.

IRON IN ANCIENT DAYS

The first use of iron in the service of man dates far back into the ages. An iron tool was found in the pyramid of Kephron which must have been used 3,500 years before Christ. However, because of the difficulty of working it, iron was not extensively employed except for swords and cutlery. The conversion of iron into steel and the tempering of steel blades grew to be an art which gave Damascus and Toledo a world-wide reputation that dates back over a thousand years.

The ancients used to smelt their iron ore in what was known as a Catalan forge because of its extensive use in Catalonia, Spain. Whether the forge was invented there or not we cannot say. Similar forges have been found in India and other widely remote places. They comprised an inclined tray leading to a pot which formed the furnace and in which a charcoal fire was kindled. The ore and charcoal were placed on the tray and from time to time were raked down into the furnace and air was forced into the bottom of the furnace by means of bellows. In an improved form of the Catalan forge air was furnished by means of an air compressor operated by a stream of water. This has already been referred to and illustrated on page 90. Limestone served as a flux to melt the earthy matter. The iron obtained from these primitive furnaces was not heated sufficiently to flow as a stream, but was merely reduced to a pasty mass which was then hammered into shape by the blacksmith. Ten or twelve pounds of metal per day was considered a fair output for one of these forges.

DISCOVERY OF COKE

It was not until the middle of the 14th Century that a blast furnace, crudely similar to those we have to-day, was first built and with it a temperature was obtained that was high enough to turn the metal into a liquid which could be cast in molds. Charcoal continued to be the fuel used until about four centuries later, when Abraham Darby discovered that by baking coal to remove its free gases, he could produce a new fuel known as coke which was a good substitute for charcoal. This gave a wonderful impetus to the iron industry in England where there were ample deposits of coal adjacent to the iron mines. Shortly after that, Mr. Henry Cort of Gasport, England, invented the processes of puddling and rolling the product of the blast furnace, thus converting the iron into a tough, malleable metal.

ALLOYS OF CARBON AND IRON

We must pause here to learn the difference between cast iron, wrought iron and steel. Iron, as we know, has a high affinity for oxygen. When exposed to air and moisture it oxidizes, rusts very quickly. The iron we find in nature is largely oxidized. In other words, it is rusty. It is also found in combination with other elements as well. The object of putting iron ore through a furnace is to rid it of oxygen and this is most readily accomplished by melting it in a carbon fire. The highly heated carbon combines with the oxygen and passes off as carbon dioxide and carbon monoxide gas. But a certain amount of carbon unites with the iron and it is this alloy of carbon and iron that makes cast iron so stiff and brittle. The less carbon present the softer is the metal and pure iron is very ductile.

It was to rid cast iron of its carbon content that Cort invented the puddling process. As the metal came out of the blast furnace it ran into a “reverberatory” furnace where, without coming in contact with coke or other carbon fuel, it was exposed to flames from an adjoining furnace which burned out the carbon, and then the carbon-free iron was cast into large pieces known as blooms which were hammered to rid them of slag. The final product was known as wrought iron. Wrought iron then differs from cast iron in having no carbon. Steel, on the other hand, stands half way between wrought iron and cast iron in having a small percentage of carbon. How steel is made will be described later.

MECHANICAL HANDLING OF ORE

Of course machinery plays a large part in the modern iron industry. It would be an endless task even to load one of the big blast furnaces by hand and then the enormous output of molten metal—40 tons for every pound produced by the old Catalan furnaces—could not be handled without ponderous machines whose huge arms and fingers are not scorched and blistered by the intense heat. Along the Great Lakes vast loading machines fill the holds of ore vessels and at the plant there are enormous unloading machines that travel on rails. These have long bridgelike arms that reach out over the ore boat and drop huge clam-shell buckets into their holds. The buckets quickly unload the boats and dump the ore on shore where other buckets pick up the ore, carry it back and pile it up in big heaps that look like mounds of reddish earth.

THE MODERN BLAST FURNACE

Blast furnaces are towering cylindrical structures of steel lined with fire brick. They are loaded from the top with alternate layers of coke and ore. Limestone also is added to act as a flux for the earthy matter, as explained above. Running around the furnace near the base there is a large pipe known as the “bustle” pipe. Through this air is fed to a set of “tuyeres” which lead to the base of the furnace and admit blasts of air requisite to maintain combustion at an intense heat. The gases of combustion are not allowed to escape freely into the atmosphere. The top of the furnace through which the ore and fuel is admitted is closed by an air lock and the flaming hot gases are led into “stoves” where they give up a large part of their heat to preheat the air which is pumped to the blast furnace. The gases being mainly composed of carbon monoxide are further combustible and may be used for heat, light, and power purposes. In fact, they are commonly used to drive the air compressors which feed the blast furnaces.

The blast furnace has two openings, one above the other. Through the upper one slag is drawn off while the molten iron which trickles down and collects at the bottom of the furnace is tapped off through a hole near the base of the furnace. The fiery stream pours out into a lot of small trough-shaped molds and is thus formed into “pigs.” These pigs are all connected to the main body of the metal stream and must be broken off. To save the time of cooling and of breaking off the pigs a machine is used which consists of a series of molds connected to form an endless belt. The molten iron is poured into these molds which in their course dip into a trough of water. Here the iron is cooled and solidified. The molds then run up an incline and finally dump the pigs directly into railway cars which haul them away.

BURNING OUT THE CARBON

The production of steel economically and on a large scale dates back to the inventions of Henry Bessemer. While searching for an improved method of making big guns, Bessemer hit upon the idea of forcing a blast of air through the molten iron and thus burning away carbon, silicon, and manganese in the cast iron. No fuel was supplied except the carbon and silicon in the iron itself. In burning out this carbon sufficient heat was generated to keep the metal fluid.

When Bessemer made the announcement of his new process before the British Association in 1856, his paper met with skepticism, but he was able to demonstrate by actual experiment that cast iron could be converted into malleable iron in this way. However, when several firms operating under licenses from the inventor endeavored to reproduce his experiment on a commercial scale they were unsuccessful, and after costly experiments the process was given up as a failure. Bessemer, however, persisted in his efforts and succeeded eventually in producing malleable iron of a quality equal if not superior to that on the market. But iron makers after the failure of the first experiments would have nothing to do with the new process until Bessemer began to turn out quantities of iron at $100 a ton below the prevailing market price. Then iron makers woke up and Bessemer had no difficulty in placing his process with numbers of firms on a very profitable royalty basis.

This process pertained to the making of iron and not steel. When Bessemer tried to produce steel he was confronted with serious difficulties. The steel he obtained was very brittle. He tried purer ores with little better success. Then a solution of his problem was offered by Robert F. Mushet, who discovered a compound which would be added to the molten metal to purify it. This compound which is known as “spiegeleisen” is composed of iron, carbon, and manganese. It removes the oxide of iron and the sulphur and regulates the amount of carbon in the steel.

A Bessemer converter furnishes by far the most spectacular operation in steel manufacture. The converter consists of a large bottle-shaped vessel lined with refractory material. In the bottom of the vessel there are openings through which the air blast is admitted. The molten metal is poured into the flask and then the air blast is turned on. The metal begins to boil violently. A dazzlingly brilliant blast of flame and sparks comes roaring out of the mouth of the converter. Bubbles of metal are thrown high into the air where they burst into showers of sparks. The effect is similar to that of a volcanic eruption. In from ten to twenty minutes the eruption subsides and then a quantity of spiegeleisen is added. The converter is mounted on trunnions so that when the operation is completed the vessel is tilted over and the charge of molten metal now converted into steel is poured out.

OPEN HEARTH FURNACES

While the Bessemer converter provides a very economical and expeditious method of converting cast iron into steel, it is difficult to regulate the carbon content with great accuracy and hence the use of the open-hearth furnaces which furnish a slower method of burning out the carbon. Figure 74 is a diagrammatic representation of such a furnace. Below the hearth of the furnace there are two pairs of chambers, A, B and C, D, filled with a checkerwork of bricks. Gas is passed through one chamber A, and air through the other B, and they combine to form a very intense flame above the hearth E in which the metal is placed. The burnt gases pass over and through the other pair of chambers, C, D, on their way to the stack. By this means the bricks in the latter chambers are raised to a white heat. Then the process is reversed; air flows through the hot checkerwork of bricks in the chamber C and gas through the hot checkerwork in chamber D, and after combustion in the furnace the burnt gases are drawn through the bricks of the first pair of chambers. By alternating the direction of flow the air and gas fed to the furnace are always preheated by the stored-up heat of the previously burned gases. While it takes but a few minutes to convert cast iron into steel in the Bessemer converter, the open-hearth process occupies from eight to twelve hours.

The steel produced in the open-hearth furnaces is poured into ingot molds. These are approximately rectangular in section and slightly larger at the bottom than at the top. They are open at the top and bottom, but at the bottom rest upon a base plate. As soon as the steel has hardened the plunger of a stripping machine holds down the glowing ingot while a pair of hooks lift off the mold, leaving the ingot resting on the base plate.

ROLLING INGOTS INTO RAILS

In the manufacture of railroad rails the ingots are placed on a traveling “table” consisting of a series of rapidly turning rollers. These carry the ingot to a pair of large steel rolls between which it passes. The rolls compress the ingot slightly and it is automatically turned over and passed through a second pair of rolls. After passing through four “stands” of rolls, turning over between each stand, it is considerably reduced in cross-sectional area and correspondingly lengthened. It is now termed a “bloom.” The bloom goes through a series of rollers which gradually reduce its section until it is some forty feet long. Then it is cut in two and each section passes through other rolls, until finally it is reduced to the required rail section. Each section is then about a hundred and twenty feet long and the glowing writhing rail passes on to the saws where it is cut into ten-yard lengths. A similar process is employed in rolling other forms of rails and in making steel plates and sheets.

STEEL FOR BIG GUNS

The largest machines employed in the steel industry are those used for the manufacture of armor and big guns. A modern large high-powered gun is not a single solid casting or forging, but is made up of a series of steel tubes that are shrunk one upon another so that the inner tube is compressed. The reason for this is that the explosives used are so powerful that they would expand the inner tube or lining of the gun beyond its elastic limit and in that way enlarge the bore. By having it compressed to start with it can expand farther without exceeding the elastic limit. This expansion takes place so suddenly that the lining rebounds or returns to its original dimensions before the outer tubes have felt the full pressure and they too are thus prevented from being expanded too far. In some cases the compression is effected by winding the gun with a heavy wire of rectangular cross section.

SQUEEZING OUT THE “PIPES”

Steel for guns and armor is made in the open-hearth furnace where the quality of the metal may be regulated to a nicety. Gun tubes are cast in vertical molds and during the cooling of the ingot it is subjected to pressure so as to prevent segregation and the forming of “pipes.” Pipes are cavities that are liable to form in the center of the ingot due to contraction during cooling. Steel, as we have learned, is not pure iron, but an alloy, and the various constituents have different temperatures of solidifying, consequently they exhibit a tendency to segregate. It is to overcome such tendencies that a so-called “fluid” compressor is used. This is virtually a hydraulic press with a plunger that bears down on the fluid metal as it is solidifying. Modern big guns are enormously large. A sixteen-inch 50-caliber gun, for instance, is nearly seventy feet long, consequently the ingot must be even longer than this and the fluid compressor for so large a piece must be correspondingly powerful. After the ingot has been cast and cooled, the ends are cut off and it is bored to form a tube. Then it is placed in a furnace and raised to a white heat, after which a bar or mandrel is inserted in the bore and the tube is placed under the hydraulic forge press. This is a very powerful machine with an immense hammer that is actuated by hydraulic pressure. The stroke of the hammer is carefully regulated so that the forging as it is turned in the forge is subjected to equal blows. In the forge the tube is roughly formed to the dimensions it is eventually to have when finished. The process of forging subjects the metal to strains which must be relieved and so the tube has to go to the annealing oven where it is raised to a temperature which destroys crystallization. In this oven it is allowed to cool very slowly, letting the molecules of the metal adjust and rearrange themselves. When the temperature of the tube has been lowered to a certain point it is taken out and plunged into a bath of oil. This sudden cooling tempers the metal, giving it a high degree of elasticity and tensile strength. Again the tube must be annealed to relieve it of any strains occasioned by the tempering, and then it goes to the shop to receive its finish boring and turning.

The process as here briefly described seems simple enough, but we must not forget the enormous size of these pieces and their tremendous weight. They would be difficult enough to handle when cold, but much of the work is done while the pieces are at a white heat so that the men who control and operate the machinery that handles the big forgings must keep their distance. The casting, annealing, and tempering operations are performed with the piece in vertical position, and lofty machines and cranes are required to deal with these tall castings. A visit to a plant which manufactures big guns is bound to impress the visitor with awe and give him increased respect for the men who are able to handle such huge masses of metal and also for the men who have conceived and developed such gigantic operations.

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