William L. Ilgen

178. Refining Pig Iron.—Two distinct methods have been adopted for the conversion of pig iron into wrought iron, each depending upon the kind of furnace used. They are called the open-hearth or finery process, and the puddling process. The chemical reactions are similar in both processes, being based on the oxidation of the impurities in the metal. This is accomplished both by means of the oxygen in the air supplied and by the oxide of iron in the fluxes that are added to assist the operation.

179. The Open-hearth or Finery Process.—This is carried on in what is sometimes termed a “bloomery” from the product which is called a bloom. The pig iron is placed in direct contact with the fuel on the hearth which is formed of cast-iron plates exposed to a current of air to keep them cool. This mixture of the iron with the fuel is objectionable, because while the fuel acts as a reducer the excess air decarbonizes the product only partly, besides prolonging the process considerably; by the addition of hammer scale and rich slag, the operation is hastened greatly. However, if some carbon is supposed to be contained in the product, making it of a steely nature, then the open-hearth process is considered a good method of refining.

Fusion is allowed to take place gradually, so as to expose the metal for a long period to the oxygen of the blast. At the moment of fusion the foreign elements are rapidly oxidized and form a fusible slag. After the slag becomes neutral and has been partly removed, fresh basic slag and hammer scale are added, to hasten the operation. Then the mass of iron, which is now of a white spongy texture, is lifted up in the furnace to a level with the tuyÈre, in order that the combined carbon may be completely oxidized. It is then formed into balls of about 60 to 80 pounds each, after which it is removed and formed into a bloom by means of a squeezer or hammer. This furnace is not illustrated, because most of the wrought iron is produced by the puddling process. An open-hearth furnace, such as is used in producing steel, is somewhat similar to the one here described and is shown in Fig. 163.

180. The Puddling Process.—The greatest amount of wrought iron is produced from pig iron by this process, owing to the superior quality of the product. The term “puddling” was originally applied to the process of working iron that had never been completely melted, but had only reached a puddled or pasty state. But later, when refined or pig iron was similarly treated, it was discovered that it would melt perfectly and boil up freely. The process was then termed “pig boiling.”

The furnace used in this process is of the reverberatory type; the fuel does not come in contact with the iron. (See Fig. 150.) It is built in a rectangular form; the fireplace A is located at one end, next to it is the hearth C where the metal is placed, and beyond are the flue B and the chimney D.

From the fireplace the heat is supplied and directed upon the metal by the top or roof, which is curved downward from the fireplace toward the flue and chimney. The fireplace also is separated from the hearth by a partial partition wall E, called the fire bridge, which prevents the fuel from coming in contact with the metal. Another similar partition F, located between the hearth and the flue, prevents the metal from going into the latter and is called the flue bridge.

Fig. 150.—Puddling Furnace of a Reverberatory Type.

Both of these and all interior portions that come in contact with the heat and metal are constructed of fire brick. The bridges are built over hollow iron castings, through the openings of which there is a circulation of water provided to keep them cool. The bottom of the hearth is formed of iron plates rabbeted together; this and the sides are sometimes provided also with hollow castings for water circulation.

The hearth is lined with blue billy and the sides with bulldog. The former is a fusible silicate, chiefly ferric oxide, and is produced from tap cinder; it does not readily unite with silica when heated. Bulldog is made from burnt pyrites, a quality of ore used for the manufacture of sulphuric acid; the resulting oxide is sometimes called blue billy, but more frequently bulldog, to distinguish it from the former class of oxides. Both of these linings are known as fettlings.

The flue slopes down toward the stack; the draft is regulated with a damper, located in the top and connected by a chain, which hangs within reach of the operators. Various forms of furnaces are used, such as stationary and rolling furnaces; but whatever the style of furnace, the process is based on the decarbonization of the metal, and the charge of pig iron does not come in direct contact with the fuel, as in the open-hearth process. An advantage gained in using the puddling furnace is that various kinds of fuel can be employed without injury to the product of iron, also various labor-saving devices, which have recently been invented, can be better used.

In the pig-boiling process the furnace is first lined with the fettlings and charged with about 500 pounds of white foundry or forge pig iron. The refining process is divided into four distinct stages known as melting down, mixing, boiling, and balling.

A very high temperature is desired during the first stage, which usually lasts about thirty-five minutes. During this time the melting down occurs, and a partial removal of the silicon from the pig iron is effected.

In the second or mixing stage, which lasts about seven minutes, a comparatively low temperature is maintained by lowering the damper in the stack, while the charge is being thoroughly mixed with the oxidizing fluxes or cinders that are added. The puddler draws down the metal from around the sides into the center, where it will become more rapidly refined and mixed.

During the third or boiling stage the damper is raised to increase the temperature. At this time a violent reaction occurs, caused by the release of carbonic oxide, which is formed when the oxygen unites with the carbon in the pig iron. The gas escapes through the slag on the surface of the metal, thus causing it to appear as though it were boiling, from which action the process derives its name. During this stage, which lasts from twenty to twenty-five minutes, a large portion of the manganese, sulphur, and phosphorus contained in the pig iron is removed.

The oxidation is assisted by the constant stirring or rabbling of the metal by the puddler, done for the purpose of bringing it under the oxidizing influence of the air. The boiling gradually ceases, and the surface of the charge “drops,” as it is called, and the whole mass lies in a pasty state on the bed of the furnace, where it is worked by the puddler as thoroughly as possible so as to allow the flames to pass uniformly over it.

The fourth or balling stage requires from fifteen to twenty minutes. This consists of breaking up the contents into balls weighing from 60 to 80 pounds each. After they have been formed, they are rolled near the fire bridge to receive a final welding heat before they are removed to the squeezer, or hammer, where the slag is expelled and the bloom formed.

The blooms from either the open-hearth or puddling process are treated similarly in what is termed the forge; this includes hammering, rolling, and shingling.

Fig. 151.—Rolling Tool Steel.

Squeezers or hammers are used for forming the bloom and expelling the inclosed slag. The bloom is then put through the largest groove of the roughing rolls and passed back through the next smaller, and so on until it is rolled down to the desired size. Figure 151 shows 14-inch rolls in use which, although somewhat similar to those employed for rolling iron, are larger and generally made with more rolls.

The product of this first rolling is not usually considered of superior quality, so, in order to refine it more thoroughly, the bars are cut up into short lengths, piled into bundles, reheated, and again welded. This process is called shingling and is done two or three times, depending upon the desired quality of iron. This shingling produces the laminÆ of the iron referred to in section 60. For ordinary bar iron the piles are made about 2 feet long by 4 inches square, and for larger sizes they may be made 5 or 6 feet long by 10 or 12 inches square.

The rolls are of various kinds. All shapes and sizes of bar iron used in blacksmithing may be produced in this manner. Rolling machines are known as two, three, and four high, meaning that they are provided with that number of rolls, one above the other. Universal rolling machines have two pairs of rolls in one machine; one pair runs on horizontal axes and the other on vertical axes. Each pair can be opened or closed independently, thus giving the machine a wide range.

181. Steel.—The word “steel” means very little to those who are uninformed as to its different qualities and the causes of the distinctions between them. People are generally familiar with the various purposes for which steel is used, but know very little about its nature. There are, however, great differences in the qualities, and definite reasons for them.

Formerly any combination of iron and carbon that would harden by sudden cooling or quenching was considered steel. But since modern methods of manufacturing have been adopted, tons of metal, which would have been classed as iron if judged by the cooling test, are at present known as mild or soft steel.

Steel may properly be defined as an alloy of iron with carbon, the latter not exceeding 1.8 per cent; the materials are completely fused and poured into molds, allowed to cool, and then rolled into shape. In the processes of making wrought iron the materials are only partly fused and are not cast into molds, but are taken out of the furnace in a soft, pasty condition suitable for immediate working.

The older process of producing “blister” or “cementation” steel is not generally employed now. By this method the bars of iron were put through a soaking or prolonged heating, while they were packed in charcoal. It was similar to the casehardening process, explained in section 90.

We have at present three notable processes of making steel; namely, the crucible, Bessemer, and open-hearth.

182. The Crucible Process.—Crucible furnaces are flat structures containing from two to twenty holes, each one capable of receiving four or six crucibles. The crucibles are earthen vessels made of fire clay, mixed with refractory materials for withstanding intense heat. Each one is capable of receiving from 70 to 80 pounds of metal. (See Fig. 152.) In this furnace the gas and air supply may be applied independently to each hole, practically making each one a separate furnace, but all of the holes are connected with one main stack or chimney. A sectional view of a four-hole furnace is shown in Fig. 153, where the crucibles are shown in position.

This process is the most simple. It consists of melting the stock in the crucibles and pouring it, when completely fused, into molds, as shown in Fig. 154, forming what is known as ingots or steel castings. For that reason it is very frequently called cast steel. The stock is carefully selected and weighed so as to produce the required grade. After the ingots are cooled, the piped or hollow ends caused by shrinkage are broken off and graded by the appearance of the granular structure and luster of the fractured parts. They are then marked and piled away for future use. On the ingots shown in Fig. 155, the piped ends can be seen.

The ingots are heated in an ordinary heating furnace, and rolled or hammered into suitable bars, the sizes being fixed both by the amount of carbon contained in the ingots and by the dimensions required for the manufacture of special tools. Figure 151 shows the workmen in the act of rolling tool steel; in Fig. 156 they are seen drawing octagon tool steel with the tilting hammer.

Special alloys of crucible steel such as Mushet, blue chip, high speed, or other special brands are made by the same process, the secret of the difference lying entirely in the selection of the stock.

183. The Bessemer Process.—This consists of blowing air through molten pig iron in a vessel called a converter, sectional views of which are shown in Figs. 157 and 158. A converter is a pear-shaped structure hung on trunnions A, A, so that it can be tipped forward. The air is forced through one of the trunnions, which is hollow, and is connected with a pipe which conveys the air to the air chamber f at the bottom of the converter. The bottom grate or tuyÈre plate is located directly above the air chamber, and through the openings j, j, in the tuyÈre plate, the air passes up through the metal.

The converter is tipped forward into a horizontal position while the molten metal is poured into it. The air is then turned on, and the converter is raised to a perpendicular position. The air passes up through the entire charge of iron; consequently the metal is thoroughly acted upon, while in the open-hearth process it is not. The Bessemer process is based on oxidation; it produces a very high temperature and keeps the charge in a liquid state during the time of blowing. This is continued until the sulphur and phosphorus are removed or the charge becomes decarbonized,—a condition termed burned steel, owing to the presence of dissolved oxygen. This condition is then changed or recarbonized by adding manganese alloys, such as spiegeleisen or ferromanganese, which give the necessary amount of carbon. By these additions the iron is changed into steel.

The Bessemer process requires a very short time in comparison with the puddling process. Three tons of pig iron can be refined in about twenty minutes, while by the puddling process the same amount of metal requires about twenty-four hours.

Considerable excitement was caused at the time the process was invented, not only on account of the time saved, but also because there was such a great saving in fuel.

After the metal has been poured from the converter into molds similar to those shown in Fig. 159, and has cooled sufficiently to become solid, the molds are stripped off, as shown in Fig. 160, and the ingots of metal placed in the soaking pits, Fig. 161. These pits are somewhat similar to a crucible furnace and are used for reheating ingots before they are slabbed or rolled. Such a furnace is generally made of the regenerative type and is divided into several compartments, each one capable of receiving several ingots which are inserted on end.

From the pit furnace the ingots are taken and rolled into slabs, rails, blooms, or other forms suitable for use. When the plant is equipped with both blast furnace and converter, this is all done without additional heating, but when the plant is not so equipped, the pig iron is melted in a cupola furnace before being put into the converter. A blooming mill is shown in operation in Fig. 162.

184. The Open-hearth Process.—Fig. 163. Here again the process depends on the type of furnace. Open-hearth steel is produced with a reverberatory furnace, and the heat is supplied by regenerative gas and air. The furnace is built mostly of brickwork with the exception of the supporting beams, doors, tie rods, and hearth castings, which are made of cast iron, wrought iron, or steel. All brickwork that comes in contact with the intense heat is made of silica brick, manufactured from rock crystals, flint, or other varieties of quartz rock with about two per cent of quicklime. The roof of the furnace slopes toward the center, so that when the air and gas enter they are directed downward on the charge of metal. The bottom or hearth is constructed of heavy steel plates riveted together and supported on I beams. This bottom is first covered with a layer of brick, then sand is applied to about the thickness of one inch and well rammed down, then other layers of brick and sand are added until the thickness is about 14 to 16 inches. This bottom requires repairing with more sand between successive heats. Figure 164 shows a cross section through the center of the charging and discharging openings.

When the furnace has been charged, the gas and air are allowed to enter at intervals of fifteen minutes, first from one side, then from the other. When the air and gas enter one side, the exhaust or waste gases pass out through the other side. The reversing is done by means of levers which open and close the valves. A sectional view is given in Fig. 165, showing the air and gas chambers and the brick checker work through which the air and gas pass and are heated. The broken lines represent the passages leading to these chambers; the valves are also shown.

When the metal has been fused sufficiently, a sample is dipped out and analyzed, so that its composition may be known and sufficient carbonizing material added to produce the desired quality. This is not possible with the Bessemer process. It is finally tapped into a large ladle, from which it is poured into molds forming the ingots, which are treated in the same way as described in the Bessemer process. The discharging is shown in Fig. 166.

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