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592

BESSEMER'S PROCESS-MANUFACTURE OF STEEL.

soon as the carbon is burnt off; when this occurs the melted iron nearly freed from carbon is run off into moulds. A great loss of iron is, however, incurred in this operation; a copious slag of oxide of iron, mixed with a little silicate, is produced, and in this a large quantity of metallic iron is entangled: not less than 20 per cent. of the metal is thus wasted, and the malleable iron still retains nearly all the phosphorus and much of the sulphur originally present.

But though the process of Bessemer has not been attended with the important results which were anticipated from its employment in refining the ordinary pig-iron obtained by the smelting of clay iron-stone with coal, it is stated to have been eminently successful when applied to the pure Swedish charcoal pig-iron, which has by its means been converted by a single operation of short duration into cast steel of the finest quality: as much as 5 tons of iron are commonly operated on at one fusion.

(750) Production of Wrought Iron direct from the Ore.—The pure ores, which consist of magnetic oxide, or of peroxide of iron, are frequently converted at once into wrought iron, without the production of cast iron. This process is practised in the Pyrenees, by what is termed the Catalan forge, and still more largely by the bloomery forges of North America. In the American bloomery forge either the hot or the cold blast may be employed :-The ore having been first reduced by stampers to a coarse powder, is placed on the top of the coal in the forge which has been kindled for its reception; a high heap of coal is kept on the fire, and a gradual supply of ore is maintained; as the metal is reduced, it sinks to the bottom in a pasty state; when sufficient has been added to form a bloom, or ball, the metal is collected on an iron bar, heated before the blast-pipe, and then hammered, rolled, and welded, as if it had come from the puddling furnace (Overman's Metallurgy, p. 544). This method yields a very pure iron when charcoal is employed, but the consumption of fuel per ton of metal is much greater than in the blast furnace; a large portion of the ore is also wasted in the form of slags, which are very rich in oxide of iron. The iron produced by this process frequently contains sufficient carbon to give to it some of the properties of steel; for instance, it becomes much harder when heated and suddenly cooled. Iron of this description is valuable in the manufacture of plough-shares, and heavy articles requiring both toughness and hardness.

(751) Manufacture of Steel.-Iron, when combined with a smaller proportion of carbon than is contained in cast iron, fur

MANUFACTURE OF STEEL.

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nishes the valuable compound well known as steel, of which there are several varieties. The quantity of carbon in good steel varies between 07 and 17 per cent.; but steel which possesses the greatest tenacity has been found to contain from 13 to 15 per cent. of carbon, and about o'1 of silicon. Natural steel is produced directly from the best cast iron by heating it by means of charcoal on the refining hearth, as in the operation which precedes the process of puddling; the oxygen burns off a portion of the carbon from the cast iron, and steel is left. In some of the Welsh iron works steel is now made upon the bed of the puddling furnace itself, by carefully arresting the operation at a stage short of the complete oxidation of the carbon. The preparation of natural or puddled steel is, therefore, like the Bessemer steel, an intermediate stage in the conversion of cast into wrought iron. Iron which contains manganese is best fitted for the preparation of this kind of steel. The mass thus obtained is rendered homogeneous by forging. It yields a steel of inferior quality, which is employed for making agricultural implements and springs for machinery.

For more delicate purposes blistered steel is made use of: this is obtained by means of cementation, which is an operation just the reverse of that by which natural steel is formed.

This process is carried on in a furnace into which two rectangular boxes of brickwork or stoneware, for the reception of the bars of iron which are to be converted into steel, are built; the fire-grate is between these boxes, around which the flame circulates freely. This conversion is effected by heating the iron in contact with powdered charcoal, or with soot; the carbonaceous matter in either case is usually mixed with about a tenth of its weight of common salt and wood ashes, forming what is technically termed cement powder. In preparing a charge, the bottom of each box is covered with a layer of the cement powder to a depth of about an inch, and upon this a layer of bars of the best malleable iron is placed. The bars are generally about 3 inches broad, and inch thick. The interstices 2 between the bars are also filled with cement powder, which is tightly packed around the iron; above this is a layer of the powder, then another layer of bars, and so on in succession until the box is nearly full, when it contains from 5 to 6 tons of iron. The remaining space is now covered with a layer of damp sand of from 3 to 6 inches in depth, and the fire is gradually raised to a full red heat, or to about the temperature required for melting copper; at this point it is steadily maintained. One of the bars of iron is so placed that it can be removed from time to time during the operation, for the purpose of ascertaining the process of the carbu

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BLISTERED STEEL-CEMENTATION.

ration by inspection. The process is usually complete in six or eight days; but the time required necessarily varies with the thickness of the iron bars operated on; the fire is then gradually reduced, and the furnace is suffered to cool slowly, an operation which lasts ten days or a fortnight. The steel thus obtained retains the form of the iron, but it is covered with blebs or blisters, by which the surface is rendered irregular and uneven. The mass is found to have been penetrated by carbon which has been transferred from particle to particle of the metal, the properties of which it has completely changed. In some cases these blisters probably arise from the combination of parts of the carbon with oxygen derived from particles of oxide of iron, which are apt to be mechanically retained even in the most carefully prepared bars. Carbonic oxide would thus be produced, and imprisoned in the tenacious metal, which in its softened state would be raised by it into bubbles or blebs. Great care, however, is generally taken to exclude slag and oxide of iron from bars which it is intended to convert into steel: so that in the majority of instances it is not unlikely that the blisters are occasioned by the combination of carbon with the sulphur which is still retained by the iron, and which, by forming the volatile bisulphide of carbon would produce the effect (T. H. Henry). All bar iron contains traces of sulphur; but in steel sulphur is seldom present, and there appears to be no other mode of accounting for its general absence than its removal during the process of carburation in the form of bisulphide of carbon.

By the process of cementation the iron has been combined with about 15 per cent. of carbon: it is now much more fusible than before.* It has likewise entirely lost its fibrous texture; and

* According to Fremy, steel contains also as a necessary ingredient a minute quantity of nitrogen, which it has been suggested may be in the form of cyanogen. Caron, however, maintains that this trace of nitrogen is not essential this, however, is still sub judice. Rammelsberg was unable to detect more nitrogen than 20 parts in a million of cast iron. Marchand heated both cast iron and steel with potassium in an atmosphere of hydrogen, and also heated the metal with soda-lime he also burned the metal by heating it with oxide of copper, but did not obtain more than 150 parts of nitrogen from a million parts of the metal, and often a much smaller quantity. He considered it due to the accidental presence of foreign impurities; and this is certainly the most probable opinion; no instance is known in which so minute a quantity of matter is an essential constituent of any compound. Boussingault found pure iron reduced from the oxide in hydrogen gave no trace of nitrogen, by a method of analysis which indicated in soft iron 50 millionths, and in piano wire from 70 to 86 millionths, and in cast steel 57 millionths: this steel also contained traces of sulphur. The difficulty of excluding such minute traces of nitrogen in the course of the analysis is extreme, even in the hands of one who, as in this case, is confessedly a master. (See p. 621).

VARIETIES OF STEEL.

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Steel

when broken across exhibits a close, fine-grained fracture. may also be made without direct contact with carbon, by simply heating the bars in carburetted hydrogen; but this process has not come into general use.

Blistered steel is never homogeneous, the surface being always more highly carburetted than the inner portions of the bars. This variety of steel is employed for files, tools, and hardware of all descriptions. When blistered steel is fused, it forms cast steel, which, from being more uniform in texture, is of superior quality, as the carbon is more equally distributed throughout the mass: it is employed for cutlery of the best description. Tilted steel is also obtained from blistered steel; this is first broken up into lengths of about 18 inches, then bound into fagots and raised to a welding heat in a wind furnace, where it is covered with sand, which combines with the superficial coating of oxide of iron and forms a fusible slag: the red-hot fagot is then rolled, and forged, by means of the tilt-hammer, into smaller bars. All steel is improved by this process of hammering. These tilted bars, when broken up and welded together, form shear steel.

For many purposes, the addition of a small quantity of manganese is an improvement to the quality of the steel. If about I per cent. of carbide of manganese, or of a mixture of charcoal and oxide of manganese, be introduced into the melting-pot, a steel is obtained of fine, close grain, which admits of being welded to wrought iron; a property not possessed by ordinary steel. The experiments of Faraday and Stodart led them to the conclusion that the addition of small quantities of chromium, or of rhodium, to good steel, furnished a steel of a superior kind. They found that steel may be alloyed with about a five hundredth of its weight of silver; and with platinum, as well as with rhodium, and with osmium and iridium in all proportions. The combination of 8 or 9 per cent. of tungsten with ordinary steel has been said to yield a material remarkable for hardness and elasticity, but experience does not seem to justify the expectations of its utility. (Percy's Metallurgy, vol. ii. p. 193.) A similar remark is also applicable to titanium steel (Ib. p. 168). When steel is to be used for the manufacture of dies for coining, the presence of a small proportion of phosphorus is beneficial (Brande).

When diluted nitric acid falls upon steel, a dark grey spot is produced, owing to the solution of the metal in the acid whilst its carbon remains unacted upon: the acid produces a green spot upon iron. The acid acts unequally upon different parts of the surface in certain of the finer varieties of steel, and thus produces

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QUALITIES OF STEEL-TEMPERING.

a veined appearance, such as was formerly given to the celebrated Damascus blades. The Damascus steel is more highly carburetted than ordinary steel, and if allowed to cool slowly, it separates into layers of two different degrees of carburation (Bréant); hence certain parts, when acted on by diluted acid, leave more carbon than others; the form and direction of these veins vary with the mode of forging adopted.

Wootz is a finely damasked, hard cast steel, of excellent quality, which is obtained from India. Faraday found aluminum in a sample of this steel which he analysed, and was disposed to refer its peculiar qualities to the presence of this metal. It appears, however, from the experiments of Henry (Phil. Mag., July, 1852), that aluminum is not always present in wootz. He gives the following as the composition of a bar of genuine Indian wootz, of specific gravity 7.727 :

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Other analysts have also failed in finding aluminum in wootz. The physical properties of steel differ materially from those of iron. As already mentioned, steel is granular in texture, brittle, and more easily melted than iron. Its most characteristic property, however, consists in its power of assuming a hardness scarcely inferior to that of the diamond when heated to redness and then suddenly cooled by plunging it into water, mercury, or oil. After this treatment it is rendered extremely brittle, and almost perfectly elastic. It can then no longer be attacked by the file.

This extreme hardness and brittleness may be removed by the process of tempering, which is a peculiar mode of annealing; it consists in heating the steel moderately, and then allowing it to cool. The tempering of steel is an operation of great practical importance, as from the variety of purposes to which steel is applied, it is required of very different degrees of hardness, and upon the due adjustment of this quality much of its utility depends. The degree to which the temperature is raised in the second heating, regulates this point: the higher the heat, the softer is the steel. In practice, the workman judges with sufficient accuracy of the temperature to which the metal has been

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