316

PRODUCTION OF STEEL BY CEMENTATION.

which is employed almost exclusively in this country consists in combining bar-iron with the requisite amount of carbon by what is technically known as cementation, the bars being imbedded in charcoal and exposed for several days to a high temperature.

The operation is effected in large chests of fire-brick or stone, about 10 or 12 feet long by 3 feet wide and 3 feet deep.

Two of these chests are built into a dome-shaped furnace (converting furnace, fig. 235), so that the flame may circulate round them, and the furnace is surrounded with a conical jacket of brick-work in order to allow a steady temperature to be maintained in it for some days. The charcoal is ground so as to pass through a sieve of inch mesh, and spread in an even layer upon the bottom of the chests. Upon this the bars of iron, which must be of the best quality, are laid in regular order, a small interval being left between them, which is afterwards filled in with the charcoal powder, with a layer of which the bars are now covered; over this more bars are laid, then another layer of charcoal, and so on until the chest is filled. Each chest holds 5 or 6 tons of bars. One of the bars is allowed to project through an opening in the end of the chest, so that the workmen may withdraw it from time to time and judge of the progress of the operation. The whole is covered in with a layer of about 6 inches of damp clay or sand.

The fire is carefully and gradually lighted, lest the chests should be split by too sudden application of heat, and the temperature is eventually raised to about the fusing point of copper (2000° F.), at which it is maintained for a period varying with the quality of steel which it is desired to obtain. Six or eight days suffice to produce steel of moderate hardness; but the process is continued for three or four days longer if very hard steel be required. The fire is gradually extinguished, so that the chests are about ten days in cooling down.

On opening the chests, the bars are found to have suffered a remarkable change both in their external appearance and internal structure. They are covered with large blisters, obviously produced by some gaseous substance raising the softened surface of the metal in its attempt to escape. It is conjectured either that the small quantity of sulphur present in the bar-iron is converted into bisulphide of carbon during the cementation process, and that the vapour of this substance swells the softened metal

SHEAR STEEL-CAST STEEL.

317

into bubbles as it passes off; or that the blisters are caused by carbonic oxide produced by the action of the carbon upon particles of slag accidentally present in the bar. On breaking the bars across, the fracture is found to have a finely granular structure, instead of the fibrous appearance exhibited by bar-iron. Chemical analysis shows that the iron has combined with about 1.5 per cent. of carbon, and the most remarkable part of the result is that this carbon is not only found in the external layer of iron, which has been in direct contact with the heated charcoal, but is also present in the very centre of the bar. It is this transmission of the solid carbon through the solid mass of iron which is implied by the term cementation. The chemistry of the process probably consists in the formation of carbonic oxide from the small quantity of atmospheric oxygen in the chest, and the removal of one-half of the carbon from this carbonic oxide, by the iron, which it converts into steel, leaving carbonic acid (2CO - C CO2) to be reconverted into carbonic oxide by taking up more carbon from the charcoal (CO2 + C = CO), which it transfers again to the iron. Experiment has recently shown that soft iron is capable of absorbing, mechanically, 4.15 volumes of carbonic oxide at a low red heat, so that the action of the gas upon the metal may occur throughout the substance of the bar. The carbonic oxide is retained unaltered by the iron, after cooling, unless the bar is raised to the temperature required for the production of steel.

The blistered steel obtained by this process is, as would be expected, far from uniform either in composition or in texture; some portions of the bar contain more carbon than others, and the interior contains numerous cavities. In order to improve its quality, it is subjected to a process of fagotting similar to that mentioned in the case of bar-iron; the bars of blistered steel, being cut into short lengths, are made up into bundles, which are raised to a welding heat, and placed under a tilt-hammer weighing about 2 cwt., which strikes two or three hundred blows in a minute; in this way, the several bars are consolidated into one compound bar, which is then extended under the hammer till of the required dimensions. The bars, before being hammered, are sprinkled with sand, which combines with the oxide of iron upon the surface, and forms a vitreous layer which protects the bar from further oxidation. steel which has been thus hammered is much denser and more uniform in composition; its tenacity, malleability, and ductility are greatly increased, and it is fitted for the manufacture of shears, files, and other tools. It is commonly known as shear steel. Double shear steel is obtained by breaking the tilted bars in two, and welding these into a compound bar.

The

The best variety of steel, however, which is perfectly homogeneous in composition, is that known as cast steel, to obtain which, about 30 lbs. of blistered steel are broken into fragments, and fused in a fire-clay crucible, heated in a wind-furnace, the surface of the metal being protected from oxidation by a little glass melted upon it. The fused steel is cast into ingots, several crucibles being emptied simultaneously into the same mould. Cast steel is far superior in density and hardness to shear steel, but since it is exceedingly brittle at a red heat, great care is necessary in forging it. It has been found that the addition, to 100 parts of the cast steel, of one part of a mixture of charcoal and oxide of manganese, produces a very fine grained steel which admits of being cast on to a bar of wrought-iron in the ingot-mould, so that the tenacity of the latter may

compensate for the brittleness of the steel when the compound bar is forged, the wrought-iron forming the back of the implement, and the steel its cutting edge.

This addition of manganese to the cast steel (Heath's patent) has effected a great reduction in its cost, allowing the use of blister steel made from British bar-iron, whereas, before its introduction, only the expensive iron of Swedish or Russian make could be employed.

After the steel has been forged into the shape of any implement, it is hardened by being heated to redness, and suddenly chilled in cold water, or oil, or mercury. It is thus rendered nearly as hard as diamond, at the same time increasing slightly in volume (sp. gr. of cast steel 7.93, after hardening, 7.66). The chemical difference between hard and soft steel appears to be of the same kind as that between grey and white cast-iron (p. 307), the great proportion of the carbon in hard steel being in combinaation with the metal, while in soft steel the greater part seems to be in intimate mechanical admixture with the iron, for it is left undissolved on treating the steel with an acid. If the hardened steel be heated to redness, and allowed to cool slowly, it is again converted into soft steel, but by heating it to a temperature short of a red heat, its hardness may be proportionally reduced. This is taken advantage of in annealing the steel or "letting it down" to the proper temper. The very hardest steel is almost as brittle as glass, and totally unfit for any ordinary use, but by heating it to a given temperature and allowing it to cool, its elasticity may be increased to the desired extent, without reducing its hardness below that required for the implement in hand. On heating a steel blade gradually over a flame, it will acquire a light yellow colour when its temperature reaches 430° F., from the formation of a thin film of oxide; as the temperature rises, the thickness of the film increases, and at 470° a decided yellow colour is seen, which assumes a brown shade at 490°, becomes purple at 520°, and blue at 550°. At a still higher temperature the film of oxide becomes so thick as to be black and opaque. Steel which has been heated to 430°, and allowed to cool slowly, is said to be tempered to the yellow, and is hard enough to take a very fine cutting edge, whilst, if tempered to the blue, at 550°, it is too soft to take a very keen edge, but has a very high degree of elasticity. The following table indicates the tempering heats for various implements :

If a knife blade be heated to redness, its temper is spoilt, for it is converted into soft steel.

In general, the steel implements are ground after being tempered, so that they are not seen of the colours mentioned above, except in the case

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watch-springs.

BESSEMER STEEL-SPIEGEL-EISEN.

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A steel blade may be easily distinguished from iron by placing a drop of diluted nitric acid upon it, when a dark stain is produced upon the steel, from the separation of the carbon.

Some small instruments, such as keys, gun-locks, &c., which are exposed to considerable wear and tear by friction, and require the external hardness of steel without its brittleness, are forged from bar-iron, and converted externally into steel by the process of case-hardening, which consists in heating them in contact with some substance containing carbon (such as bone-dust, yellow prussiate of potash, &c.) A process which is the reverse of this is adopted in order to increase the tenacity of stirrups, bits, and similar articles made of cast-iron; by heating them for some hours, in contact with oxide of iron or manganese, their carbon and silicon are removed in the forms of carbonic oxide and silicic acid, and they become converted into malleable cast-iron.

The opinion that steel owes its properties entirely to the presence of carbon is not universally entertained. Some chemists believe that nitrogen (or some analogous element) is an indispensable constituent, but the proportion of nitrogen found in steel is too minute to warrant this supposition. Titanium is alleged by some authorities to have an important influence upon the quality of steel, but this also appears to be a doubtful matter. Bar-iron may be converted into steel by being kept at a high temperature in an atmosphere of coal-gas, from which it abstracts carbon.

Bessemer steel was originally produced by arresting the purification of cast-iron in Bessemer's process (page 314), as soon as the carbon had diminished to about 1.5 per cent., when the steel was poured out in the fused state. i.e., in the form of cast steel. A steel of better quality, however, has been obtained by continuing the purification until liquid bar-iron remains in the converter, and introducing the proper proportion of carbon in the form of a peculiar description of white cast-iron known as Spiegeleisen (mirror iron), which crystallises in lustrous tabular crystals, and contains large proportions of carbon and manganese, being obtained by smelting spathic iron ore rich in manganese, with charcoal as fuel. The Spiegel-eisen is added, in a melted state, to the Bessemer iron before pouring from the converter.

The composition of a sample of Spiegel-eisen smelted from a spathic ore, found near Müsen in Prussia, is here given :

Homogeneous iron, as it is called, is really a mild steel containing a low percentage of carbon, and obtained by fusing the best Swedish bar-iron with carbonaceous matters. It is remarkable for its malleability and toughness, and, having undergone complete fusion, it is more likely to be homogeneous in composition and structure than wrought-iron produced by puddling.

Parry's steel is manufactured by melting bar-iron with fuel free from sulphur and phosphorus, so as to obtain a very pure cast-iron, which is

320

EXTRACTION OF WROUGHT IRON FROM THE ORE.

then partly decarbonised by a process similar to Bessemer's. The addition of manganese improves its quality.

Puddled steel is obtained by arresting the puddling process at an earlier stage than usual, so as to leave a proportion of carbon varying from 0.5 to 10 per cent.

Natural steel or German steel results in a similar way, from the incomplete purification of cast-iron in the refinery. The presence of manganese in the iron is favourable to its production.

Krupp's cast steel, manufactured at Essen near Cologne, and employed for ordnance, shells, &c., is a puddled steel made from hæmatite and spathic ore, smelted with coke. The iron thus obtained contains much manganese, which is removed in the puddling process. Krupp's steel contains about 1.2 per cent of combined carbon, and is fused with a little bar-iron for casting ordnance. The fusion is effected in black lead crucibles holding 30 lbs. each, of which as many as 1200 are emptied simultaneously into the mould for the largest castings. A casting of 16 tons requires about 400 men, who act together in well-disciplined gangs, so that the stream of molten metal shall flow continuously along the gutters into the mould. Such large castings must be allowed to cool very gradually, so that they are kept surrounded with hot cinders, sometimes for two or three months, till required for forging.

216. Direct extraction of wrought-iron from the ore.-Where very rich and pure ores of iron, such as hæmatite and magnetic iron ore, are obtainable, and fuel is abundant, the metal is sometimes extracted without being

Fig. 236. Catalan forge for smelting iron ores.

converted into cast-iron. It is probable that the iron of antiquity was extracted in this way, for it is doubtful whether cast-iron was known to