Nasmyth's Steam-hammer.
This machine enlarged at one bound the whole scale of working in iron, and permitted Maudsley's lathe to develop its entire range of capacity. The old 'tilt-hammer' was so constructed that the more voluminous the material submitted to it, the less was the power attainable; so that as soon as certain dimensions had been exceeded, the hammer became utterly useless. When the Great Western steamship was in course of construction, tenders were invited from the leading mechanical firms for the supply of the enormous paddle-shaft required for her engines. But a forging of the size in question had never been executed, and no firm in England would undertake the contract. In this dilemma, Mr Nasmyth was applied to, and the result of his study of the problem was this marvellous steam-hammer—so powerful that it will forge an Armstrong hundred-pounder as easily as a farrier forges a horse-shoe, and so delicately manageable that it will crack a nut without bruising its kernel!
BESSEMER STEEL.
In 1722, Réaumur produced steel by melting three parts of cast-iron with one part of wrought iron (probably in a crucible) in a common forge; he, however, failed to produce steel in this manner on a working scale. This process has many points in common with the Indian Wootz-steel manufacture.
As we have seen, to Benjamin Huntsman, a Doncaster artisan, belongs the credit of first producing cast-steel upon a working scale, as he was the first to accomplish the entire fusion of converted bar-iron (that is, blister-steel) of the required degree of hardness, in crucibles or clay pots, placed among the coke of an air-furnace. This process is still carried on at Sheffield and elsewhere, and is what is generally known as the crucible or pot-steel process. It was mainly supplementary to the cementation process, as formerly blister-steel was alone melted in the crucibles; but latterly, and at the present time, the crucible mode of manufacture embraces the fusion of other varieties and combinations of metal, producing accordingly different classes and qualities of steel.
In 1839, Josiah Marshall Heath patented the important application of carburet of manganese to steel in the crucible, which application imparted to the resulting product the properties of varying temper and increased forgeability. He subsequently found out that a separate operation was not necessary to form the carburet—which is produced by heating peroxide of manganese and carbon to a high temperature—but that the same result could be attained by simply in the first instance adding the carbon and oxide of manganese direct to the metal in the crucible. He unsuspectingly communicated this after-discovery to his agent—by name Unwin—who took advantage of the fact that it was not incorporated in the wording of the patent, and so was unprotected, to make use of it for his own advantage. The result was one of the most remarkable patent trials on record, extending over twelve years, and terminating in 1855 against the patentee—a remarkable instance of the triumph of legal technicalities over the moral sense of right.
A very important development of the manufacture of steel followed the introduction of the 'Bessemer process,' by means of which a low carbon or mild cast-steel can be produced at about one-tenth of the cost of crucible steel. It is used for rails, for the tires of the wheels of railway carriages, for ship-plates, boiler-plates, for shafting, and a multitude of constructional and other purposes to which only wrought iron was formerly applied, besides many for which no metal at all was used.
Sir Henry Bessemer's process for making steel, which is now so largely practised in England, on the continent of Europe, and in America, was patented in 1856. It was first applied to the making of malleable iron, but this has never been successfully made by the Bessemer method. For the manufacture of a cheap but highly serviceable steel, however, its success has been so splendid that no other metallurgical process has given its inventor so great a renown. Although the apparatus actually used is somewhat costly and elaborate, yet the principle of the operation is very simple. A large converting vessel, with openings called tuyères in its bottom, is partially filled up with from 5 to 10 tons of molten pig-iron, and a blast of air, at a pressure of from 18 to 20 lb. per square inch, is forced through this metal by a blowing engine. Pig-iron contains from 3 to 5 per cent. of carbon, and, if it has been smelted with charcoal from a pure ore, as is the case with Swedish iron, the blast is continued till only from .25 to 1 per cent. of the carbon is left in the metal, that is to say, steel is produced. Sometimes, however, the minimum quantity of carbon is even less than .25 per cent. In England, where a less pure but still expensive cast-iron—viz. hæmatite pig—is used for the production of steel in the ordinary Bessemer converter, the process differs slightly. In this case the whole of the carbon is oxidised by the blast of air, and the requisite quantity of this element is afterwards restored to the metal by pouring into the converter a small quantity of a peculiar kind of cast-iron, called spiegeleisen, which contains a known quantity of carbon. But small quantities of manganese and silicon are also present in Bessemer steel. The 'blow' lasts from 20 to 30 minutes. Steel made from whatever kind of pig-iron, either by this or by the 'basic' process, is not sufficiently dense, at least for most purposes, and it is accordingly manipulated under the steam-hammer and rolled into a variety of forms. Bessemer steel is employed, as we have said, for heavy objects, as rails, tires, rollers, boiler-plates, ship-plates, and for many other purposes for which malleable iron was formerly used.
Basic steel is now largely made from inferior pig-iron, such as the Cleveland, by the Thomas-Gilchrist process patented in 1878. It is, however, only a modification of the Bessemer process to the extent of substituting for the siliceous or 'acid' lining generally used, a lime or 'basic' lining for the converter. Limestone, preferably a magnesian limestone in some form, is commonly employed for the lining. By the use of a basic lining, phosphorus is eliminated towards the end of the 'blow.' Phosphorus is a very deleterious substance in steel, and is present, sometimes to the extent of 2 per cent., in pig-iron smelted from impure ore.