Before the introduction of this furnace, attempts had been made to produce cast-steel without the use of a crucible--that is to say, on the 'open hearth' of the furnace. Reaumur was probably the first to show that steel could be made by fusing malleable iron with cast-iron. Heath patented the process in 1845; and a quantity of cast-steel was actually prepared in this way, on the bed of a reverberatory furnace, by Sudre, in France, during the year 1860. But the furnace was destroyed in the act; and it remained for Siemens, with his regenerative furnace, to realise the object. In 1862 Mr. Charles Atwood, of Tow Law, agreed to erect such a furnace, and give the process a fair trial; but although successful in producing the steel, he was afraid its temper was not satisfactory, and discontinued the experiment. Next year, however, Siemens, who was not to be disheartened, made another attempt with a large furnace erected at the Montlucon Works, in France, where he was assisted by the late M. le Chatellier, Inspecteur-General des Mines.
Some charges of steel were produced; but here again the roof of the furnace melted down, and the company which had undertaken the trials gave them up. The temperature required for the manufacture of the steel was higher than the melting point of most fire-bricks. Further endeavours also led to disappointments; but in the end the inventor was successful. He erected experimental works at Birmingham, and gradually matured his process until it was so far advanced that it could be trusted to the hands of others. Siemens used a mixture of cast-steel and iron ore to make the steel; but another manufacturer, M. Martin, of Sireuil, in France, developed the older plan of mixing the cast-iron with wrought-iron scrap. While Siemens was improving his means at Birmingham, Martin was obtaining satisfactory results with a regenerative furnace of his own design; and at the Paris Exhibition of 1867 samples of good open-hearth steel were shown by both manufacturers. In England the process is now generally known as the 'Siemens-Martin,' and on the Continent as the 'Martin-Siemens' process.
The regenerative furnace is the greatest single invention of Charles William Siemens. Owing to the large demand for steel for engineering operations, both at home and abroad, it proved exceedingly remunerative.
Extensive works for the application of the process were erected at Landore, where Siemens prosecuted his experiments on the subject with unfailing ardour, and, among other things, succeeded in making a basic brick for the lining of his furnaces which withstood the intense heat fairly well.
The process in detail consists in freeing the bath of melted pig-iron from excess of carbon by adding broken lumps of pure hematite or magnetite iron ore. This causes a violent boiling, which is kept up until the metal becomes soft enough, when it is allowed to stand to let the metal clear from the slag which floats in scum upon the top. The separation of the slag and iron is facilitated by throwing in some lime from time to time. Spiegel, or specular iron, is then added; about 1per cent. more than in the scrap process. From 20 to 24 cwt. of ore are used in a 5-ton charge, and about half the metal is reduced and turned into steel, so that the yield in ingots is from 1 to 2 per cent. more than the weight of pig and spiegel iron in the charge. The consumption of coal is rather larger than in the scrap process, and is from 14 to 15cwt. per ton of steel. The two processes of Siemens and Martin are often combined, both scrap and ore being used in the same charge, the latter being valuable as a tempering material.
At present there are several large works engaged in manufacturing the Siemens-Martin steel in England, namely, the Landore, the Parkhead Forge, those of the Steel Company of Scotland, of Messrs. Vickers & Co., Sheffield, and others. These produced no less than 340,000 tons of steel during the year 1881, and two years later the total output had risen to half a million tons. In 1876 the British Admiralty built two iron-clads, the Mercury and Iris, of Siemens-Martin steel, and the experiment proved so satisfactory, that this material only is now used in the Royal dockyards for the construction of hulls and boilers.
Moreover, the use of it is gradually extending in the mercantile marine.
Contemporaneous with his development of the open-hearth process, William Siemens introduced the rotary furnace for producing wrought-iron direct from the ore without the need of puddling.
