Page:Collier's New Encyclopedia v. 09.djvu/94

STEEL 68 STEEL erally from 2.5 to about 5 per cent, of carbon, steel having from .05 to 1.80, and the proportion in malleable iron varying from .016 up to about ,34; but there are other circumstances which also go to determine the nature and quality of any of the kinds. The methods by which steel of various oualities is now produced are numerous, but they may all be included under these heads: (1) The partial decarbonization of cast iron; (2) the addition of carbon to malleable iron; and (3) the complete decarbonization of pig iron, and the ad- dition of the necessary carbon by means of pig iron or other kinds rich in carbon to the molten mass. By the first of these processes what are known as natural steel and puddled steel are produced. Natural steel is made from pure varie- ties of pig iron (white or fusible gray pig, from spathic or magnetic ores) by melting it with charcoal on the hearth of a refining furnace, as in the corre- sponding operation in the manufacture of malleable iron ; the operation being so controlled that it can be stopped when the desired amount of decarbonization has been effected. Natural steel, known also as Corinthian steel, from having been largely made in that province, was formerly in great request on the Continent for tool making. Puddled steel is a similar but less pure product, made also direct from cast iron on the bed of a puddling furnace, the puddling being stopped before the complete oxida- tion of the carbon. In this process, also, the only difference from the analogous stage in malleable iron manufacture con- sists in stopping the operation at an earlier stage. The steel of the famous works of Krupp at Essen is made by this process, and it is largely used for machine castings, railway wheel tires, and other large works. The kinds of steel produced by the imparting of carbon to malleable iron are known as cementation steel, and the varieties included under this head em- brace the most valuable classes of steel. The production of cementation steel de- pends on the fact that when malleable iron is heated to a high temperature in contact with carbonaceous matter with- out access of air, it absorbs carbon, and thus becomes converted into steel. Bar iron of the highest quality only is used for making cementation steel, and the bars are embedded in powdered charcoal and packed in a chamber heated by a furnace. The temperature is gradually raised, and kept tmiformly at a full red heat (about 2,000° F.) for about a week, at the end of which time the furnace is allowed to cool slowly down, which requires another fortnight. To make it homogeneous, the bars are faggoted, heated to welding heat, and hammered or rolled, the resulting mate- rial being "shear steel." When, instead of being hammered, the blister steel is melted in crucibles and cast in molds, it forms "crucible cast steel," the purest of all varieties of steel, and that which is principally used for files, cutlery, and fine tools generally. Siemens-Martin steel is produced by what may be regarded as a modification of the cementation process, since in it malleable iron is carburetted by means of pig iron or other compound rich in carbon. On the hearth of an open fur- nace heated by gas on the regenerative system a quantity of pig iron is melted, and to that scrap iron, old rails, etc., heated to redness, are gradually added. When by the examination of test pieces it is found that the quantity of carbon is sufficiently reduced, by the addition of malleable iron, for the kind of steel re- quired, the charge is run off'. The Sie- mens-Martin process is also conducted in a manner modified as follows: The pig iron and malleable scrap are melted to- gether, an operation which occupies from two and one-half to three hours, and thereupon a quantity of iron ore (haema- tite) is added to the furnace, which in giving off its combined oxygen decarbon- izes the entire charge. When the decar- bonization is found to be complete, from 8 to 10 per cent, of spiegeleisen (or about 2 per cent, of ferromanganese) , previ- ously raised to a red heat, is thrown into the furnace. After about 10 minutes this will be thoroughly dissolved, the car- bon disseminated through the mass, and the charge ready for being tapped off. Steel so made is now largely used for rails, wagon wheels and axles, and it is being introduced for boilers, ships' plates, and other similar purposes. Bessemer steel forms the subject of a separate article. Steel presents physical characteristics so distinct from those of malleable iron that it may be for prac- ticable purposes regarded as a distinct metal. Its most remarkable physical characteristic is its power of becoming exceedingly hard by sudden cooling from a high heat, and of passing through, many grades of hardness downward from that point by the process of tempering. The tempering is accomplished by reheating the metal to a certain degree and allow- ing it gradually to cool, and according to the heat to which it is raised is the re- sulting temper, the higher the heat the softer being the steel. Sir Joseph Whit- worth introduced a method of compress- ing with enormous force cast steel in its fluid state, and demonstrated that thereby the tensile strength and homogeneity of