446 Introduction to the Study of Science 



with different grades of iron. Such a combination is known as 

 the duplex process. 



Cementation and crucible processes. For the production of 

 the special quality of steel used in making fine tools and instru- 

 ments, the cementation process is sometimes used. The general 

 practice is to melt an almost carbonless wrought iron with 

 enough fine charcoal to give the desired grade of carbon steel. 

 Such steel contains from one to one and three fourths per cent 

 of carbon. The product is sometimes called crucible steel. 

 Strictly crucible steel, however, is made by heating molten iron 

 in graphite crucibles, from which the iron absorbs as much car- 

 bon as is desired. 



The electric furnace. The use of the electric furnace in 

 manufacturing steel has gradually increased since its first 

 introduction into this country about 1908. The annual output, 

 which was for the first year only fifty-five tons, now exceeds 

 sixty thousand tons. During this period more than twenty 

 furnaces have been constructed, some of which have as large 

 a productive capacity as the largest open-hearth furnace. By 

 the electric furnace such impurities as sulfur and phosphorus 

 are thoroughly removed and a steel of the highest quality is 

 produced. Although because of its quality and the cost of 

 production, electric steel was at first expensive, it is now possible 

 to make it as cheaply as any steel can be made. 



216. Kinds of steel. Steel is thus seen to be a semi-purified 

 form of iron. It retains or is supplied with a certain percentage 

 of silicon when it is to be used for steel castings. To secure the 

 right degree of hardness and strength it is supplied with or 

 retains a certain percentage of carbon from that supplied in 

 the reduction of the ore. Carbon determines the degree of 

 hardness, or the kind of steel produced. The smaller the per- 

 centage of carbon, the softer is the steel ; and the greater the 

 carbon content within a limited range, the harder is the steel. 

 Thus steels are classified as soft to hard, or low-carbon to high- 

 carbon steels. They are often designated, for example, as .27, 



