IRON AND STEEL 239 



Wrought iron is a tough, ductile material showing an elongation of 

 from 18 to 30 per cent in 8 in. Its tensile and compressive strength 

 at the elastic limit is about 28,000 lb./in. 2 for high-grade wrought 

 iron, and about 23,000 lb./in. 2 for common wrought iron. Its max- 

 imum tensile strength varies from 44,000 lb./in. 2 to 64,000 lb./in. 2 . 

 The material is much more elastic than cast iron, its modulus of 

 elasticity in tension being about 28,000,000 lb./in. 2 , and in shear 

 about 10,000,000 lb./in. 2 . 



191. Manufacture of steel. Tool steel is made by recarbonizing 

 wrought iron by heating it in a charcoal fire for several days at a 

 temperature of about 3000 F. During this process part of the carbon 

 is absorbed by the iron, the product being known as Ulster steel. 

 This is then melted and cast into ingots, from which the merchant- 

 able bars are rolled or hammered. The two steps in this process are 

 usually combined into one. 



Open-hearth steel is obtained by mixing molten pig iron with scrap 



iron or scrap steel in an open-hearth furnace. The added scrap is low 



in carbon, and thus lowers the percentage of carbon in the mixture. 



To offset this, the desired amount of carbon is introduced by adding 



/eleisen. 



Bessemer steel is made directly from pig iron in a Bessemer con- 

 verter, no additional fuel other than the impurities in the metal 

 being used. These impurities are burned out to the desired extent 

 by forcing jets of hot air through the liquid metal. Since in this 

 method the molten iron is taken directly from the blast furnace, a 

 considerable saving in the cost of production is effected, by reason of 

 which the Bessemer process has revolutionized the steel industry. 



In both the open-hearth and Bessemer processes the liquid steel is 

 cast into ingots, which are rolled into the desired shapes. 



192. Composition of steel. The physical properties of steel are 

 largely modified by the relative proportions in which the various 

 ingredients are present. 



Carbon. Increasing the amount of carbon in steel has, in general, the 

 effect of increasing its modulus of elasticity and its ultimate strength. 

 From a series of tests made on carbon steel, in which the percentage 

 of carbon varied from 0.08 to 1.47, Professor Arnold found that the 

 elastic limit varied from 27,300 lb./in. 2 to 72,300 lb./in 2 . ; the tensile 



