356 STRENGTH OF STEEL. 



shown on Fig. 174, and it will be noticed that it is similar 

 in appearance to that of a fine-grained cast iron. On 

 annealing a broken half of this bar, and again testing it, 

 the fracture was restored to the appearance of Fig. 175. 

 The compression stress on this tyre, 26 tons per square 

 inch, was in excess of that required. 



The fractures, before and after treatment, under a 

 more moderate compression, are shown on Figs. 176 and 

 177. 



THE STRENGTH OF STEEL. 



1 85 . As regard s its chemical con stituents, steel occupies 

 a position midway between wrought iron, which may be 

 regarded as commercially pure iron, and cast iron, which 

 contains a comparatively large proportion of carbon, 

 either combined or mechanically mixed. The percentage 

 of carbon present in cast iron varies from 1*5 to 6'0 per 

 cent., and in steel begins where cast iron ends, the hardest 

 containing 1 P 5 per cent, of carbon, and the softest kinds 

 about 0-1 per cent. 



Steel may be broadly divided into two classes, viz., 

 " hard " or " cast " steel, used in the manufacture of tools 

 of various kinds, and springs ; and what is known as 

 " mild steel," employed in the construction of boilers, 

 bridges, ships, and most large metallic structures. The 

 former of these two classes is generally made by the 

 carburisation of bars of wrought iron, afterwards melted 

 in crucibles, cast into ingots, and forged and rolled into 

 bars. On the other hand, the mild variety is made by 

 several processes, in all of which cast iron is decarburised. 

 Briefly, the processes used in the production of mild 

 steel are as follows : 



(a) The ordinary Bessemer process, in which the 

 silicon and carbon are burnt out of the cast iron, and 

 afterwards a sufficient amount of carbon is added in 

 the form of ferromanganese or some other compound of 

 iron, carbon, and manganese. In this process the 

 converter or vessel in which the process is carried out is 

 supplied with an acid lining. 



