CAST IRON. 381 



having the properties of wrought iron, except that they 

 are homogeneous and without the appearance of fibre. 

 These give a tensile strength of about 27 tons per square 

 inch. 



When added to steel, aluminium makes it possible to 

 produce much sounder castings that without it. 



The Effect of Temperature on the strength of steel is 

 shown graphically on Fig. 184. 



CAST IRON. 



202. The chief mechanical properties of cast iron, as 

 distinguished from wrought iron and steel, are its non- 

 malleability at all temperatures, brittleness, fusibility, high 

 compressive strength, and low tensile strength. 



The chemical constituents of this material are 

 chemically pure iron, carbon (either combined or mechani- 

 cally mixed), silicon, sulphur, phosphorus, manganese, and 

 arsenic. Of these, the sulphur, phosphorus, arsenic, and 

 silicon remain as impurities, which are not eliminated 

 during the processes of manufacture; the carbon and man- 

 ganese remain as important ingredients, whose presence 

 largely determines the mechanical qualities of the iron. 



The percentage of carbon present in cast iron, either 

 combined or mixed, varies from l - 5 to 6*0 per cent, 

 according to the process by which the iron has been 

 produced. It is customary for ironfounders not to make 

 the castings from one kind of pig only, but to mix two or 

 more kinds in varying proportions, so as to produce a 

 finished material having the necessary degree of hardness, 

 toughness, or tensile strength, as the case may be. 

 The following table gives the constituents of eight 

 samples of pig from different districts : 



