FARM MACHINERY 197 



structure and it can not be forged or welded. It is 

 shaped by machine tools, by drilling, turning, or planing. 

 It is used for the heavy parts of machines, for gears or 

 where irregular shapes are desired, which may be obtained 

 by casting molten iron. Cast-iron may be usually de- 

 tected by the lines and roughness given to it by the sand 

 mold in which it is cast. It is easily detected upon breaking 

 by the crystalline structure. 



Chilled Cast-iron. Where a particularly hard surface 

 is desired, a special kind of cast-iron is used, obtained by 

 making a part of the mold of heavy iron, which chills the 

 molten metal as soon as it comes in contact with it and 

 makes it very hard. 



Malleable iron is cast-iron which has been annealed 

 and relieved of a part of its carbon by heating in furnaces 

 for several days. Malleable iron is soft, tough, and some- 

 what ductile, and is used to replace cast-iron where these 

 characteristics are required. When broken, malleable iron 

 shows a soft malleable surface and a crystalline center. 



Cast Steel. Cast steel is, in brief, cast-iron less a part of 

 the carbon. It is less brittle than cast-iron, and is used for 

 gears and other parts subject to severe stresses. 



Mild and Bessemer Steel. Most of the material now 

 used in the construction of farm machinery is mild or Besse- 

 mer steel, which is made by a special process. It is a very 

 tough metal whose stiffness can be regulated by the manu- 

 facturer by varying the carbon content. It can be easily 

 forged, but does not weld as readily in an open fire as 

 wrought iron. 



Wrought Iron. Wrought iron is nearly pure iron. It 

 is very ductile and can be easily forged or welded. The 

 purest and best grade of wrought iron is known as Norway 

 or Swedish iron. 



