44 FARM MACHII 2RY 



the pitman of a mower. Iron, and especially steel, when 

 subjected to shocks tends to become crystallized. This 

 reduces its strength very much. 



66. Cast iron is used for the larger castings and most 

 of the^ gears used in farm machines. At one time cast iron 

 was used to a larger extent than at the present time, as 

 it is being superseded by stronger but more expensive 

 materials. Cast iron is of a crystalline structure and can- 

 not be forced or have its shape changed in any other way 

 than by the cutting away of certain portions with" 

 machine tools. Cast iron has a high carbon content, buT 

 the carbon is held much as a mechanical mixture rather 

 than in a chemical combination. 



67. Gray iron is the name applied to the softer and 

 tougher grade of cast iron, which is easily worked by 

 tools ; and white iron to a very hard and brittle grade. 

 White iron is used for pieces where there are no changes 

 to be made after casting. 



68. Chilled iron. When it is desired to have a verv 

 hard surface to a casting, as the face of a plow, the in- 

 side of a wheel box, or other surfaces subjected to great 

 wear, the iron is chilled when cast by having the molten^ 

 iron come in contact with a portion of the mold made 

 up of heavy iron, which rapidly absorbs the heat. Chilled 

 iron is exceedingly ham! 



69. Malleable iron is cast iron which has been annealed 

 and perhaps deprived of some of its carbon, changing it 

 from a hard, brittle material to a soft, tough, and some- 

 what ductile metal. The process of decarbonation usually 

 consists in packing castings with some decarbonizing 

 agent, as oxide of iron, and baking in a furnace at a high 

 temperature for some time. Malleable iron is much 

 more expensive and more reliable than common cast 

 iron. 



