198 AGRICULTURAL ENGINEERING 



Soft-Center Steel. The ability of carbon steel to be 

 hardened depends largely upon the percentage of carbon 

 it contains. When hardened, it will take a most excellent 

 polish, as is desired for plows, but hardened steel is brittle 

 and will not stand shocks. To overcome this shortcoming, 

 soft-center steel has been invented, which consists of a layer 

 of soft steel between two layers of high-carbon steel. This 

 soft, low-carbon steel lends toughness to the whole plate. 

 Soft-center steel is quite generally used at the present time 

 in the manufacture of shovels and plows. 



Tool Steel. Tool steel contains a rather high percentage 

 (0.6 to 1) of carbon, is capable of being hardened and tem- 

 pered, and has a very close, dense structure. It is used in 

 the manufacture of hand tools, such as hammers, chisels, 

 etc. A discussion of the strength of materials will be found 

 in Part VII. 



QUESTIONS 



1. Why is it important that a good quality of material be used in 

 the construction of farm machinery? 



2. Discuss the merits of wood as a material for farm machinery. 



3. Describe some of the special uses for wood. 



4. Why is hickory used for wagon axles and buggy spokes? 



5. Compare white oak with hickory. 



6. Suggest some good uses for maple, beech, birch, poplar, and 

 white and yellow pine. 



7. What are some of the properties of cast-iron? 



8. Describe the process of making chilled iron. Malleable iron. 



9. For what purposes is cast steel used? 



10. Why are Bessemer and mild steel used so largely in the con- 

 struction of farm machinery? 



11. Describe soft-center steel and its uses. Also tool steel. 



12. What is tool steel, and mention some of its properties? 



Note: Samples of the various materials used in the construction of 

 farm machinery should be collected, and machines should be examined 

 to determine the materials used. 



