382 APPLIED SCIENCE 



Another serious drawback in the use of cast iron is its 

 liability to initial stresses from inequality in cooling after it is 

 poured into the molds. That is, if one part of the casting is 

 very thin and another very thick, the thin part cools first, 

 and in cooling contracts; the thick part, cooling afterwards, 

 causes stresses in the thin part, which may be sufficient to 

 break it; or if not quite that strong the stress may be so great 

 in the thin part that a small additional force will break it. 



In the construction of machines and structures, no metal 

 used by engineers varies so much in strength and soundness 

 as cast iron; hence it is particularly important to know its 

 physical properties. Cast iron may, if of inferior quality, 

 have a tensile strength of only 5 tons per square inch, or even 

 less. When this is the case, it has no value where strength is 

 required; it may, however, be used for balance weights, 

 foundation blocks, or for purposes where weight alone is of 

 consequence. Some samples may have a strength as high as 

 19 tons, but the average strength is 7 tons. The strength of 

 iron castings may be increased by the addition of vanadium. 

 The high compressive strength of cast iron makes it desirable 

 for use in columns and posts of buildings. 



Questions 



1. Why is it necessary to design a machine carefully before 

 building it? 



2. What arc Tcrgings? Why are they used? 



3. What is a pattern? 



4. Where is iron obtained? 



5. Name some of the principal iron ores. 



6. How is free iron obtained from the ore? What is smelting? 



7. What is a blast furnace? 



8. What is a flux? 



9. Draw a sketch of a blast furnace. 



