Steel and Iron 431 



total value of the metal production of $823,483,390. The value 

 of the iron alone amounted to about $425,115,390, or more 

 than 50 per cent of the total. In the year 1913, the United 

 States' production of iron was approximately 61,924,000 tons. 



I. PROPERTIES OF IRON 



206. Why iron is useful. What are the properties of iron 

 which make it the most useful of metals? The commonly 

 recognized properties are : (1) tensile strength ; (2) crushing or 

 compression strength ; (3) bending resistance ; (4) hardness ; 

 (5) relatively light weight ; (6) rigidity ; and (7) fusing or 

 melting point. 



These properties may be considered here in the most general 

 manner. Examples of each property in some steel or iron 

 structure should be found to make the subject definite and 

 concrete. 



The tensile strength of wrought iron, which is almost pure iron, 

 is, on the average, such that a bar with a cross section area of 

 one square inch will support a weight of about 50,000 pounds. 



The crushing strength of cast iron, as used in the pillars of a 

 building, is estimated at about 40 tons to the square inch, or 

 about five times the tensile strength of cast iron. What is 

 meant by the tensile strength of cast iron ? 



Bending resistance may be suggested by a girder or beam 

 supported at both ends, as in a bridge or a floor, and the weight 

 applied at the middle. Iron or steel supports twice the load 

 of any other metal in common use. 



Hardness is necessary in almost every use of iron or steel. It 

 must be hard enough to withstand a certain amount of wear- 

 through friction, and resist being scratched or marred excessively 

 by other hard objects. Hardness is tested by the amount of 

 pressure exerted upon a diamond point, and the depth of the 

 scratch it makes in the steel. 



Iron is one of the lightest of the common metals. Volume for 

 volume (page 343), it is only four fifths as heavy as copper, three 



