294 METALS AND THEIR COMBINATIONS. 



from 0.16 to 2 per cent,, wrought- or bar-iron but very small quanti- 

 ties, of carbon. Wrought-iron is made from cast-iron by the process 

 known as puddling, which is a burning-out of the carbon by oxida- 

 tion, accomplished by agitating the molten mass in the presence of 

 an oxidizing flame. Steel is made either from cast-iron by partially 

 removing the carbon, or from wrought-iron by recombining it with 

 carbon i. e., by agitating together molten wrought- and cast-iron in 

 proper proportions. 



Properties. The high position which iron occupies among the useful metals 

 is due to a combination of valuable properties not found in any other metal. 

 Although possessing nearly twice as great a tenacity or strength as any of the 

 other metals commonly used in the metallic state, it is yet one of the lightest, 

 its specific gravity being about 7.7. Though being when cold the least yield- 

 ing or malleable of the metals in common use, its ductility when heated is such 

 that it admits of being rolled into the thinnest sheets and drawn into the finest 

 wire, the strength of which is so great that a wire of one-tenth of an inch in 

 diameter is capable of sustaining TOO pounds. Finally, iron is, with the ex- 

 ception of platinum, the least fusible of all the useful metals. 



For certain articles, such as armor plate, rock breakers, lathe tools, etc., steel 

 is hardened by alloying it with small quantities of certain other metals, chiefly 

 with chromium, nickel, or manganese. 



Iron is little affected by dry air, but is readily acted upon by moist air, when 

 ferric oxide and ferric hydroxide (rust) are formed. 



Hardening and tempering steel. Steel contains carbon both in the ele- 

 mentary state as graphite, and chemically combined as iron carbide. Within 

 certain limits the tenacity of the metal, and its hardness after having been 

 heated and suddenly cooled, bear a direct ratio to the amount of combined 

 carbon. The more of the latter is present the harder is the steel and vice versd. 



If steel be heated to redness and suddenly chilled it has attained its maxi- 

 mum hardness; if, however, it be permitted to cool slowly after heating, it 

 becomes soft. Any degree of hardness between these extremes can be obtained 

 by the process known as tempering or " letting down." It consists in carefully 

 reheating the previously hardened metal to a certain temperature and then 

 plunging it into cold water. To the experienced worker the required temper- 

 ature is indicated by a series of colors appearing successively on the surface of 

 the steel. These colors are due to a gradually thickening film of iron oxides 

 while the iron softens. The colors pass successively from pale yellow through 

 several shades of darker yellow to brown, purple, blue, and bluish black. The 

 highest temperature gives the least hardness and vice versd. 



In hardening steel, prior to tempering, care should be taken not to injure 

 the metal by overheating, which causes oxidation of the carbon and blisters the 

 metallic surface, rendering a fine temper impossible. In tempering small 

 instruments a coating of some material, such as soap, is necessary to prevent 

 oxidation as far as possible. 



Elasticity and tenacity desired for specific purposes, as in the case of springs, 

 is imparted to steel by hammering. This causes a condensation of the particles 



