MAKING AND WORKING OF WROUGHT IRON 391 



436. The Effect of Drawing and Rolling. When iron bars 

 are rolled, their molecules become stretched into a fibrous con- 

 dition. Rolling gives to the metal, especially when thin as in 

 boiler plates, a greater tensile strength in the direction of its fi- 

 bers. Wire drawings and cold rolling (passing over cold rolls 

 of steel) increase the tenacity and hardness of wrought iron, 

 but annealing returns it to its original strength and softness. 



437. Case-Hardening. The process of hardening the 

 surface of iron or steel is called case-hardening. The 

 piece to be treated is first heated to a bright red and the 

 surface rubbed with prussiate of potash. When it has cooled 

 to dull red, it is immersed in cold water. When the piece is to 

 be highly finished, the finish may be obtained by heating it 

 in a cast iron pot containing red-hot lead. A good case-hard- 

 ening mixture may be made with three parts prussiate of 

 potash and one of sal ammoniac. The temperature for cherry 

 red is given at 1832 F. Machine nuts, set screws, wrenches, 

 and collets are samples of such work. 



438. Characteristics of Wrought Iron. Wrought iron 

 possesses one of the most valuable properties of metals; small 

 masses of it will weld or unite into one. No other metals 

 except platinum and aluminum possess this property. If 

 two pieces of iron are heated to a white heat, they become 

 sticky or viscous so that when hammered together they ad- 

 here and may be perfectly united by forging. This kind of 

 iron is used by blacksmiths and, unlike cast iron, may be ham- 

 mered into any desired shape. Because of its greater strength, 

 wrought iron is used for making bars, plates, wires, structural 

 material, and parts of machinery. It is tougher, stretches 

 more, and gives more warning before fracture than does 



