MAKING AND WORKING OF WROUGHT IRON 397 



448. Thermit Welding. Thermit welding depends upon 

 the chemical combination of certain substances which pro- 

 duce a great heat. It is particularly successful in combining 

 broken parts of machines. The work is carried out by build- 

 ing around the two ends, a mold, into which the molten 

 metal or steel generated by this process is poured. New metal 

 is thus added to the ends and the parts are united. 



449. Drop Forging. The bending and shaping of large 

 pieces of iron and steel is called drop forging. An oblong 

 block of steel case in a steel foundry is called an ingot. As 

 soon as the steel is set in the mold, the hot mass is taken by 

 a crane from the ingot mold and placed in a soaking pit to 

 be annealed. While the metal is setting the exterior of the 

 ingot becomes chilled. The soaking pit is a brick mold with 

 all air excluded. When the ingot is placed in the mold, the 

 interior heat of the ingot reheats the exterior surface, and 

 causes it to soften or anneal. When this process is finished, 

 the ingot is carried by a crane to the steam hammer, where 

 it is drawn out roughly to shape. The number of times 

 it may be necessary to heat the ingot depends upon the 

 amount of work to be done on it and the speed at which 

 that work is to be accomplished. 



450. Steam Hammer. The steam hammer is a forge 

 hammer consisting of a steam cylinder placed vertically over 

 an anvil. The trip or hammer-head rises and falls by the 

 power of steam. The trip is controlled by a lever, which is 

 generally operated by a hammer boy. The work is carried 

 to the hammer by means of hooks, and held in place by 

 special tongs. A large portion of steam hammer work is 

 drawn out to templates. 



