396 APPLIED SCIENCE 



on the anvil. Another form of forge or pressure welding is 

 seen when two pieces of metal are heated by an electric 

 current, and then forced together. This latter process is 

 known as butt or spot welding. 



445. Autogenous Welding. Autogenous welding is ap- 

 plied to welds which are made by heating metals to such a 

 temperature that they will fuse together on contact, without 

 the application of pressure. The difference between forge 

 welding and autogenous welding lies mainly in the difference 

 in the temperature of the metals. In the autogenous weld, 

 the metal is heated to a state of fluidity, and the two pieces 

 flow together. This name is also applied to the process of 

 adding molten metal to other metal pieces, thus building up 

 or filling defects in the latter. There are different methods 

 of autogenous welding by the electric arc, by the oxyacety- 

 lene flame, by the thermit method. 



446. Electric Arc Welding. Electric arc welding consists 

 in joining two pieces of metal by filling in the cracks with 

 molten metal. This is done by attaching or laying upon the 

 steel or metal to be welded one wire of the current, and at- 

 taching the other wire to a piece of carbon or metal which the 

 welder holds. The current passes from the piece which is to 

 be welded to the electrode which the operator holds. Great 

 heat is generated and consequently the electric arc is one of 

 the most efficient methods of producing welding heat. 



447. Oxyacetylene Welding. Oxyacetylene welding is 

 one of the most practical methods of welding. The heat is de- 

 rived from burning acetylene in oxygen gas. Oxyacetylene 

 flame is also used in cutting metal. The flame actually cuts 

 the metal by melting it very rapidly. 



