THE MAKING AND WORKING OF STEEL 403 



treatment removes all the carbon, since the oxygen in the 

 air combines with the carbon of the iron to form carbon 

 dioxide. From 5 to 10% manganese is then added and the 

 blowing is resumed long enough to incorporate the mixture. 

 The molten mass is ladled into iron molds to form ingots. 

 As these are more or less porous they are reheated and run 

 through a clogging mill or steam hammer and are finally 

 rolled or forged into shape. This kind of steel is named 

 from Bessemer, the inventor of the process, and is largely 

 used for structural purposes. 



The disadvantage of this process is that no sulphur or phos- 

 phorus is removed from the metal, so that it is necessary to 

 use a grade of iron very nearly free from these impurities, 

 if the steel is to have the required properties for tools, etc. 



460. Basic Bessemer Process. A modification of the 

 foregoing method, by which the phosphorus is removed 

 from the iron, is known as the basic Bessemer process. Its 

 special feature is that calcined or burnt lime is added to 

 the charge just before pouring. The lime unites with the 

 phosphorus forming phosphate of lime, removing the 

 phosphorus from the steel and bringing it into the slag. 

 The removal of the phosphorus makes it possible to use an 

 inferior grade of iron. 



461. Siemens-Martin Steel. In the Siemens-Martin 

 process, steel is produced either by melting a certain quan- 

 tity of pig iron in the hearth of a reverberatory furnace and 

 adding wrought iron till the bath attains the desired degree of 

 carbonization, or by mixing cast iron and certain kinds of iron 

 ores. The oxides of the ores are removed and carbon and 

 manganese are added by the introduction of a small quantity 



