CASES IN METALS 195 



inolLun and degassed, the ingots are p(jured by tilting the entire 

 furnace first to one side and then to the other. 



Rohn claims that by dividing the iron core into several parts, a 

 homogeneous field is obtained while there is only a small induction 

 effect in the metal casing. The primary coil, also, is composed of 

 four separate units which can be connected in series or energized 

 separately. Energizing only one unit of the primary, it is claimed, 

 causes vigorous stirring of the molten charge, thus facilitating the 

 degasification. When the units are energized in series, the uniform 

 field obtained causes but little stirring. 



One of the chief obstacles it was necessary to overcome during the 

 development of these large furnaces was lack of a satisfactory vacuum 

 casting procedure. Rohn and his co-workers found that, if a normal 

 casting procedure were followed after all the gases were removed from 

 a charge of metal, ingots were obtained with such large shrinkage 

 cav'ities that working them was impossible. When using iron or 

 sand molds the formation of these cavities could not be prevented. 

 A vertical water-cooled, copper mold was developed, however, that 

 was satisfactory when the melt was poured slowly. 



Another obstacle encountered was in obtaining a satisfactory lining 

 for the vacuum furnaces. A moistened material, tamped into place, 

 could not be used because of the difficulty in removing water vapor. 

 The method adopted consisted of packing the regular refractory, as a 

 dry powder, between the outside furnace wall and a template made of 

 the same metal as the charge to be melted. The temperature of the 

 charge was then so controlled that the refractory powder sintered 

 before the template melted. 



The Advantages of Vacuum Melting 

 In addition to freedom from blowholes in castings, one of the main 

 improvements effected by using vacuum furnaces for melting metals 

 is the degree of quality and composition control which can be attained. 

 With this method, no gas can interact with constituents of the melt 

 to cause composition changes, and the melt can therefore be kept 

 liquid for long intervals of time. This allows the suspended particles 

 of slag and oxides to rise to the surface, and results in ingots freer 

 from inclusions. Furthermore, the usual deo.xidation methods can 

 be dispensed with. For instance, the iron oxide in molten steel is 

 completely eliminated by reaction with carbon, and therefore, no 

 deoxidation is necessary. Likewise, for non-ferrous metals, no de- 

 oxidizers are needed, for during vacuum melting no oxidation of the 

 melt can occur. Thus, inclusions from these sources also are avoided. 



