450 BELL SYSTEM TECHNICAL JOURNAL 



the iron-nickel solid solution. The alloy can therefore be hot-rolled 

 to small section under controlled conditions to develop a desired grain size, 

 and then cold-worked to separate the individual crystals. Grain size 

 depends upon the degree of refinement in the hot-rolUng operation and 

 upon the distribution of the sulphide film around the grain boundaries. 

 Final pulverization is accomplished in an attrition mill, and the product 

 is generally annealed to soften the particles. 



The powder is then treated to cover each of the particles with an insu- 

 lating film that is generally of the ceramic type. The cores are then pressed 

 at about 100 tons per square inch to develop proper density and strength. 

 There is no sintering treatment performed on this type of material after 

 pressing, but the cores are generally annealed to remove pressing strains 

 and restore magnetic quality. 



Fig. 5 — Relative size of cores for a given duty in a telephone circuit. A is a core made 

 of electrolytic iron powder; B is made of 80% Ni-permalloy powder; C is 2-81 Mo- 

 permalloy.**^ 



This powder metal process is thus different from those previously de- 

 scribed and is one of the specialty group of materials which cannot be 

 prepared by any other methods. Except for the deliberate coating of 

 the metal particles with an insulating fihn, and the avoidance of a sintering 

 operation, however, the procedure is that normally followed in preparing 

 powder metal articles. 



In addition to the permalloys described, a number of other magnetic 

 powders are used in pressed core form for various applications. These 

 include electrolytic iron, carbonyl iron, and several types of magnetic 

 oxides. Carbonyl iron, in particular, has been used extensively for radio 

 cores where the spherical shape and small size of the particles have been 

 factors in their successful utilization. 



Advantages of the Powder Metallitrgy Process 



Throughout this paper, numerous advantages of the powder metallurgy 

 process have been indicated as well as some of the limitations. Outlined 



