MAGNETIC ALLOYS 115 



impurities. The magnetic properties recorded in this paper have 

 therefore been confined mostly to those obtained on materials pro- 

 duced by standard metallurgical methods. 



In the commercial method of producing these alloys the best grades 

 of commercial iron, nickel, and cobalt are used. The melting is done 

 in an electric furnace, and after the mechanical fabrication into suitable 

 shapes these alloys are heat-treated to develop the desired magnetic 

 properties. 



Early in an investigation of these alloys it was found that some of 

 them required special heat-treatments to develop the desired magnetic 

 properties. For some the slow cooling incident to the ordinary process 

 of annealing was not suitable, and a rapid cooling was necessary. 

 For another group the slow cooling in the annealing process was not 

 slow enough, and the best results were obtained when the alloys were 

 held at a constant high temperature for a considerable time. It was 

 evident that to determine the most suitable temperature of heating 

 and rate of cooling for each alloy would require more time than was 

 warranted in the exploratory work. Three methods of heat-treatment 

 that, in a general way, would separate the alloys into groups, were 

 developed. These heat-treatments are designated in this paper as 

 "annealing," "quenching," and "baking." 



The annealing process consists of heating the samples in closed 

 containers to a temperature of 1,000 degrees centigrade, and cooling 

 with the furnace. The cooling ordinarily requires 7 hours before room 

 temperature is reached. This heat-treatment is primarily for the 

 purpose of removing the effects of mechanical strains necessarily 

 resulting from the rolling and stamping of the alloys into suitable 

 shapes. All the alloys discussed in this paper received this heat- 

 treatment before any of the more special processes were applied. 



The quenching process consists of heating the alloys for a short time 

 at 600 degrees centigrade, and cooling in air at room temperature for 

 small samples with large surfaces, and in oil for larger samples. The 

 rate of cooling attained by these methods is approximately 40 degrees 

 centigrade per second. It has been found that the best rate of cooling 

 for maximum permeability does not always develop the highest initial 

 permeability. The difference, however, is not large, and often is 

 masked by other variations in the manufacturing process. 



The baking process consists of heating the alloys for 24 hours at 

 425 degrees centigrade, and then slowly cooling to room temperature. 

 The rate of cooling does not affect the development of the magnetic 

 properties unless it is so rapid as to introduce mechanical strains. 



