30 



BELL SYSTEM TECHNICAL JOURNAL 



Effects of Heat Treatment 



The manner in which the magnetic properties of this composition 

 are affected by the rate of cooHng is illustrated in Figs. 5-7. The 

 measurements plotted in these figures are from three rings, air 

 quenched, annealed and baked, respectively. For weak fields there 

 are large differences in the magnetic properties for these rings. The 

 initial permeability for the quenched ring is more than twice that of 

 the baked one. With increased field strength this difference decreases 

 and disappears for fields over 50 gauss. The permeability variations 

 as the strength of the field increases also show the remarkable change 



1100 



900 



M 700 



500 



300 



1 2 3 4 5 



H 



Fig. 6 — Permeability curves for Perminvar (45% Ni — 25% Co — 30% Fe) 



which heating for a long time in the critical temperature range produced 

 in these alloys. For the quenched ring the permeability increased 

 from 620 to 800 for an increase of field strength from to .5 gauss. 

 For the baked ring the permeability remains constant for fields up 

 to 2.5 gauss. 



The hysteresis loss and the shapes of the loops also are affected 

 greatly by the heat treatment. This is illustrated in Fig. 7, where 

 loops for a number of flux densities are plotted for two sample rings, 

 one baked at 425° C. and the other air quenched, and in Fig. 3 where 

 loops for the same maximum flux densities are plotted for an annealed 



