Effect of annealing on the magnetic characteristics of iron-27% cobalt alloys

This paper describes the changes in magnetic properties of iron-27% cobalt alloys undergoing various annealing treatments. Two commercial heats (0.2 and 0.1 mm thick respectively) of similar compositions were studied. The anneals were carried out in both dry hydrogen and vacuum for a variety of temperatures up to 1200°C. After annealing, the magnetic properties were measured, the samples were examined metallographically, and the residual carbon and sulfur contents were determined. Results indicated that annealing in a dry hydrogen atmosphere was superior to heat treating under vacuum. The best annealing temperature to obtain good overall magnetic properties was 900°C. A slightly higher annealing temperature, 925°C, caused severe degradation in magnetic properties. It is suggested that the poorer properties are related to the presence of the alpha/gamma phase transformation at 925°C. Annealing at temperatures above 925°C caused improvement in some of the magnetic properties. Coercive force (Hc) seemed most sensitive to annealing conditions, with a strong positive correlation being found between Hcand the carbon plus sulfur contents after annealing.