Effect of stress on d.c. magnetisation properties of Permendur

The magnetic properties of Permendur (49% Co 49% Fe 2% V) are very dependent on heat treatment. Common practice is to anneal laminations between 720°C and 800°C for use in machine stators or other devices where high saturation induction is desirable. Annealing below 700°C develops exceptional mechanical properties at the expense of magnetic properties. The material might be useful as rotor laminations of aircraft generators which operate under high centrifugal stress; so to test the suitability of the magnetic properties d.c. measurements were made after annealing at temperatures between 450°C and 875°C on samples subjected to mechanical stress of up to ± 300MN/m2. It was found that tension always improves the B/H curve, and compression makes it worse. Annealing at 650°C gave material which exhibited both magnetic and mechanical properties suitable for high-speed rotors. Changes in magnetic properties are discussed in terms of structural changes affecting the stress sensitivity of the domain structure.