A low cobalt ternary Cr-Co-Fe alloy for telephone receiver magnet use

The processing, structure, and properties of a low-cobalt, ternary Fe-28Cr-10.5Co isotropic magnet alloy (Chromindur II), developed for use in the telephone receiver, are described. The new alloy is easy to process, and permits a convenient low-temperature recrystallization anneal of heavily cold rolled strips leading to a fine-grained, ductile, single-phase structure suitable for cold forming into the desired magnet shape prior to aging. Optimization of the microstructural control process during spinodal decomposition is discussed with a particular emphasis on solving the problem of critical control during aging required for desirable magnetic properties. It is shown that, in order to reproducibly achieve an optimum combination of wavelength and amplitude during decomposition, two drastically different continuous cooling curves, connected at some intermediate temperature, must exist. This newly devised, two-stage aging technique permits a reasonably wide heat treating window, and accomodates heat-to-heat compositional variations. Satisfactory isotropic magnet properties are reproducibly obtained by industrial heat treatment leading to a successful commercial application of the new alloy in the ring armature type telephone receiver.