Recent experiments have indicated that a torque transducer can be constructed based on local bands of naturally stabilized remanent circumferential magnetization within a hollow steel shaft. A strong coercive force along with the crystalline anisotropy combine to stabilize the circumferential magnetization of the sensing area of the shaft. A torqued shaft has its magnetic axis tilted into a helical orientation, which generates an axial field signal linearly proportional to the applied torque that can be sensed externally to the shaft. Our research has shown that the transducer function can be improved by appropriate sequential heat treatments in a helium atmosphere. Subsequent to all heat treatment steps, the test shafts were measured to determine the changes in axial and circumferential magnetic hysteresis properties and to track the development of the magnetic sensing area to applied torque ͑sensitivity͒. Although little change was produced in the originally narrow axial hysteresis loops, a 10%-20% decrease was observed in both axial coercive force and remanent magnetization, and correspondingly, a 50% increase was observed in sensitivity, accompanied by a higher linear saturation limit. Additionally, the heat treatment cycle significantly widened the originally broad circumferential hysteresis loops, resulting in a substantial increase in circumferential coercive force, which improves the stability of the sensory region.

Magnetostriction effects were investigated for three different materials by using a simple, reproducible, and cost-effective method recently developed in our laboratory. The magnetostriction effects were generated by a large oscillating magnetic field produced by a high current 60 Hz ac welder power supply, capable of reaching saturation levels for the material, and then detected by a change in capacitance between a hollow cylindrical sample and a concentric brass ring. This capacitance change was continuously monitored at a high frequency rate by a standard laboratory capacitance bridge meter. The output voltage of the bridge was fed into a storage cathode-ray oscilloscope and its voltage versus time signals were then analyzed by a computer program. Two ferromagnetic rings, constructed of high-speed steels 4620 and 4340, which have proven applicability for use in magnetoelastic torque sensing, were used as the samples for investigating the magnetostriction effects, while a paramagnetic aluminum ring was used for the control sample. Our study showed that the 4340 ring, which had higher nickel, cobalt, and chromium content than that of the 4620 ring, had the largest magnetostriction effect, and that the aluminum ring displayed no magnetostriction effect, as was expected. We have found this experimental method to be both reproducible and sufficient to rank different ferromagnetic materials by their magnetostriction level, which is a significant consideration in producing effective magnetoelastic torque sensors.

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