Cooling Rate Effect on Initial Permeability of 4–79 Molybdenum Permalloy

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Abstract
This paper shows the combined effects of Ni and Mo on the optimum cooling rate of 4–79 molybdenum Permalloy and provides an empirical equation for calculating this rate (CRopt) for highest 60 cycle initial permeability at B=40 gauss (u0). Cold‐rolled strip was obtained from heats of the Carpenter Steel Company's HyMu 80 with Ni between 79.29 to 80.63% and Mo between 4.08 to 4.44%. Before cooling rate studies were run, duplicate ring lamination samples from each heat were thoroughly purified by annealing in hydrogen at 2050°F to remove carbon and other nonmetallics. This was done to obtain the true effects of Ni and Mo on the u0 —cooling rate relationship. Magnetic measurements were made after each four hours at temperature until the u0 of each heat stabilized. An average cooling rate of 150°F/hr through the critical ordering temperature range was employed. As annealing time increased, u0 gradually increased and then stabilized for each heat at various values between 30 500 and 43 250. The stabilized samples, when repeatedly heated to 2050°F in hydrogen and cooled at different rates between 60° and 810°F/hr, displayed values of CRopt between 100° to 600°F/hr. As the CRopt decreased, the peak of the u0—cooling rate curve became more abrupt. The following regression equation was calculated: CRopt (°F/hr) = −26201.2+365.0 (%Ni) −633.3 (%Mo). The standard deviation of the difference between the observed and the predicted values was calculated to be 45°F/hr. The Mo has nearly twice the influence as Ni on the CRopt but in an opposing direction.

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