1,076,168. Determining hardness of metal magnetically. BRITISH IRON & STEEL RESEARCH ASSOCIATION. June 9, 1964 [June 12, 1963], No. 23393/63. Heading G1N. Metallurgical properties (e. g. hardness) of a moving strip, rod or sheet of magnetizable material are determined by measuring its retentivity. In Fig. 1 a strip 18 successively passes through a treatment section 19, past a saturating coil 20, a first fluxgate detector 21 with an output meter 24, a demagnetizer 22, and a second fluxgate detector 23 with a meter 26. The apparatus may be operated by simply energizing the coil 20 by means of a direct current sufficient to saturate the strip, de-energizing the coil and detecting the residual magnetism by means of the fluxgate 21 the output of which may also be fed to a process control unit 25. In an embodiment of the above method the saturating coil 20, the demagnetizer 22 and fluxgate 23 only are used; the strip being first saturated, and then the demagnetizer is energized by a predetermined non-saturating current of opposite sense to that of the coil 20 and the output of the fluxgate 23 is applied to the meter 26. The demagnetizer does not completely demagnetize but rather the constant current energization thereof represents a reference level or standard of mechanical hardness, while the meter 26 indicates the deviation of the strip from such level. Alternatively the demagnetizer can be energized through a current controller 27, the output of the fluxgate 23 being applied thereto to vary the energizing, current of the demagnetizer until there is zero residual magnetism in the strip. This action is that of a null seeking servo. An ammeter 28 will indicate an approximation to the coercive force, an output from the current controller 27 or the fluxgate 23 may be used to control a process. Fig. 2 (a) shows the arrangement of a resiliently mounted head carrying a magnetizing coil 33 fluxgate 34, demagnetizing coil 35 and fluxgate 36. The head is automatically retractable away from the moving strip, by means of a trip bar arrangement 73, should a welded join approach thereto, which join might otherwise damage the head.
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