tailieunhanh - Volume 17 - Nondestructive Evaluation and Quality Control Part 4

Tham khảo tài liệu 'volume 17 - nondestructive evaluation and quality control part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Fig. 57 Current and flux density curves during demagnetization projected from the hysteresis loop. See text for discussion. In using this principle the magnetizing force must be high enough at the start to overcome the coercive force and to reverse the residual field initially in the part. Also the incremental decrease between successive reductions in current must be small enough so that the reverse magnetizing force will be able on each cycle to reverse the field remaining in the part from the last previous reversal. Demagnetization With Alternating Current. A common method of demagnetizing small to moderate-size parts is by passing them through a coil through which alternating current at line frequency is passing usually 50 to 60 Hz . Alternatively the 60-Hz alternating current is passed through a coil with the part inside the coil and the current is gradually reduced to zero. In the first method the strength of the reversing field is reduced by axially withdrawing the part from the coil or the coil from the part and for some distance beyond the end of the coil or part along that axial line. In the second method gradual decay of the current in the coil accomplishes the same result. Passing a part through an ac coil is usually the faster preferred method. Small parts should not be loaded into baskets and the baskets passed through the coil as a unit because alternating current will not penetrate into such a mass of parts and because only a few parts on the outside edges will be demagnetized and these possibly only partially demagnetized . Small parts can be demagnetized in multiple lots only if they are placed in a single layer on a tray that holds them apart and in a fixed position with their long axes parallel to the axis of the coil. Large parts are not effectively demagnetized with 60-Hz alternating current because of its inability to penetrate. Alternating current with 25-Hz frequency is more effective. Machines that provide decaying alternating current have