Special Techniques

T.O. 33B-1-1 3-83 3.6.7 Limitations. a. When steel is heated it passes through its Curie point, approximately 770°C (or about 1418°F) for soft steels. Above the Curie point it is no longer ferromagnetic. When the steel cools to room temperature in the absence of a magnetic field it will contain no residual magnetism. Other means of demagnetizing always leave some residual field. Complete demagnetization is not possible even though it is often specified. b. The earth's field will always affect the residual magnetism in a ferromagnetic part and will often determine the lower limit of practical demagnetization. Long parts, or assemblies of long parts, such as welded tubular structures, are especially likely to remain magnetized, at a level determined by the earth's field, in spite of the most careful demagnetizing technique. c. Many articles and parts become quite strongly magnetized from the earth's field alone. Transporting parts from one location to another may produce this effect. Long bars, demagnetized at the point of testing, have been found magnetized when delivered to the point of use. It is not unusual to find that parts of aircraft, automotive engines, railroad locomotives, or in fact, any parts made from steel of fair retentivity, are quite strongly magnetized after having been in service for some time, even though they may never have been near any artificially produced magnetic field. Parts also become magnetized by being near electric lines carrying heavy currents, or near some form of magnetic equipment. 3.6.8 Special Techniques. Where the size, shape or method of magnetization of a part make demagnetization difficult, there are several techniques, which may be used effectively. Through the use of the techniques that follow, most difficult parts can be demagnetized to the extent required for service. 3.6.8.1 General. a. Sometimes parts that are difficult to demagnetize can be effectively demagnetized by striking the part with a hammer during the demagnetizing operation. To use this technique, the part is placed in the demagnetizing coil and the current is turned on. The part is then hammered with a rubber mallet and withdrawn from the coil field while the hammering is continued. Care must be taken that the part is not damaged by the hammering. b. Demagnetizing coils sometimes work better if they are positioned so that the path of the part, as it is drawn through the coil, is in an east-and-west direction rather than north-and-south. This is particularly true for long parts that may be influenced by the earth's magnetic field. c. Sometimes the residual field from heavy parts can best be removed by a technique known as the transient method of demagnetization. To perform this technique, the part is placed in the demagnetizing coil and the current turned on and off five to ten times. The current is then turned on and left on while the part is withdrawn from the magnetic field of the coil. 3.6.8.2 Short Parts. When a short part is being demagnetized in an AC coil by the method of withdrawing the part along the line of the axis of the coil, it is helpful to rotate the part both around the axis parallel to and transverse to the coil's axis. This should be accomplished while the part is in the coil as well as during the entire time of withdrawal. This procedure is also effective in demagnetizing short, hollow or cylindrical parts. A short part with an L/D ratio of one or less can sometimes be better demagnetized by placing it between two pole pieces of soft iron of similar diameter but longer than the part. This combination is then passed through the coil as a unit. It has the effect of increasing the L/D ratio and facilitates the removal of the field in the part.