T.O. 33B-1-13-108magnetic field. They are true indications but, since thereis no unintentional interruption of the material, they do notaffect the usefulness of the part. It is important that the inspector know how and why these non-relevant indications areformed and where they can occur.3.7.5.2 Classes.3.7.5.2.1 MagneticWriting.This is a condition caused by a piece of steel rubbing against another piece of steel that has been magnetized. Sinceeither or both pieces contain some residual magnetism the rubbing or touching creates magnetic poles at the points ofcontact. These local magnetic poles are usually in the form of a line or scrawl, and for this reason the effect is referredto as magnetic writing. In Figure 3-69 the part in the top view is magnetized with a circular field. If another partmade of magnetic material is rubbed against or comes into contact with the magnetized part, as in the second view, aweak field will be induced into the smaller part. After the smaller part has been removed, the circular field in theoriginal part will be altered or distorted to some extent as shown in the bottom view. Since there is no force to changethe direction of the altered field, there will be some leakage at the point of distortion that will attract magnetic particles.Figure 3-69. Creation of Magnetic Writing3.7.5.2.2 LongitudinalMagnetization.When a part is longitudinally magnetized in a coil, there are always magnetic poles at the ends of the piece. Magneticmaterial such as chips or magnetic powder or paste will be attracted to these poles. The same situation occurs when ayoke is used to create a magnetic field; poles are induced on the part in the areas where the yoke touches the part.3.7.5.2.3 ColdWorking.Cold working consists of changing the size or shape of a metal part without raising itstemperature before working.When a bent nail is straightened by a carpenter witha hammer, the nail is being cold worked. Cold working usuallycauses a change in the permeability of the metalwhere the change in size or shape occurs. The boundary of the area ofchanged permeability may attract magnetic particles when the part is magnetized.3.7.5.2.4 HardorSoftSpots.If there are areas of a part which have a different degree of hardness than the remainder of the part, these areas willusually have a different permeability. When a part with such areas of different permeability is inspected with magnetic
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