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Because most of the knowledge and information pertaining to magnetic particle inspection methods seemed to have been confined to small parts, and because of so many conflicting claims for various methods used, an investigation was made in an effort to determine some basic information regarding the different methods of magnetization—field intensity and sensitivity of powders and pastes—in connection with the inspection of large forgings. The investigation was confined to surface defects only, such as forging folds and laps, thermal cracks extending to the surface, and inclusions. Inclusions occurring within a few mils of the surface, if they are of sufficient size, can be detected with the same magnetizing values as surface defects. Several forgings of different sizes and known to contain thermal cracks were used for test. These forgings were thoroughly demagnetized between each series of tests. The result of each test was recorded by Scotch tape pictures, some of which are shown in the following illustrations. Figure 1 shows one of the partially-machined rotor forgings that was used as a test specimen. This rotor has a maximum diameter of 44 1/4 in. and is slightly over 8 ft. long. Figure 2 shows the Scotch tape record of thermal cracks found in the test forgings at different diameters.
Boyle, C. J.
General Electric Co., Schenectady, N. Y.