Table 3-4. Relative Permeabilities for Some Ferromagnetic Materials

T.O. 33B-1-1 3-42 3.3.12.6.4.2 Much of the information contained in paragraphs 3.3.12.6.2 through 3.3.12.6.3.6 on solenoid coils also pertain to cable wrapped coils. The rule-of-thumb given in paragraph 3.3.12.6.3.1 for a part lying in the bottom of a coil may be used to estimate the current requirement. However, since the cables are likely to be closely placed around the part, the full current will not be required. Sometimes less than one half of the estimated current will be sufficient. 3.3.12.7 Applications. Longitudinal magnetization is used to inspect ferromagnetic components having material permeabilities of about 500 or greater. This includes most steel alloys. (See Table 3-4). A simple test to determine whether or not a part is sufficiently magnetic is to place a permanent magnet against a part to be tested. If the attraction of the magnet can be felt, the part is sufficiently magnetic for magnetic particle inspection. Table 3-4. Relative Permeabilities for Some Ferromagnetic Materials. Ferromagnetic Materials Relative Permeability* Iron (99% annealed in H) 200,000 Iron (99.8% annealed) 6,000 Iron (98.5% cold rolled) 2,000 Nickel (99% annealed) 600 Cobalt (99% annealed) 250 Steel (0.9% Carbon) 100 + Excerpt from Nondestructive Testing Handbook, Vol. 6, American Society for Nondestructive Testing, 2^nd Ed., 1988. *Relative to air, which has a permeability of 1.0 3.3.12.7.1 Discontinuities detected by the longitudinal method are those which lie generally in a direction transverse or crosswise to the direction of the applied field. The depth at which a discontinuity can be detected depends upon the size and shape of the discontinuity relative to: (1) the size of the cross section in which it is located; (2) the length to diameter ratio (L/D) of the part; and (3) the strength of the applied magnetizing field. For a given coil and coil current amperage, the smaller the L/D ratio, the lower will be the magnetic flux density in the part, and the weaker will be the leakage fields over discontinuities. In other words, the smaller the L / D ratio, the greater the coil current amperage must be to produce the same flux density or field strength in the part. Coil amperages become impracticably large for L/D ratios of 3 or less. Small L/D ratios of 3 or less can be effectively increased by using pole pieces of magnetic material (refer to paragraph 3.3.12.6.3.7), one on each side of a part. All three pieces must be lined up in the direction of the applied field or coils axis. Very long parts having L/D ratios greater than 15 should receive multiple inspections along the length of a part. The most effective field in a part extends about 6 to 9 inches on each side of a coil. For multiple inspections, a coil must be repositioned at intervals of from 15 inches to 18 inches along the part. Rule-of- thumb formulas for estimating current longitudinal magnetization using coils, for parts having L/D ratios up to l5, and diameters not exceeding about 1/10 that of the coils, are given in paragraph 3.3.12.6.3. 3.3.12.7.2 Longitudinal magnetization of coated parts may be accomplished depending upon the kind and thickness of the coating. Metallic plating generally should not exceed 0.005 inch in thickness unless it is known that the discontinuities being looked for can be detected through greater thicknesses. Nonmetallic coatings such as paint or other protective coatings require removal only if they are excessively thick or damaged to the extent that particles can be trapped mechanically. Any oil or grease must be removed since such materials contaminate the liquid media. Any loose scale or rust must also be removed from parts before inspection since they also can interfere with formation of indications and are a contaminant in a liquid bath.