Copyright Goodheart-Willcox Co., Inc. Chapter 18 Weld Inspection and Repair 269 A fl uorescent penetrant test requires an ultravi- olet light (black light) to observe the test results. It may be necessary to shield the viewing area from ambient light in order to properly read the test results. Magnetic Particle Test A magnetic particle test (MT) is a nondestruc- tive method of detecting cracks, seams, inclusions, segregations, porosity, or lack of fusion in magnetic materials. This test can detect surface defects that are too fi ne to be seen with the naked eye or that lie slightly below the surface. When a magnetic fi eld is established in a ferro- magnetic material, minute poles are set up at any defects. These poles have a stronger attraction for magnetic particles than the surrounding material has. In a magnetic particle test, the ferromagnetic material is magnetized by an electric current, and iron particles are applied to the magnetized area. If the magnetic fi eld is interrupted by a defect, the iron particles form a pattern on the surface. The pattern is the approximate size of the defect. Figure 18-6 shows how magnetic particle tests are performed. Small, portable, permanent magnets can be used for thin-gauge materials. Heavier material requires power from transformers, generators, or rectifi ers. A typical magnetic particle unit is shown in Figure 18-7. The magnetic particle test can be performed using either the wet or the dry method, depending on the individual application. The wet method, in which the particles are suspended in a fl uid, is generally more sensitive than the dry method. Wet magnetic particle inspection allows for a more even distribution of particles over a large area and is better for detecting very small discontinuities on a smooth surface. The dry method, which uses fi nely divided dry particles that are dusted onto a magnetized surface, is better for rough surfaces. Either red or gray particles can be A B Penetrant Penetrant Apply penetrant on the inside of the vessel. Apply developer on the outside of the vessel. Figure 18-5. Minute leaks in tanks or vessels can be located using penetrant inspection. A—Dye penetrant inspection of an open-top tank. B—Dye penetrant inspection of a closed tank. Yoke Part Weld Magnetic lines of force Figure 18-6. Magnetic particle tests. One pole of the yoke is placed on each side of the weld. Figure 18-7. Magnetic particle units are very useful on small weldments and require only 110 volts for operation. (Magnaflux, A Division of Illinois Tool Works)