Figure 3-53. Magnetic Particle Indication of a Seam on a Bar.
b. Laminations. (See Figure 3-54.) Laminations in rolled plate or strip are formed when blowholes or
internal fissures are not fused during rolling, but are enlarged and flattened into sometimes quite large
areas of horizontal discontinuities. Laminations may be detected by magnetic particle testing on the cut
edges of plate. The laminations do not give indications on plate or strip surfaces, since they are internal
and parallel to the surface. Ultrasonic mapping techniques are used to define them.
Figure 3-54. Magnetic Particle Indications of Laminations
Shown on Flame-Cut Edge of Thick Steel Plate.
c. Cupping. This is a condition created in drawing or extruding when the interior of the metal does not
flow as rapidly as the surface. Segregation in the center of the metal usually contributes to this
occurrence. The result is a series of internal ruptures that are severe defects whenever they occur. They
may be indicated with magnetic particles if the ruptures are large and are near the surface of the part.
The cupping problem can be minimized by changing die angles. (See Figure 3-55.)