T.O. 33B-1-12-84breaks the surface for its entire length or a partially filled seam will also produce an intermittent linear indication asshown in Figure 2-32c.2.7.6.2.1.3 RoundorDotIndications.Rounded indications are characterized as having a length and width of approximately equal dimensions. Porosity orrelatively small areas of unsoundness in metal components usually form rounded indications. However, the actualsurface opening may be irregular in shape. Deep discontinuities, such as weld crater cracks, may appear rounded dueto the large volume of entrapped penetrant. Figure 2-32d illustrates the appearance of large and small roundedindications.Figure 2-32. Typical Penetrant Indications.2.7.6.2.2 ManufacturingDiscontinuities.Many discontinuities result from the manufacturing and repair processes. These will probably be detected each timethe part is reinspected. The NDI inspector must, therefore, be familiar with their appearance and cause, in order tomake valid interpretations of inspection results. Some of the common types of manufacturing discontinuities aredescribed and illustrated in the following paragraphs.2.7.6.2.2.1 Porosity.Porosity is common to all cast parts, particularly aluminum and magnesium. Porosity occurs when gases are entrappedin the molten metal during pouring and solidification and may also occur during welding. It does not always break thesurface, and internal porosity is not detected by penetrant inspection. Porosity can be very small and distributedthroughout the material, in which case it is called microporosity. Microporosity may or may not cause a penetrantindication. When it does, it produces an overall background. The larger pores are called macroporosity. In castings,porosity is not usually considered a defect, unless it is extensive enough to cause a structural weakness or allow theleakage of a fluid intended to be contained by the casting.
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