T.O. 33B-1-1Table 6-21 Deleted6.8.12.1The inspector reading radiographs should be acquainted with the exposure technique used, materialradiographed, conditions of processing, and the geometry of the exposure setup. In this way he can judgemore accurately the radiographs produced and interpret the discontinuities more accurately. To determine ifthe part is rejectable or acceptable he will generally consult with the structural or design engineer unlessstandards have been established. To properly apply information obtained through radiography, the materialinspected must also be accurately identified with respect to the object radiographed. Lead letters or numbersare radiographed with the object to provide identification. This identification must be recorded and keyed tothe object.6.8.13TypicalRadiographicDiscontinuities.The radiographic inspection process is comparatively expensive and should be used for evaluation of internaldiscontinuities that cannot be evaluated by more economical methods. Therefore surface discontinuities of amagnitude to be considered detrimental to the function of the part should be evaluated by visual inspection orother NDI methods that are more economical than radiography. The major use of radiography is to revealinternal discontinuities.6.8.14Castings.The process of forming various shapes of metal by pouring molten metals into molds accounts for aconsiderable share of the critical components of an aircraft. These castings are made by melting ferrous andnonferrous alloys. The majority of the castings encountered requiring X-ray inspection are made of lightalloys; that is, aluminum and magnesium alloys. There are a number of inherent difficulties in thismanufacturing technique which plague the foundry. Since the molten metal occupies a larger space than thesame material after it freezes or cools, precautions must be taken to prevent the metal from shrinking toorapidly and forming voids which are called shrinkage or from rupturing the metal to cause hot cracks. Themolten metal also traps considerable gases from the air. These may result in tiny regular shaped bubbles inthe solid metal casting. Some metals such as aluminum accumulate gas on the surface of the molten metal.This may be trapped in the casting if adequate precautions are not taken to prevent pouring the gas into themold. In addition, sand may wash from the walls of the mold into the casting forming inclusions that reducethe strength of the castings.Change 36-97/(6-98 Blank)
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