T.O. 33B-1-1SECTION VIMT PROCESS CONTROL1.6MAGNETICPARTICLEINSPECTIONPROCESSCONTROL.1.6.1PurposeandScope.This section provides information on the procedures necessary to assure a high quality performance for themagnetic particle inspection system. This section discusses the reasons for process control, controlrequirements, the use of the Quantitative Quality Indicators and the system effectiveness check.1.6.2General.1.6.2.1NeedForProcessControl.The presence of magnetic particle indications confirms the existence of discontinuities in the part. However,the absence of indications does not guarantee the absence of discontinuities. Flaws can be present and not beindicated for a number of reasons. The reasons for incorrect inspection results are deficiencies in either thematerials used or application of the process. Of the two, the latter is far more common. These deficienciesare insidious since they may not be readily evident during the inspection of a part. It is necessary toperiodically examine the materials, equipment and process parameters to be sure they are as required foradequate inspection results.1.6.2.2NewMaterials.Magnetic particle materials are subjected to testing during their formulation to assure their propercomposition. However, materials that do not perform satisfactorily can be received. If unsatisfactorymaterial performance is not discovered until some number of parts have been processed, then extra time andexpense is required to reinspect the suspect parts. Unsatisfactory materials can result from a number ofcauses. The cost of verifying adequate material performance is extremely low and the required tests can beperformed at any field laboratory.1.6.2.3In-UseMaterials.Some inspection processes use the magnetic particle materials only once. In these processes the materials areusually applied by spraying or dusting and only the amount of material required for the inspection is applied.The materials are stored in closed containers until they are used. These processes minimize the possibility ofmaterial contamination or degradation during use. More often, however, the materials are used in opentanks where the excess materials are allowed to drain from the part back into the tank. This methodprovides numerous opportunities for contamination, deterioration and changes in concentration and suchmaterials must be checked periodically to be sure they are functioning satisfactorily.1.6.3CausesofMaterialsDegradation.1.6.3.1Contamination.Contamination is a primary source of magnetic particle bath performance degradation. There are a numberof contaminants and their effect on performance can vary. Some of the common contaminants frequentlyencountered are:a.Water is a common contaminant in petroleum based baths. It can occur because of condensation,leaks or moisture carryover on parts.b.Organics such as paint, lubricants, oils, greases and sealants, are another source of contamination.These materials are usually introduced into the magnetic particle bath by parts to be inspected andcan react with or dilute a bath so that it loses some or all of its ability to function.c.Organic solvents such as degreaser f luid, cleaning solvent, gasoline and antifreeze solution, are alsopotential contaminants. These materials can mix with the inspection bath or f loat on top of itreducing the bath’s effectiveness.Change 31-43
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