SECTION VI MT PROCESS CONTROL

T.O. 33B-1-1 SECTION VI MT PROCESS CONTROL 1.6 MAGNETIC PARTICLE INSPECTION PROCESS CONTROL. 1.6.1 Purpose and Scope. This section provides information on the procedures necessary to assure a high quality performance for the magnetic particle inspection system. This section discusses the reasons for process control, control requirements, the use of the Quantitative Quality Indicators and the system effectiveness check. 1.6.2 General. 1.6.2.1 Need For Process Control. 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 be indicated for a number of reasons. The reasons for incorrect inspection results are deficiencies in either the materials used or application of the process. Of the two, the latter is far more common. These deficiencies are insidious since they may not be readily evident during the inspection of a part. It is necessary to periodically examine the materials, equipment and process parameters to be sure they are as required for adequate inspection results. 1.6.2.2 New Materials. Magnetic particle materials are subjected to testing during their formulation to assure their proper composition. However, materials that do not perform satisfactorily can be received. If unsatisfactory material performance is not discovered until some number of parts have been processed, then extra time and expense is required to reinspect the suspect parts. Unsatisfactory materials can result from a number of causes. The cost of verifying adequate material performance is extremely low and the required tests can be performed at any field laboratory. 1.6.2.3 In-Use Materials. Some inspection processes use the magnetic particle materials only once. In these processes the materials are usually 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 of material contamination or degradation during use. More often, however, the materials are used in open tanks where the excess materials are allowed to drain from the part back into the tank. This method provides numerous opportunities for contamination, deterioration and changes in concentration and such materials must be checked periodically to be sure they are functioning satisfactorily. 1.6.3 Causes of Materials Degradation. 1.6.3.1 Contamination. Contamination is a primary source of magnetic particle bath performance degradation. There are a number of contaminants and their effect on performance can vary. Some of the common contaminants frequently encountered 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 and can 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 also potential contaminants. These materials can mix with the inspection bath or f loat on top of it reducing the bath’s effectiveness. Change 3 1-43