MT PROCESS CONTROL
MAGNETIC PARTICLE INSPECTION PROCESS CONTROL.
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.
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.
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.
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.
Causes of Materials Degradation.
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
Water is a common contaminant in petroleum based baths. It can occur because of condensation,
leaks or moisture carryover on parts.
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.
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 baths effectiveness.