What’s up guys this is Michael with Michael Talks Metal.
Our topic today is another of the inspection methods that can be used to check a steel part for defects without damaging the component being examined.
Magnetic Particle Inspection. MP or MPI for short; is one of several methods of non destructive testing used to detect surface and sub-surface defects in steels and other ferromagnetic alloys. This technique does not work for the austenitic stainless steels since they are non-magnetic or only slightly magnetic.
Most of you have probably seen the field generated by a magnet made visible using iron filings scattered around the magnet sometime. Grade school? High School?
When we impose a magnetic field in the steel component and it encounters a flaw we will also disturb the magnetic field much like a rock in a stream disturbs the otherwise smooth flow. In a stream, we can see the disturbance created. In our inspection procedure we use magnetic particles applied to the surface to visulize the field and any disturbance caused by a flaw.
Magnetic particles can be applied as either a dry powder or suspended in a liquid carrier. Particles may be colored or fluorescent to enhance the visibility.
Surface finish is also a consideration. Rough machined surfaces may not be suitable for producing an acceptable test result; defects could be hidden by the machined texture of the surface and how it may affect the applied magnetic field.
Parts may become magnetized by the process and require demagnetizing after inspection.
In aircraft quality steels, AMS standards require that material is sampled and tested by the magnetic particle method to assure the material meets requirements for internal cleanliness. This preliminary material testing assures that the material is suitable for any later magnetic testing performed on parts. In these tests,any inclusions are rated and limited by both the frequency and the severity of any indications found in the samples.
All Non Destructive Evaluation (NDE) is intended to confirm the quality of a part and requires careful planning. Many parts require inspections at multiple steps in fabrication.
If you are a regular viewer of our video series, then you know that things can get very complicated in a hurry. So do not forget to ever, EVER – CHECK THE SPECS !!!!!!!!
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