Magnetic Particle Inspection (MPI) (MT)
Magnetic particle Inspection (MPI) is a non-destructive testing (NDT) process for detecting surface and shallow subsurface discontinuities in ferromagnetic materials such as iron, nickel, cobalt, and some of their alloys. The process puts a magnetic field into the part. The piece can be magnetized by direct or indirect magnetization. Direct magnetization occurs when the electric current is passed through the test object and a magnetic field is formed in the material. Indirect magnetization occurs when no electric current is passed through the test object, but a magnetic field is applied from an outside source. The magnetic lines of force are perpendicular to the direction of the electric current, which may be either alternating current (AC) or some form of direct current (DC) (rectified AC).
The presence of a surface or subsurface discontinuity in the material allows the magnetic flux to leak, since air cannot support as much magnetic field per unit volume as metals.
To identify a leak, ferrous particles, either dry or in a wet suspension, are applied to a part. These are attracted to an area of flux leakage and form what is known as an indication, which is evaluated to determine its nature, cause, and course of action, if any.
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MPI ASTM E 1444 15-5 MAT’l Linear indications
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MPI ASTM E 1444 15-5 MAT’l Linear indications
updated 1 month, 4 weeks ago 2 Members · 2 Posts
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We had a lot of 127 pc sent to our finishing house for plating and NDT. We were notified that 57/127 pc were rejected for linear indications. We have made various other part numbers out of this same material and have not had a rejection previously. What would be some things to look into as a root cause per the attached photo? Any help steering me in the right direction would be helpful.
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I would suggest metallographic cross-sectioning to verify crack presence and depth, and then acid etch the metallographic mount as well as remnants of the destroyed part to see if grinder burn/cracking occurred. First suspicion would be grinder check, which can be caused by poorly-controlled machining process.
