MT Magnetic particle testing (MT)
Magnetic particle testing (MT) · Inspección y ensayos · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
On the drawing, “MT ISO 9934 on gear high-stress area” states a magnetic particle testing (mt) requirement.
What controls
Non-destructive test applicable only to ferromagnetic materials: the part is magnetized and magnetic particles are applied, which accumulate wherever the field is distorted, revealing surface and slightly subsurface discontinuities. It is more sensitive than penetrant testing for fine cracks and detects some that are not open to the surface, but it only works if the material can be magnetized and if the field is oriented transverse to the defect.
How to verify
Magnetization by yoke, prods or coil depending on geometry, application of dry or wet particles, evaluation under white or ultraviolet light depending on the system, and prior verification of setup sensitivity with a field indicator. Indications are sized and classified and compared against the applicable acceptance criterion. Qualified personnel and a test record including the magnetization parameters.
Common mistakes when interpreting it
- Applying it to austenitic stainless or non-ferrous materials and concluding that the part is sound.
- Magnetizing in only one direction and assuming the whole surface is covered.
- Forgetting demagnetization when the drawing or the next operation requires it.
How to get it in the workshop / if it comes out of tolerance
It does not work on austenitic stainless steels, aluminum, copper or most non-ferrous materials: in those cases the equivalent test is penetrant testing. Since the field only reveals defects transverse to it, the part must be magnetized in two roughly perpendicular directions to cover all orientations. If the drawing calls for it, demagnetize at the end, because a magnetized part attracts chips and interferes with instruments.
No indications where they were expected: check the field direction, because a defect parallel to the field is not revealed. Background saturated with particles: excessive field or a surface with abrupt section changes. Arc burns from contact prods on the part: they are a defect in themselves and are not permitted on many materials. And do not forget to demagnetize if the part is going to be machined or assembled near sensitive components.
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