Magnetic Testing (MT) / Magnetic Particle Inspection (MPI)
Magnetic Particle Inspection also known as Magnetic Testing (often abbreviated MPI or MT) is a surface method of non-destructive testing.
MPI is widely used as a relatively quick surface test for a weld and heat affected zone for ferritic materials. It is used on steels where a volumetric test is not possible or is impractical.
The test area is first sprayed with a white lacquer to provide a contrasting background.
It is then sprayed with a magnetic ink – usually black or red in colour. The ink is comprised of a bath such as a thin paraffin liquid and very fine iron particles.
The test area is then magnetised with a yoke or permanent magnet. When a defect is found to be present, the lines of magnetic force will create a what is called a flux leakage.
This allows the particles in the ink to accumulate around the edges of the defect. In doing so, any surface defects will show as a highlighted indication against a
contrasting white back ground.
MPI is an effective and efficient way for location of surface breaking and slight sub-surface defects. These include a lack of toe fusion in welds, cracks, pores, cold lap, and so on. This test method provides very good defect resolution.
The advantages of this method is that it is relatively quick and easy. Results are instantaneous. Plus, there is usually little or no surface preparation needed, provided the surface is clean and free from any contamination that could interfere with the interpretation.
The major limiting factor is that the test material must be magnetic.There is basically no limit to the size of parts that can be inspected using the magnetic particle inspection method. There can however be limitations due to the capacity of the test equipment. High currents are typically required to inspect large parts.
Inspections need to be performed in two directions to insure detection of discontinuities in the longitudinal and circumferential directions.
No pre-cleaning of parts is required before MPI, usually only solvent degreasing is needed to remove oils. Paint, heat-treat scale, grease should be removed to deliver the most accurate results of the test. Inspections can be performed before and after plating. It is regular practice to demagnetize parts following inspection and post-clean the parts to remove magnetic particles and the bath.
All Weldtest technicians hold the mandatory qualifications required and have the necessary experience to provide the principal or designer with the assurance that the relevant quality requirements of the standard have been met.