Magnetic Particle Inspection to Find Discontinuities

Magnetic particle inspection successfully detects surface and near surface defects in ferromagnetic materials such as steel and iron. This is just one of a few types of nondestructive testing or NDT inspection methods so named because materials and products are tested or inspected without damage. Some other nondestructive testing processes include liquid penetrant inspection, ultrasonic testing, radiographic or X-ray inspection, visual inspection and hydrostatic pressure testing.

The Process 

Magnetic particle inspection is performed by spreading fine magnetic particles over the surface of a test piece as it is being magnetized, causing a buildup of particles at a discontinuity such as a crack, lap, inclusion or seam. These particles have high magnetic permeability, so they are easily magnetized and attracted to small magnetic leakage fields caused by discontinuities.

During magnetic particle inspection, the particles can be applied in either of two ways: 

• Dry as a powder 

• Wet as a suspension in a petroleum-based or water-based carrier

Dry particles are most commonly used for convenience in field inspections. Fluorescent wet inspection is considered best for detecting very small discontinuities because much smaller particles are used and the liquid carrier allows the particles to flow easily to small leakage fields to form indications. A wet magnetic particle inspection has an advantage over a dry inspection because the equipment used in this process can quickly and easily spray a uniform layer of particles over the surface of the material or part to be inspected.

The magnetic field can be produced by the use of direct current (DC) circuits or by alternating current (AC) circuits. AC circuits produce fields that are confined to the near surface of the test sample and are useful in finding surface cracks. DC circuits provide magnetic fields that penetrate a shallow distance into the test sample and are useful for also finding discontinuities below the surface.

Magnetic particle inspection can be applied to raw materials, parts or products during various stages in the manufacturing process. It is also applied during equipment or machinery maintenance and overhaul to find fatigue cracks.

Advantages and Limitations 

There is relatively no limit to the size of parts that can be inspected using the magnetic particle inspection method, except for limitations due to the capacity of the test equipment. High currents are usually needed to inspect large parts. Inspections need to be performed in two directions to insure detection of discontinuities in the longitudinal and circumferential directions.

No extensive or elaborate pre-cleaning of parts is required before inspection; usually only solvent degreasing is needed to remove oils. Paint, heat-treat scale, grease, etc. should be removed to provide the most accurate test results. Magnetic particle inspections can be performed before and after plating. It is standard practice to demagnetize parts following inspection and post-clean the parts to remove magnetic particles.

Standards and Specifications 

A variety of standards and specifications are used as the basis for performing magnetic particle inspection. These include sections of American Society of Materials Engineers (ASME) Codes, Aerospace Material Specifications (AMS), Society of Automotive Engineers (SAE) Specifications, American Society for Testing and Materials (ASTM) Standards, various military standards, and the American Society for Nondestructive Testing (ASNT) Recommendations.

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