Magnetic Particle Inspection

Magnetizes 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 type of nondestructive testing (NDT) performed at Laboratory Testing Inc. NDT methods allow materials and products to be tested or inspected without damage. LTI also provides liquid penetrant, ultrasonic, radiographic (X-ray) and visual inspection, as well as hydrostatic pressure testing. These NDT methods will be discussed in the next few issues of LabNews.

The Process

Magnetic particle inspection is performed by spreading fine magnetic particles over the surface of a specimen 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 from discontinuities. A trained and experienced technician will interpret the indications that might be observed during the inspection.

Fluorescent wet magnetic particle inspectionThe magnetic particles can be applied dry as a powder or wet as a suspension in a petroleum-based or water-based carrier. Dry particles are most commonly used for inspections in the field. Fluorescent wet inspection is considered best for detecting very small discontinuities because it uses much smaller particles and the liquid carrier allows these particles to flow easily to small leakage fields to form indications.

A wet 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.

Inspections at LTI

Magnetic particle inspection for surface flawsAt LTI, the most common method used for magnetic particle inspection is the fluorescent wet continuous method using full wave direct current (FWDC). The part is placed into one of our three wet fluorescent horizontal units and then the magnetic field is applied while the suspension of magnetic particles is sprayed or poured over the test sample. Our horizontal units range in size from 6 ft. to 12 ft. long. Larger size parts are inspected using cables, yokes and portable equipment.

Advantages and Limitations

There is relatively no limit to the size of parts that can be inspected using the magnetic particle 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. 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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