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.
The 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
At 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.