Page 106 - SSAB Welding Handbook Edition 2

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18.0 Non destructive evaluations Welding handbook

18.3 Magnetic particle testing The choice of magnetizing instrument depends on
the requirements of the specific application. Aspects
©SSAB
Magnetic particle testing is a frequently used such as productivity issues and access limitations
method for detecting surface breaking discontinuities to the inspected area are to be considered when
in the joint. As a secondary feature, this method also selecting a suitable instrument. Two examples are
allows for the detection of embedded discontinuities an electromagnet and a prod technique, which are
that lay just below the surface of the joint. illustrated in fig. 18.3 and fig. 18.4.

The inspection area should be clean and free from The magnetic particles that are applied are normally
contaminants prior to inspection. These contaminants, iron or iron oxide. These particles have a specific
including substances like oil, mill scale, grease, or color or a coating of fluorescent material. They are
loose or thick layers of paint should be removed using applied either as a dry powder or in a mixture with
appropriate measures such as brushing or application liquid. Both alternatives can be used depending on
of solvents. The cleanliness requirements are somewhat the situation. A liquid solution promotes a more
lower than for penetrant testing since magnetic particle even distribution on the surface while the dry
testing can reveal discontinuities just under the surface. powder supports applications on hot surfaces.
Therefore, the shop primer can be left on as well as a
thin layer of paint, depending on the total thickness of
the paint. A total thickness of paint up to 50 μm does
not normally affect detection with this method. The
shop primer used by SSAB has an approximate
thickness of 10-15 μm.

In the subsequent phase, the joint is magnetized with
external equipment that creates a magnetic flux in a
certain direction. If a discontinuity is detected, the
direction of the magnetic flux changes due to a flux
leakage and new magnetic pools are created around
the discontinuity. The direction of the magnetic flux
is revealed by adding small ferromagnetic particles
to the surface of the structure. These particles will Fig. 18.3: Magnetization of a fillet joint with an electromagnet.
orient along the magnetic flux. This means that if a
discontinuity is found, the particles will form along
the magnetic flux created around it. Lines of magnetic force
Prod
However, if the discontinuity is situated near the Prod
direction of the magnetic flux, the disturbance will
be low or absent and the discontinuity may not be
revealed. The joint is, therefore, magnetized and
evaluated at least two times, each time allowing
the magnetic flux to be set in a different direction. Electric
After the completion of inspection, residual current
magnetism may be present in and adjacent to the (-) (+)
joint. If so, the residual magnetism can be removed Perpendicular indications Parallel indications
by a demagnetizing equipment. will show may not show

Fig. 18.4: Magnetization of a butt joint with a prod technique.

106 This handbook contains general suggestions and information without any expressed or implied warranty of any kind. SSAB hereby expressly disclaims
all liability of any kind, including any damages, in connection with the use of the information and for their suitability for individual applications.
It is the responsibility of the user of this brochure to adapt the recommendations contained therein to the requirements of individual applications.