Page 107 - SSAB Welding Handbook Edition 2

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Welding handbook 18.0 Non destructive evaluations

When using non-fluorescent magnetic particles, paint 18.4 Ultrasonic testing
is typically applied to the inspected area in order to
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create a higher contrast between the parent metal and Ultrasonic testing is a method that can detect
the magnetic particles. An example of a detected crack embedded discontinuities in joints that have plate
is illustrated in fig. 18.5. A characteristic of magnetic thicknesses up to the maximum single plate
particle testing is that the marked width of a detected thickness of the Hardox and Strenx grades. This
discontinuity is larger than the actual discontinuity. testing method can detect small discontinuities and
allows for their individual size and character to be
determined. The location of the discontinuity can
also be identified both horizontally and vertically.

Additional aspects to consider with ultrasonic
testing are that the operator needs to be considerably
skilled and the equipment requires careful and
regular calibration.

Before explaining the procedure of this method, the
principals of ultrasonic waves are described. In the
vast majority of cases, a pulsed echo technique is
used during the inspection of joints. This technique
is based on distributing ultrasonic waves in a certain
Fig. 18.5: Detection of a crack with magnetic flux inspection. The direction through the joint and its surroundings.
magnetic particles are black in this picture.
These waves are not audible since the frequency is
very high. During inspections of joints in the Strenx
Identification of potential discontinuities requires and Hardox grades, they are normally in the range
good lighting conditions when using colored magnetic of 2-4 MHz . Lower frequencies promote larger
particles and UV light when the fluorescent type is penetration depths while higher frequencies support
used. The latter type can achieve a somewhat higher the ability to find smaller discontinuities. The
sensitivity than the previous type since the magnetic ultrasonic waves will attain a high degree of
particles glow in this case. consistency when distributed in an area where there
are no discontinuities.
After the inspection is made, excess metal particles
and paint, if applicable, are removed in accordance Sonic waves that strike the interfaces between the
with their instructions. steel structure and the surrounding air are reflected
back to the steel due to the large differences in
Demagnetization of the welded structure may be acoustic characteristics between the steel and the
required if the magnetism is too high. This procedure air. However, when ultrasonic waves strike a foreign
is commonly done by applying a magnetic flux that substance in the steel, the sound waves are disturbed
constantly changes direction. This gradually reduces and reflected either more or less. When present, a
the level of magnetization in the structure. One discontinuity represents a substance other than the
method is to wipe the magnetized area with an steel structure.
A.C. yoke.

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. 107
It is the responsibility of the user of this brochure to adapt the recommendations contained therein to the requirements of individual applications.