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This
equipment is specifically for use in detecting surface and near
surface discontinuities in ferrous billets of sizes from 120mm
square to 200mm square, using a non-contact magnetic particle
inspection method.
Billet
Sizes 120mm square to 200mm square
Billet
length can vary but is not significant
Billet
out of straightness maximum 100mm in 10m
Non-contact
and therefore no wearing parts
Fast
throughput continuous operation, approx inspection speed 500mm
per second
Improves
test integrity over traditional methods
Relatively
low capital cost and running cost through lower electrical current
requirements
Magnetic
Particle Inspection of hot rolled steel billets is a long established
technique for the detection of surface breaking longitudinal cracks
as part of the mill production routine quality control process.
This
system aids grading and provides indication of severity of defects
which are the result of the rolling process. The common method of applying this technique was to pass high
amperage current along the length of the billet to induce circular
magnetic flux in it.
Whilst
the above method is an excellent system it suffers from several
drawbacks.
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For
the range of lengths of billet normally tested exceedingly high
output current from the power pack are necessary.
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The
high current is passed in to the billet by contact pads. It
is essential, in order not to decrease the available current
yet further, that the pads are in very firm contact with the
billet. Billet ends are normally ragged, sheared faces lacking
the flatness necessary for good contact.
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It
unlikely that all billets are of an identical length it is necessary
that at least one of the pads needs to be moved a significant
distance to suit the actual length of the billet.
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The subdued light viewing booth - and the ultraviolet illumination,
must be at least the length of the maximum billet which requires
the inspector moves along the billet to view the upper half
of it.
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The
length, along the line, required for this type of system is
extensive and the capital cost is high.
A
method to overcome these shortcomings but be equally effective -
requiring no high current contact, indeed no contact at all, with
the billet for magnetising purposes has been developed.
This
is the yoke system in which progressively only a short length of
billet is magnetised circumferentially and inked whilst the billet
moves axially through the system. With the special viewing arrangements
adjacent to the magnetising station it is thus not necessary to
stop the billet even for inspection purposes. Transverse flux is
induced by the yoke - in square billets two yokes are necessary.
Each
yoke magnetises two opposite faces by generating A.C. flux between
the pole piece ends.
A.C.
flux with its attendant skin effect will traverse the two appropriate
sides creating a transverse magnetic field in the required direction.
Indicating ink applied immediately before each yoke will allow particles
to move through the fluid layer to the defect edges during the magnetising
phase.
Immediately
after each magnetising position will be a viewing station. The viewing
can take either of two forms. Each of the two viewing stations can
have a man to view the upper surface and, with the aid of a specially
designed mirror system, the lower surface.
The
operator would be provided with a remote operated marking device
to mark defects appropriately.
Since the yokes are operating continuously with A.C., as the billet
moves from the yokes it is subjected to a reducing A.C. reversing
polarity field and therefore is automatically demagnetised.
FOR
MORE INFORMATION PLEASE SEE:
AN IN-LINE AUTOMATIC BILLET MAGNETIC PARTICLE INSPECTION SYSTEM,
AUTOMATIC DEFECT RECOGNITION
IN MPI - INLAND STEEL RESEARCH PROJECT and
AUTOMATIC
DEFECT RECOGNITION IN MAGNETIC PARTICLE INSPECTION APPLICATIONS
(all in PDF format) from our Technical Papers Library
To obtain this
information as an Adobe Acrobat document Click
Here.
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