4.2.7 Weld Inspection
The inspection of welds is a very sophisticated
subject. The inspector must have a deep
understanding of the material, processes, and application. Inspection of welds carried out by robots is
far easier in an assembly line as long as the process parameters are well
adhered to. However, the quality of the
welding carried out manually, even when process parameters are well adhered to,
will significantly depend only upon the skill of the welder. It is practically impossible to examine all
welds, along entire route lengths, on a finished product.
The weld inspection can be broadly classified,
based on non-destructive testing and destructive testing of the weld under
observation.
4.2.7.1 Non-Destructive Testing
All welds carried out at works and the site,
need to be inspected to establish that they are fit for purpose. The inspection regime will depend upon the
application to which the component is put to use. The welds are subject to inspection as per
the inspection plan that is either specified in the standards or agreed to
between various stakeholders.
Depending upon the type of Busbar System, welding
may be required to be carried out both at works and at the site where it is
installed. The quality assurance plan
for manufacture at works and installation at the site are different. The entire welding inspection plan may
comprise a combination of different testing procedures.
·
Visual
Examination – A visual examination by a trained inspector is the most
cost-effective way of carrying out this activity. But it can only recognize the surface defects
that are visible and draw subsequent inferences. For internal quality and defects, other types
of tests are required for certification.
Surface
Crack Detection – Dye penetrant test (DPT) and magnetic particle test (MPT) are the
most common testing procedures followed for the detection of surface cracks.
Dye penetrant test is also referred to as, liquid penetrant test (LPT). This test can only detect the surface
breaking discontinuities or hairline cracks on the surface. (Any discontinuity that is embedded inside
the weld cannot be detected by this method).
It is also not suitable for porous or rough surfaces where the
interpretation of the test result could be misleading. It cannot detect a lack of fusion in a
weld. It is one of the most prevalent
methods of testing in the bus industry and can be used for weld inspection of
ferrous as well as non-ferrous material.
The test is performed as per the following procedure:
The weld area is scrubbed to remove
slag / foreign matter and thoroughly washed, cleaned, and dried.
The dye penetrant is applied by
spraying, brushing, or dipping the weld into it. The colour of the dye is
normally deep red.
The dye is allowed to sit and be fully absorbed into the surface. This may take a couple of hours depending upon the complexities of the weld.
After a specified duration, the excess penetrant is removed from the surface using a solvent or water.
A developer is then applied (or
sprayed) on the surface of the weld to draw the dye to the surface. The developer is usually white to have a high
degree of contrast with the dye. The
penetrant will bleed out from within the discontinuities and the surface
defects can be located. (Instead of the coloured dye, a fluorescent penetrant
can also be used and a blacklight can be used to detect the
defects in the weld.)
Dye penetration testing procedure
(Figure – 4.7)
Magnetic particle test (MPT) can be
performed only on Ferro-magnetic material.
The test object is subjected to a uniform magnetic field. On a homogeneous test piece, the flux lines
will have a streamlined flow. Any
Non-magnetic object or a void that is embedded in its path will distort the
flux lines. A magnetic probe, scanning
the surface can detect these discontinuities.
These can be observed by the field patterns or accumulations of the
magnetic particles.
Some of the test pieces may get magnetized
during testing and retain residual magnetism.
If it were to affect the performance of the object under service conditions,
then it has to be demagnetized.
(Figure – 4.8)
Inner Crack-Voids-Foreign Object Detection - Defects that have been sealed by welds need to be detected by
radiographic and ultrasonic weld inspection.
These are expensive tests. X-rays
from tube or gamma rays from radioactive isotopes are directed on to the
weld. A photographic film placed beneath
the weld is developed after exposure to the rays through the weld. The amount of energy absorbed will depend
upon the thickness and the density of the metal. The remnant energy will be absorbed by the
film and these areas will appear dark when developed. Since the parent metal also gets exposed on
the same film, defects can easily be identified by comparison. This test is possible when both sides of the
weld are accessible.
Radiographic interpretation requires skill and
experience in the field. A false
interpretation can be very expensive. A
radiographic test provides a permanent record of the test. It is essential that trained and qualified
personnel, who understand the procedures and health & safety issues,
conduct the test.
In the ultrasonic test, a beam is directed
towards the object through a couplant.
When the beam meets a discontinuity, it is reflected back. The study of the incident and reflected beams
diagnoses the size, depth, and shape of the discontinuity. Needless to say that this procedure also
requires skilled personnel to handle the equipment and interpret the
results. For conducting this test,
accessibility to one surface is sufficient.
4.2.7.2 Destructive Testing
This method of testing is carried out for the
certification of welders and failure analysis studies. It is also used in research & development
establishments and for sampling inspection.
There are various procedures adopted for conducting destructive
tests. Among others, it includes micro
etching, macro etch testing, fillet
weld break test, transverse tension test, and guided bend test.
4.2.7.3 Welder Qualification
It is imperative to have the welders qualified and certified to undertake the work they are assigned. While a welder may be qualified from a technical institute, he has to be certified by an accredited agency for the specific type of work undertaken by him for his organization.
Welders will have to be certified for Steel,
Aluminium & Copper welding and also for the degree of difficulty. The welders are to be monitored to ensure
that they continue to possess the skill that they have been certified for.
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