Page 86 - Quick Guide to Welding and Weld Inspection by S.E. Hughes, Clifford Matthews

Non-destructive and Destructive Testing

Advantages
. Quick and simple to use.
. Can find slightly subsurface defects (under certain condi-
tions).
. Portable when permanent magnets used.
Disadvantages
. Can only be used on ferromagnetic materials.
. Cannot be used on austenitic stainless steels (non-
magnetic).
. Electric prods can cause arc strikes.
. The component may need demagnetising on completion of
testing.
Ultrasonic testing (UT)
Ultrasonic testing is used to find internal defects within a
weld or body of a component being tested. A probe emits a
sound wave that is passed through the material. If this sound
wave hits a defect then all or part of it gets rebounded back
to a receiver in the probe and the size and position of the
defect can be plotted on a graph by a skilled operator. Figure
6.3 shows the arrangement.
Angled probes send the wave into a weld at angles suitable
for the weld preparation bevel angles used. A zero degree
‘compression’ probe is used first to check for any laminations
in the parent material that could deflect angled waves and
mask defects in the weld.
The surface of the component must be clean and smooth
and a couplant applied to exclude air from between the probe
and component. The couplant must be suitable for use on the
material being tested and then be thoroughly cleaned off
afterwards to prevent any risk of corrosion or degradation of
the component in service. Wallpaper paste is often used as a
couplant because when it dries it can be easily peeled off.

71

Woodhead Publishing Ltd – A Quick Guide to Welding and Weld Inspection
Data Standards Ltd, Frome, Somerset – 17/9/200906QG Welding chap6.3d Page 71 of 87