Page 82 - Quick Guide to Welding and Weld Inspection by S.E. Hughes, Clifford Matthews

Chapter 6

Non-destructive and Destructive
Testing

Liquid penetrant testing (PT)
Liquid penetrant examination, often called dye penetrant or
penetrant testing (PT), is used to find surface breaking
defects only. It involves the use of a cleaner (degreaser), a
liquid penetrant and a developer. The most common PT
system on site involves the use of these three materials from
cans and is referred to as the ‘three can system’. Figure 6.1
shows the idea.
A typical colour contrast procedure involves preparing the
surface to remove any spatter, slag or other imperfections
that could retain the penetrant and mask relevant indica-
tions. The surface is then thoroughly cleaned, using the
cleaner, to remove any surface oil or grease, which could
prevent the red liquid penetrant being drawn into surface
breaking cracks or indications. It is then dried using air or
lint free cloths.
The penetrant is then applied by spray or brush and left for
a dwell time as specified in the procedure. This dwell time
must be long enough to enable the penetrant to be drawn into
surface breaking defects by capillary action. Excess penetrant
is then removed using cloths dampened with the cleaner. The
cleaner must not be sprayed directly on to the component
otherwise penetrant could be washed out of relevant
indications.
Developer (a white chalk like substance) is then lightly
sprayed on to the surface causing the red penetrant to be
drawn out of any indications by reverse capillary action and
the blotting effect of the developer. Any indication high-
lighted by the red penetrant against the white developer can
then be assessed. In effect, PT is a form of enhanced visual

67

Woodhead Publishing Ltd – A Quick Guide to Welding and Weld Inspection
Data Standards Ltd, Frome, Somerset – 17/9/200906QG Welding chap6.3d Page 67 of 87