Page 262 - CITS - Welder - Trade Theory

WELDER - CITS

to receive them. The main application of transmission method is the inspection of plate for cracks or laminations
that have relatively large dimensions compared to the size of the search unites.
Testing of Welds : Before actually commencing the examination of welds it is necessary that aspects like visual
conditions of the weld surface, couplants & the parent metal in the scanning zone be considered.

The visual appearance of the weld should be checked for shape of the weld, e.g. surface curvature, degree of
root penetration, backing ring, different parent metal thicknesses, and extent of the reinforcement, presence
undercuts, weld finish and alignment of parts.
Couplants : A coolant, usually a liquid or semi-liquid is required between the face of the probe and the test
surface to permit transmission of the acoustic energy from the transducer to the material under test. Typical
couplants include water, oil, grease and glycerin.
Defect Detection : To detect all possible defects, the weld is to be examined over its entire cross section and
along the length specified. For the detection of longitudinal defects the shear wave prove is placed on contract
surface and kept perpendicular to the weld centerline. To examine the entire weld, the probe is moved over the
scanning zone.
Defect Location: The accurate determination of the position of a defect in a welded joint is important not only
when repairs have top be made but, in an ultrasonic examination it can give, together with defect orientation
useful information for the determination of the type of defect. The position of a defect is determined by the
distance (Projected path length between prove index and reflector) and depth from the surface. The distance and
the depth can be easily calculated from the path length prove angle and thickness.
Defect Identification: Important indication with regard to the shape and orientation of a defect can be obtain
from the appearance of the signal and its behavior when the defect is scanned from various directions. The echo
signals obtained from the planar defects, such as cracks and lack of sidewall fusion will mostly appear as sharp
and narrow indications. They are characterized by their directionality to the incident beam in both directions Thio-
Sulphate are the two chemicals popularly used. Hardening agents are added top harden the gelatin which is very
soft until this treatment.
Thorough washing in flowing water removes the chemicals embedded in the gelatin. The films are dried in a Just
free chamber at 50° C and they are ready for evaluation.
Image Quality Indicators (IQI)
As a check on the adequacy of the radiographic technique, a standard test piece, called a pentameter, (IQIs) is
placed on the source side of the specimen. The pentameter is of a simple geometric shape, made of a material
radio graphically similar to the specimen itself, and usually contains some simple structure such as strips wires
etc. Its thickness is a definite proportion (e.g.2%) of the specimen thickness. The radiographic technique may be
considered satisfactory if the penetrameter and its structures are show clearly in the radiograph. If the radiographic
procedure has been able to demonstrate a 2% difference in specimen thickness, (i.e. it shows the structure of a
2% penetrameter) the penetrameter sensitivity is considered satisfactory. Wire type, step wedge and plate hole
are the widely used types of penetrometers and are covered by different codes.
It should be remembered that even if a certain hole in a penetrometer is visible in a radiograph, the cavity of
the same diameter and thickness in the specimen may not be visible, the penetrometer holes having sharp
boundaries, given un abrupt through small change in metal thickness, while a natural cavity with more or less
rounded sides gives a gradual change. Therefore the image of the hole will be sharper and more easily seen in
the radiograph than the image of the cavity. Similarly, a fine crack may be of considerable extent, but if the X- rays
happen to pass from tube to film normal to the plane of the crack, its image on the film may not be visible because
of the very gradual transition in photographic density. Thus a penetrameter is used to indicate the quality of the
radiographic technique and not as a measure of sizes of the cavity which can be shown. A thin stock or a uniform
block may be used to keep the penetrameter on, when the object to be radio graphed is not suitable to keep the
IQI directly over it.
Radiographic Interpretation:
The interpretation of a radiograph involves the following three distinct stages:

Verification that the pattern of the radiographic image in conformity with the shape of the part and that it is related
to the particular specimen under consideration.

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CITS : C G & M - Welder - Lesson 83 - 97