Back to Article: Weld Inspection UT2
SCHEMATIC REPRESENTATION OF GMAW DISCONTINUITIES
The Schematic illustrates the main discontinuities found in GMAW welded joints.
The descriptions that follow are the foundations for the operator's evaluation of the signals.

Non-depicted cases: Non-fusion Root - Stacked Defects - Bead Offset (or wander) - Transition (counterbore)

Operator Evaluation Checklist
Characteristic ------------------------------------- Comment
Indication over threshold
Channel(s) affected
Symmetry (US & DS)
Transit Time
Length
Yes
Root & LCP
Yes
At calibration target distance and smooth
Acceptable/Rejectable

Defects

Nonfusion Root: Several variations can occur.

Misfire: The internal welding head did not fire or sputtered. No metal is deposited. Ideally this presents 2 smooth root faces, however, welders have been known to see this from the outside and the Hot Pass bug can be run over the area twice. This can cause some metal to penetrate and reduce the surface area of nonfused root face.


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Operator Evaluation Checklist
Characteristic ------------------------------------- Comment
Indication over threshold
Channel(s) affected
Symmetry (US & DS)
Transit Time
Length
Yes
Root only
No
At calibration target distance and smooth
Acceptable/Rejectable
Nonfusion Root:
Missed edge: Due to misalignment of the internal head or high low conditions, one side of the root bevel may not get metal deposited on it. The Main Figure shows a missed edge on the right side of the weld. Undercut, shown on the left, is the depression caused by melting of the parent metal next to the weld edge. It would need to be very deep to detect but we could not reasonably see a difference between U/C (U/C=undercut) and a missed edge.


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Operator Evaluation Checklist
Characteristic ------------------------------------- Comment
Indication over threshold
Channel(s) affected
Symmetry (US & DS)
Transit Time
Length
Yes
Root
No
At calibration target distance and smooth
Acceptable/Rejectable
Nonfusion Root:
Nonfusion Root: It may be possible for the root bead to be placed symmetrically but due to oil or arc redirection, an area of the parent metal does not get melted to fuse with the weld puddle. The bead will appear acceptable on the inside surface but nonfusion still exists. Although it does not appear to be surface breaking it is considered a surface defect


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Operator Evaluation Checklist
Characteristic ------------------------------------- Comment
Indication over threshold
Channel(s) affected
Symmetry (US & DS)
Transit Time
Length
Yes
Root & LCP
No
At calibration target distance and broken
Acceptable/Rejectable
Nonfusion Root:
Nonfusion Root and LCP: If a condition of High Low exists or debris is caught between the root bead and corner of the weld prep. at the land area, the nonfusion may span 2 zones. Such a flaw is difficult to evaluate as one or another condition. If seen mostly by the LCP probe it is called LCP and if seen mostly with the Root probe but with some LCP component it is called a Root indication


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Operator Evaluation Checklist
Characteristic ------------------------------------- Comment
Indication over threshold
Channel(s) affected
Symmetry (US & DS)
Transit Time
Length
Yes
LCP
Yes (usually)
At calibration target distance
Acceptable/Rejectable
LCP:
Lack of Cross Penetration both identifies the defect and is the term used to identify the zone at the weld land area. This can be associated with the internal welding machine not depositing the bead deep enough, the hot pass weld not penetrating deep enough or is often associated with conditions of high low. With welder problems the condition may be more symmetric (seen with approximately equal length and amplitude upstream and downstream) whereas high low conditions could cause one side to be more pronounced that the other. The adjacent channels (Root and hot pass 1) are usually examined to see if the LCP extends inwards or outwards.

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Operator Evaluation Checklist
Characteristic ------------------------------------- Comment
Indication over threshold
Channel(s) affected
Symmetry (US & DS)
Transit Time


Length
Yes
HP1 &/or HP2
No
At calibration target distance.
HP1 may have some associated LCP signal.
HP2 may have some associated Fill 1 signal.
Acceptable/Rejectable
Nonfusion Hot Pass:
With its 45° orientation, the hot pass bevel was a difficult problem for radiographic testing. Due to the surface length of this area it has been divided into 2 zones, hot pass 1 and hot pass 2. UT signals here are clear and due to the large angular difference between the 45° hot pass bevel and the 90° LCP below it and the 85° Fill above, signals from the hot pass zone are not confused with adjacent zone defects. (If they were detectable by RT these would have been called LFSS).


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Operator Evaluation Checklist
Characteristic ------------------------------------- Comment

Indication over threshold
Channel(s) affected
Symmetry (US & DS)
Transit Time
Length
Yes
Fill 1
No
At calibration target distance
Acceptable/Rejectable
Nonfusion Fill 1:
The sources of this defect are the same as for any nonfusion defect in the fill passes. Fill 1 nonfusion is often associated with the corner where the hot pass bevel and the fill bevel meets.





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Operator Evaluation Checklist
Characteristic ------------------------------------- Comment
Indication over threshold
Channel(s) affected
Symmetry (US & DS)
Transit Time



Length
Yes
Fill 2 or 3
No
At calibration target distance
(If this is also the cap zone and
no indication occurs on next lower
Fill a possible undercut may exist).
Acceptable/Rejectable
Nonfusion Fill 2:
This defect can be simple sidewall nonfusion, or it may have a component of cold lap to the Fill 1 pass. In lightwall pipe only 2 fill pass probes are used to cover the Fill 1, Fill 2 and Cap passes. Undercut , if deeper than 1 mm would occur in the Fill 2 zone as well. The operator cannot discriminate where the nonfusion exists. This must be determined by the hand scanner who would have to plot any defects called for repair. If during the manual evaluation no undercut is seen it is assumed the flaw is subsurface. For heavier wall pipe where 3 or 4 fills are required the concerns are the same as for fills 1 & 2.


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Operator Evaluation Checklist
Characteristic ------------------------------------- Comment
Indication over threshold
Channel(s) affected
Symmetry (US & DS)
Transit Time

Length
Yes
2 or more adjacent channels
Not necessary but can occur
At calibration target distance
or slightly after (cold lap).
Stop/starts will be short Geometry or welding machine
difficulties may result in long stacked defects
Acceptable/Rejectable
Stacked Defects:
When a welder stops a weld in the middle of the process (to clean a cup or clear bad wire, etc., they must then restart the weld in the same area. If the arc stutters or the stop was not properly cleaned out a vertical component will be seen that can extend to 2 or more zones. If the chart indicates the situation exists on 3 or more adjacent zones, it is recommended to investigate further using manual ultrasonics even if length is less than 25mm.


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Operator Evaluation Checklist
Characteristic ------------------------------------- Comment
Indication over threshold
Channel(s) affected
Symmetry (US & DS)
Transit Time

Length
Yes
Root & LCP
Not necessary but can occur
Less than calibration target distance
and maybe smooth or rough
Acceptable/Rejectable
Burnthrough:
This occurs when the heat of the melt is sufficient to weaken the weld metal laid down previously and pokes through thereby removing a volume of metal from the inside surface of the weld/pipe. The amount of material may be very small, typically 5-6 mm diameter and only be sufficient to remove the bead surface. This would produce little or no indication on the root probe channel. However, if the situation is more pronounced, metal could be removed from the hot pass and bead as well as material from the parent metal. This would be seen on Root, LCP and 1 or 2 hot pass zones. Associated with this stacked indication would be an arrival time sooner than the normal nonfusion and some degree of symmetry.

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Porosity:
Operator Evaluation Checklist
Characteristic ------------------------------------- Comment
Indication over threshold
Channel(s) affected
Symmetry (US & DS)
Transit Time

Length
Yes
Root and LCP
Usually
Very irregular varying from calibration
target distance to 1-2 mm beyond
Acceptable/Rejectable
Caused by gasification of impurities on the weld prep. surface or by loss of shielding gas, porosity is seen by the standard probes in the Root and LCP zones. For fill and hot pass occurring porosity, a special probe is incorporated into the system. Signals will typically have irregular amplitudes in the amplitude gates and irregular arrival times in the time gates although with such short time intervals as exist in the Root and LCP this is more difficult to see. Some symmetry is often noted and if the porosity is heavy, the geometry signal seen from the weld surface will be reduced or even eliminated.

Porosity Bead (See image part: Root Bead Porosity)

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Operator Evaluation Checklist
Characteristic ------------------------------------- Comment
Indication over threshold
Channel(s) affected
Symmetry (US & DS)
Transit Time


Length
Yes
Porosity channel and others from HP1 up
Usually
Occurs at varying distances and usually
begins at base of pass affected. Associated
cap geometry signal may drop back
Acceptable/Rejectable
Porosity Fill (See image part: Typical Porosity)







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Operator Evaluation Checklist
Characteristic ------------------------------------- Comment
Indication over threshold
Channel(s) affected
Symmetry (US & DS)
Transit Time


Length
Yes
Fill 1 &/or 2 &/or 3
Yes
Beyond calibration target distance
corresponding to centre notch or hole and
slightly irregular
Rejectable any length
Centreline Cracking:
If too much weld metal is deposited too quickly the heat gradient in the weld nugget will cause shrinkage cracking to occur as it solidifies (also called solidification cracking or shrinkage cracking). This is not likely to occur in the root pass but may occur in any pass made from the outside surface. As this is likely to pass right through the weld nugget its amplitude is usually large and symmetry exists. On a radiograph the crack edges are very fine and may not show clearly, it would then be misinterpreted as LCP as it is approximately centred.

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Geometry
Operator Evaluation Checklist
Characteristic ------------------------------------- Comment
Indication over threshold
Channel(s) affected
Symmetry (US & DS)
Transit Time

Length
Yes
Root and LCP
No
Long transit time on high side associated
with short transit time on low side
NA
High-Low:
Not a defect, high low is a geometry condition caused by ovality or poor mechanical fit. Care must be taken by the operator to ensure the indications originating from this are not called for repair. However, this condition can cause genuine defects and the two must be discernible. Large differences in root transit times are usually an indication of high low.


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Operator Evaluation Checklist
Characteristic ------------------------------------- Comment
Indication over threshold
Channel(s) affected
Symmetry (US & DS)
Transit Time







Length
Yes
Root and LCP
No
Transit time for root probes move in
opposite directions. If bead wander then
movement of transit times is gradual and
across the centreline. If bead off-set then
one or more internal welding heads are seen
to have transit times long on one side and
short on the other and will exhibit a sudden
jumping of position from the centre
NA
Bead Offset (or wander): Internal welding is performed using 6 welding heads arranged to align with the weld centreline. If not correctly set, one or more heads will deviate from the central position. This may be a head starting on one side and crossing the centreline or it may be a head that moves straight but is offset upstream or downstream. Root transit time indicates this condition.






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Other
Operator Evaluation Checklist
Characteristic ------------------------------------- Comment
Indication over threshold
Channel(s) affected
Symmetry (US & DS)
Transit Time



Length
Yes
Any channel can be affected
No
If sloped the test cannot be carried out from that side.
If too thick signals will usually have length
as well as long transit times. All signals
must be evaluated manually
NA
Transition (counterbore): To allow heavywall and lightwall pipe to be joined the heavy wall pipe is counterbored to the same thickness as the light wall. If the counterbore is made as a taper the counterbored side cannot be UT inspected as the skip angles are no longer correct. If the counterbore is parallel inspection is possible but ovality may result in some areas being thicker than they should be. This results in the gated regions being incorrect for the sound path traveled.


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Copyright © Rolf Diederichs, rd@ndt.net 1. Apr 1998