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Schematic Representation of GMAW discontinuities. The Schematic illustrates the main discontinuities found in GMAW welded joints. Figure 28: Separate Page
An ultrasonic technique for weld inspection should investigate the total weld volume plus a region either side of the weld considered to be the heat affected zone. When geometry permits, the weld should be inspected from at least two sides. A normal beam scan is usually performed over the metal on which the shear wave probe will traverse. This provides thickness information and indicates areas where plate laminations might exist and interfere with the shear wave beam.
Scan patterns for manual weld inspection are usually zigzag with some overlap to ensure 100% coverage. Some technicians add a pivoting motion to the forward and backward zigzag pattern to enhance detection of off angle flaws. Others prefer to scan in 3 directions. First perpendicular to the weld centreline then +45° to the weld centreline then -45° to the weld centreline. Scanning speed should be slow enough to maintain coupling and ensure the operator can monitor all events on the CRT. The volumetric zone coverage and scan patterns are shown in Figures 1 and 2.
Weld reinforcements such as crown, unground root and root backing bars will limit access for scanning and provide a source of geometry signals.
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Click on a joint and access to Figure 3 to 14
Some solutions require special equipment. An example is very high temperature environments. Probes may be expected to perform simple thickness testing functions but the operating temperatures may be sufficiently high to melt solder joints in wiring and depolarize normal piezoelectric material. For such applications special probes are designed using high temperature piezoelectric materials and electrical contacts are maintained by spring loading mechanical contacts against the element.
Some codes do not adequately address the needs of some industries. An example exists in API and CSA codes relating to ultrasonic examination of electric resistance welded (ERW) pipe seams Primary calibration targets are corner reflectors from a through wall hole but a common and potentially critical defect occurs midwall and does not break either surface. Standard 45° refracted angles best adapted to maximize response off the corner reflectors will not adequately detect midwall defects by simply positioning a probe midway between ID and OD exit point maximums. See Figure 10-22.
Ultrasonic Testing of Pipeline Girth Welds |
Ultrasonic Testing of Pipeline Girth Welds