Weld-seam defects

Weld-seam defects. Failure to fuse the metal fully along the weld line in welded tubes may result in a linear open seam or crevice. 

Weld-seam defect. This appears as a linear groove or crevice running along the seam formed in a welded component. 

Figure 14.4 Tubercle capping a pit located in a weld-seam defect.

Seams. As defects, seams are distinct from weld seams and may be found in nonwelded (seamless) tubes. They are caused by crevices that have been closed by some rolling process but remain unfused. 

Seams. As a defect, a seam is distinct from the seam resulting from a welding process. Seam defects can be found in nonwelded (seamless) tubes. They can originate from blow holes or nonmetallic inclusions in the ingot and are caused by crevices that have been closed by some rolling process but remain unfused. At times, they will appear in a spiral pattern in tubes. Seams can be very tight and appear as hairlines on the surface. They can cause failure when the component is pressurized. 

Figure 14.1 illustrates one of many similar pinhole perforations found in this and other tubes within the exchanger. Figure 14.2 shows the typical appearance of the internal surface immediately after removing the tube from the exchanger. Laboratory acid cleaning of the internal surface revealed a defective weld seam (Fig. 14.3). 

Tank fabricators and tank inspectors check new and repaired welds on floors of low-pressure tanks using soap suds. They commonly use a vacuum box to check tank floor seams. The inspection team coats a short section of weld seam with soapy water. An inspector places a custom-built rectangular box that has a clear window on the top of the soapy weld seam. A specially designed air eductor, which uses compressed air, evacuates the box. If soap bubbles appear in the box, the inspector marks the defective weld for repairs. 

It should be observed that fusion welding processes are often characterized by defects due to the melting and subsequent soUdification of the metals and to impurities characterizing the welding seam. In this way, processes in which the melting of the metals is not reached can... 

Leaks can originate from a number of sources such as defects in a welded seam, threaded opening, or pits. To check for leaks, the cylinder must be pressurized and examined. All seams and pressure openings must be coated with a leak detection solution, then observed under pressure. Leaks can be detected by the formation of bubbles as gas escapes from the leak. The solution should be compatible with the gas being tested. Any visible leakage except at threaded openings is cause for rejection and condemnation. 

The acceptability standard is satisfied if no cracks or other defects of over 3.2 mm or half the wall thickness, if this is lower, appear after bending in the weld seam or fusion zone in each direction. No cracks starting from the edge of the samples, if smaller than 6.4 mm measured in each direction, should be taken into consideration, unless accompanied by other defects. 

To find and possibly prevent welding defects, the operator must acquire familiarity with the form and dimension of the weld pool and its relation to the form and appearance of the finished weld seam. 

Longitudinal alignment of perforations in a welded tube is a strong indication that a seam may be defective. 

The subject of weld defects is quite extensive, and complete coverage is well beyond the scope of this chapter. Therefore, this chapter will focus on specific types of weld defects of general concern in cooling water systems. Defects of seam-welded tubes are considered under material defects in Chap. 14. 

In 1979 a number of surface marks were noted in the plates adjacent to seam welds on Calder Reactor 4. Two of the features had the appearance of crack-like defects (Figure 4 but more detailed investigations using a hydraulic manipulator with television cameras, eddy current probes, ultrasonic probes and a crack depth gauge, revealed them to be surface imperfections and fabrication flaws. 

The ultrasonic system comprises two scanners and one control console and was designed and constructed by Risley Nuclear Laboratories of the UKAEA. The design intention was for defect signals to be reported and sized in accordance with ASME XI code requirements for the 50 mm thick vessel shell. Weld profiles of the main seam welds were used to define probe angles and movements and a novel permeable membrane, irrigated by water at a rate of 0.04 litres / hour was developed to provide the necessary ultrasonic coupling. The main features of the scanners and the control console are as follows. 

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