Seam defect

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. 

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. 

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.

Figure 14.5 Severe seam defect apparent after acid cleaning of the internal surface.

Various types of stitch and seam defects are discussed below ... 

The second misconception involves the perception of what constitutes a defect. A defect is not simply a visually observable discontinuity such as a hole, lap, or seam in a component. Defects, from a failure-analysis standpoint, may also be such things as a high residual stress that may lead to cracking or unfavorable microstructures that can lead to either... 

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. 

Surface defects, if sufficiently severe, may result in failure by themselves. More commonly, they act as triggering mechanisms for other failure modes. For example, open laps or seams may lead to crevice corrosion or to concentration sites for ions that may induce stress-corrosion cracking. 

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). 

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

No other evidence of deterioration, such as corrosion, was apparent on either surface. The cracks are probably material defects. They may be laps or seams that were present on the external surface prior to the fin-rolling operation and were exaggerated during the rolling process. 

Figure 14.6 Material defect, perhaps a lap or seam, on the external surface of a finned copper tube.

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. 

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. 

RE etch back or CMP may be used to remove the polysilicon overburden. When RE etch back is used, however, a center seam is etched into the trench. In addition, the RE etch is not self-arresting, and therefore leads to a step at the oxide-polysilicon edge. These defects reduce planarity and make it difficult to reliably cover the trench with the strap film. Center seam and edge step defects in deep trench formation are analogous to the same defects that form when RE etch back is used to remove the... 

It must be sufficiently ductile to enable it to be formed into wire or strip without cracks, seams, laps or other mechanical defects. 

Sub-conformal and conformal electrodeposition of copper in vias and trenches, on the other hand, occurs in an additive-free acid sulfate solution. These deposits have two types of defects voids and seams (Fig. 3 in Ref. 22). 

Defect classifications (weight, seams, seals, wall thickness, pin holes, etc.). 

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