Specific Weld Defects

General description. Burnthrough as discussed here specifically refers to the melting of tube metal in the vicinity of the weld such that a cavity is formed. If burnthrough is severe, a continuous channel may be produced that can cause leakage. 

Locations. Burnthrough in cooling water equipment may affect welded tube-to-tubesheet joints. 

Criticai factors. Burnthrough may result from the use of excessive welding heat relative to the thickness of the tube. 

Identification. If accessible, defects from burnthrough may be visually identified as fused holes in the tube wall. Various nondestructive testing techniques, such as radiography and ultrasonics, may also detect this defect. The defect generally causes leakage soon after affected equipment is placed in service. 

Eiimination. Careful use of appropriate welding procedures and techniques, as these relate to metal temperature, is necessary, especially when welding thin-walled tubes. 

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In anticipation of the waiver, DOT requested assistance from NBS In evaluating the fracture mechanics analysis and the nondestructive evaluation methods used to detect and determine the dimensions of specific weld defects. The model used in the waiver request by Alyeska was actually more conservative than others that were generated In the evaltiation by NBS scientists. 

For NDT of new construction this implies that, the more one knows about the material properties and operational conditions, the better the acceptance criteria for weld defects can be based on the required weld integrity and fine-tuned to a specific application. In pipeline industry, this is already going to happen. 

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. 

The report of the inquiry [111] criticised the design and fabrication of the alterations made to the original pontoon. The actual cause of the accident was the failure of some tie bars in the detail around the jacking points. The failure was due to brittle fracture which initiated from severe notches such as a small radius curve at the fillet between the spade end and the shank of the tie bar. Weld defects and fatigue cracks were also present in tie bars subsequently recovered from the sea bed. The tie bars had been flame cut to shape and had weld repairs visible to the eye. There had been no post welding heat treatment of the steel. The steel complied with the original specification but tests showed low Charpy V notch impact values. Photo elastic tests indicated a stress concentration factor of 7 at the fillet between the spade end and the shank. The fracture was initiated in the opinion of the inquiry tribunal by the low ambient temperature of around 3°C. 

The integrity of welded stmctures depends on the integrity of the welds, and much attention is given to testing methods, such as destmctive tests, nondestmctive tests, and general weld inspection. An objective of many tests is to determine whether welds contain specific defects, such as porosity, slag inclusions, cracks, or lack of fusion (14,15). 

Weld inspection duties of personnel responsible for judging the quahty of welding with regard to specifications have been treated (15). Some of these duties involve the visual inspection of welds to determine if they are of the proper size, location, and type and are free of defects. Specifications of materials used must be checked, as must equipment and procedures. 

There is no guarantee that crack-free joints will automatically be obtained when fabricating weldable metals. This is a result of the fact that weldability is not a specific material property but a combination of the properties of the parent metals, filler metal (if used) and various other factors (Table 9.7) . The consequence of the average structural material possessing imperfect weldability is to produce a situation where defects may arise in the weld deposit or heat-affected zone (Table 9.8 and Fig. 9.27). 

It is always worth while to pay attention to the quality of welded pipe by quoting the correct specification and by adequate shop inspection. API 5L and ASTM A155 require electric fusion welds to be double side welds, as do DIN 1626 Blatt 3 and Blatt 4 and BS 3601 SFW (spiral weld). ASTM A134, DIN 1626 Blatt 1 and Blatt 2 and BS 3601 EFW, on the other hand, permit single side welds. The quality control measures called for in the specifications permitting single side welds do not guarantee freedom from defects, so they are not suitable for the more severe duties. 

Scientists (Ref 12) welded HSLA-65 using tungsten-base tools. Subjected to bend tests, a 10 mm (0.4 in.) thick weld passed, and a 6 mm (0.24 in.) thick weld failed when bent with the root in tension, due to the formation of surface cracks. Tensile properties of the 10 mm thick welds exceeded the specifications for the base metal. Some 6 mm thick welds exceeded the plate specifications, while others were approximately 10% below the plate specifications. Charpy V-notch (CVN) toughness at both -29 and 0 °C (-20 and 0 °F) were below the base material toughness but exceeded the minimum specification of the plate. The surface of the welded material was found to have small defects due to the roughness caused by the interaction between the shoulder and the surface of the plate. Salt spray corrosion tests indicated no preference for corrosion in the weld zone. 

The importance of adequate calibration is paramount in any ultrasonic inspection and is generally caurried out both to monitor equipment stability and to enable defect echo amplitudes to be referred to those from known standard reflectors. Figure 6 depicts the calibration block specifically designed for this work. One surface was machined concave with the same radius of curvature.as the cone at the position of the outer weld. Two 3 mm diameter flat bottomed holes (FBH) and a 1.5 mm diameter side drilled hole (SDH) were provided at a depth equivalent to the cone plate thickness and spaced sufficiently far apart that reflections could be obtained from each one independently of the others. A second 1.5 mm SDH at 15 mm depth served two purposes, firstly as euti equivalent reflector to the SDH in the standard A2 block and secondly to provide a means, in conjunction with the other SDH, of checking the probe angle. One section of the block, V thick, simulated the cone plate itself and was used for recording backwall echo amplitudes for the focused and normal probes. 

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