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Welded joints in any component or structure usually require thorough inspection. This inspection would be by one or more of the techniques that we define under the title of Non-Destructive Evaluation (NDE). The role of NDE in the metals and fabrication field is extremely important and, by extension, the approach and the methods that can be used have become highly developed.

Non-Destructive Evaluation (sometimes written as NDT for Non-Destructive Testing) comprises a number of test methods which, as the name defines, are used to test components without subjecting the component to any damage or curtailment in its operation.聽 The main advantage of NDE, as opposed to destructive tests such as destructive proof or tensile tests, is that the inspection can be performed during the fabrication or manufacturing steps. Such NDE can continue during the service of the component if 鈥渋n service鈥� maintenance checks are required by design or called out by the governing standard to which the component was built.

Any discussion of the topic of inspection by NDE has to include a clear definition of what can be defined as a defect. A defect is a discontinuity in the weld zone that creates a possible risk of failure to the structure during its designed service life, and, as such, presents a danger to the public and the environment.聽 The weld zone, by definition, includes the weld metal and adjacent Heat Affected Zone or HAZ. A component may contain several discontinuities but they are only defects if they exceed given parameters as defined by the governing fabricating standard. I.e. CSA W 59 or ASME V111. These parameters will normally; include defect type, defect distribution, length and perhaps location.

Neither metal, or the welds that join them, are free from imperfections. For example, steel plates may contain any number of inclusions depending on the steel making practice and the quality of the order required. Welds are also very rarely free from discontinuities; a SMAW weld may contain slag inclusions but a GTAW weld will not as it doesn鈥檛 form a slag. Similarly, a GTAW weld may contain a tungsten inclusion that has broken away from the Tungsten electrode but a FCAW weld will not as it does not employ tungsten as part of the process. When a weld discontinuity is located it must be identified, be sized, its position located and its significance defined.

The significance of a weld discontinuity, whether it is a defect not, will be viewed in the context of the fitness for service of the welded fabrication. A fitness for purpose evaluation and the standard drawn from it will be significantly different from that of a metal garbage bin to a metal staircase, to a steel bridge, to an aeroplane or to a nuclear reactor.

Non-Destructive Evaluation may find many discontinuities in the structure, but only those that are outside the acceptance criteria of the fabrication standard in use will be defined as a defect. Any such defect may need to be removed from the weld, subsequently repaired and inspected again on the completion of repair.

Figure 1. Diagrammatic Representation of Possible Discontinuities in a Weld.

In debating the significance of a discontinuity, let us consider the two discontinuity types shown in Figure 1 above. Either of these two discontinuities types could be found by visual inspection if on the surface or, by other forms of NDE if in the body of the weld.

On the left a linear 鈥渃rack like鈥� discontinuity is shown. This could occur in the weld zone, due to the effect of hydrogen, to produce a hydrogen induced cold crack (HICC) or could be caused by solidification mechanisms and thus would be a hot or solidification crack. It may also occur due to corrosion mechanisms, such as a stress corrosion crack. These distinctions don鈥檛 matter when reviewing the discontinuity, it鈥檚 linear and 鈥渃rack like鈥�.

On the right rounded discontinuities are shown which are commonly defined as pores. In steel, porosity is caused by聽the absorption of nitrogen, oxygen and hydrogen in the molten weld pool which is then released on solidification to become trapped in the weld metal. Nitrogen and oxygen absorption in the weld pool usually originates from poor gas shielding.

In aluminum the primary gas of concern is hydrogen, which originates from a contaminant, either a hydrocarbon (grease, skin oils) or moisture, both of which can break down in the arc plasma to produce atomic hydrogen. Hydrogen has a very low solubility in solid aluminium and so as the aluminium solidifies, hydrogen gas is evolved and forms gas pores which are then frozen within the weld bead.

Linear or crack like discontinuities are forbidden by most standards as they contain sharp edges that may continue to grow under load and hence reach a critical failure size. Conversely, rounded discontinuities are considered less likely to grow under load and, therefore, a certain amount of these kind of flaws are normally accepted by fabrication standards.

The bottom line is that these discontinuities, which will always be present, need to be found in order to negate possible premature failure of the component over time They need to be located, identified and sentenced as 鈥渁ccept鈥� or 鈥渞eject鈥� and possible repair. To find them we need one or more of the tools and techniques that Non-Destructive testing offer us.聽

These tools and techniques will be the focus of future notes in this series of articles.