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by Tony Anderson
Many characteristics of a weld can be evaluated during welding inspection, some relating to the welds size, and others relating to the presence of weld discontinuities. The size of a weld can be extremely important, as it can often relate directly to the weld's strength and associated performance, undersized weld's may not withstand stresses applied during service. Weld discontinuities can also be important. These are imperfections within or adjacent to the weld, which may or may not, dependent on their size and/or location, prevent the weld from meeting its intended performance. Typically these discontinuities, when of unacceptable size or location, are referred to as welding defects, and can sometimes cause premature weld failure through reduction of the weld strength or through producing stress concentrations within the welded component.
The inspection of welds can be conducted for a number of reasons. Perhaps the most fundamental reason is to determine whether the weld is of suitable quality for its intended application. In order to evaluate a weld's quality, we must first have some form of measuring block with which to compare its characteristics. It is impractical to attempt to evaluate a weld's quality without some form of specified acceptance criteria.
Weld quality acceptance criteria can originate from a number of sources. The welding fabrication drawing/blue print will typically provide weld sizes and possibly other welding dimensional information, such as length and location of welds. These dimensional requirements will usually have been established through design calculations or taken from proven designs that are known to meet the performance requirements of the welded connection.
Acceptable and unacceptable levels or amounts of weld discontinuities for welding inspection are usually obtained from welding codes and standards. Welding codes and standards have been developed for many types of welding fabrication applications. It is important to choose a welding standard that is intended for use within the particular industry or application in which you are involved.
Welding inspection can often require a wide variety of knowledge on the part of the welding inspector: the understanding of welding drawings, welding symbols, weld joint design, welding procedures, code and standard requirements and inspection and testing techniques, to name a few. For this reason many welding codes and standards require that the welding inspector be formally qualified or have the necessary knowledge and experience to conduct the inspection services. There are a number of welding inspection training courses available and a number of welding inspector certification programs internationally. The most popular program used in the USA is administered by the American Welding Society (AWS). This is the Certified Welding Inspector (CWI) program. Certification as a welding inspector: will typically require demonstration of an individual's knowledge of welding inspection through passing examination.
In order to further appreciate the extent of welding inspection we will need to examine specific areas of inspection techniques and welding inspection applications. I have chosen the following topics to provide this welding inspection overview:
Inspection and Testing for Welding Procedure Qualification – Types of inspection used for these requirements and how they can be an essential part of the overall welding quality system.
Visual Inspection – Often the easiest, least expensive, and probably, if performed correctly, the most effective method of welding inspection for many applications.
Surface Crack Detection – Methods such as Liquid Penetrant Inspection and Magnetic Particle Inspection – How they are used and what they will find.
Radiographic and Ultrasonic Weld Inspection – Methods known as Non Destructive Testing (NDT) and used typically to examine the internal structure of the weld in order to establish the weld's integrity without destroying the welded component.
Destructive Weld Testing – Methods used to establish weld integrity or performance, typically through sectioning and/or breaking the welded component and evaluating various mechanical and or physical characteristics.
One of the main ingredients of a successful welding quality system is the establishment, introduction and control of a sound welding inspection program. Only after the full evaluation of the weld quality requirements/acceptance criteria, the full appreciation of the inspection and testing methods to be used, and the availability of suitably qualified and/or experienced welding inspectors can such a program be established.