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Testing Joint Integrity. Nondestructive examination is an important part of assuring the quality of welded components. Some of the more common techniques for examining welds are described below.
Visual inspection is a very cost-effective method and it does not require complicated equipment. It is an ideal first step in the inspection process and can reveal flaws such as surface porosity, undercut, surface cracks and unacceptable bead contour. Visual inspection must be performed by a certified individual before, during and after welding. Various simple instruments, such as fillet and contour gauges, are used with visual inspection.
The limiting factor with visual inspection is the resolution ability of the human eye. It has difficulty determining porosity diameters less than 0.25 mm or cracks less than 0.025 mm wide. Normally, objects closer than 150 to 250 mm are hard to focus on. Also, lighting conditions must be adequate for good visibility. Fiber optics and closed circuit television are useful aids. Only surface discontinuities can be detected with this method and the surface must be cleaned of contaminants to prevent masking of those surface discontinuities.
Penetrant testing involves either the use of dye penetrant or fluorescent penetrant -- Fig. 1. In dye penetrant, a red liquid dye is applied to the surface and if cracks are present, it is drawn into them by capillary action. After a soaking time of about 15 to 30 min a developer is applied to the surface. The developer draws out the dye, leaving a red stain against a white background.
With the fluorescent method, the dye is illuminated by an ultraviolet ("black") light, which shows the flaws in a yellow-green color. The eye discerns the flaw more readily under black light than white light.
The penetrant test can be used on a variety of different materials, but it does require a waiting period before results can be discerned. It is easy to apply, but it only reveals surface discontinuities.
Magnetic particle examination relies on the attraction of externally applied iron-oxide powder to locations of magnetic flux leakage (cracks) resulting from a magnetically induced field. The powder can be applied either wet or dry, but often it is suspended in liquid to aid its application. To provide contrast with the black powder, the surface may be coated with white paint. Fluorescent compounds can be added to the magnetic particles for viewing under a UV light. The results can be seen immediately with this process, but it is only effective on ferromagnetic materials.
Radiography provides the advantage of detecting discontinuities within the material. Gamma rays or X-rays penetrate the material and expose anomalies on a photographic film. A real advantage with radiography is the permanent record it provides of the weld area. It is limited, though, because of the hazards associated with radiation and there are certain joint designs where it is difficult to position film for a reliable exposure.
Ultrasonic testing also has the capability to detect internal anomalies in a material. High-frequency sound directed at an angle into a material echos back into a probe. The sound energy is converted to electrical energy that can be visualized on a CRT display screen in the form of vertical and horizontal lines. This information can be interpreted as representations of discontinuities.
Ultrasonic examination can be used on thick sections and it can determine the depth of discontinuities. It also has the advantage of examining from one side when access is limited on other sides. It is capable of locating small discontinuities, but discontinuities in thin or small welds are difficult to detect, and large-grained metals present problems. The process does require personnel well trained in interpretation of the images.
