Materials Performance

JUN 2019

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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25 MATERIALS PERFORMANCE: VOL. 58, NO. 6 JUNE 2019 Nondestructive Testing of Corrosion-Resistant Alloys and Dissimilar Weld Materials FIGURE 5 An inspector uses an ultrasonic PA flaw detector to inspect curved tank welds. Photo courtesy of Olympus Scientific Solutions Americas Corp. FIGURE 6 Using the TRL technique with DMA probes offers the ability to steer the skew angle of the beam. Images courtesy of Olympus Scientific Solutions Americas Corp. FIGURE 4 A dual matrix array probe consists of two transducers that contain 28 PA elements each. Photo courtesy of Olympus Scientific Solutions Americas Corp. longitudinal waves to bounce off the bot- tom surface is also limited by internal clad- ding by a material dissimilar to the main component. For example, the bottom sur- face of a component cannot be used for bouncing back longitudinal waves when this surface is coated with a CRA. Because of the difficulties in inspecting the uppermost section of a weld with a lon- gitudinal wave sectorial scan, the near- surface volume of the weld can be covered using surface waves. Depending on the materials, the weld's structure, and the probe parameters, surface waves enable inspection of the first few millimeters in front of the probe. If this distance is suffi- cient, the inspection can be carried out without removing the weld cap. However, when required, the weld cap should be made flush, and a second scan may need to be performed on top of the weld itself to ensure full coverage. Electronic Focusing and Probe Selection The probe aperture, as well as the need to vary the spot size by electronic focusing, are other considerations when choosing the appropriate PA probe for a particular type of inspection. When good sensitivity and good sizing capability are required, PA probes enable control of the spot size of the UT beam. Depending on the thickness of the material, decreasing or increasing the spot size helps achieve maximum sensitiv- ity at the depth of interest. Each PA probe has a natural focusing depth, known as the near field distance. Electronic focusing and/or mechanical focusing can be used to reduce the spot size and concentrate the probe's energy at the depth of interest, which brings the focal point closer to the surface compared to the near field distance. When a thicker compo- nent needs to be inspected, however, it is not possible to lower the focal plane by electronic focusing. In this case, a PA probe with a higher aperture should be used. Instrument Capabilities When a weld is inspected from both sides, the PA instrument must drive several probes simultaneously for a single-pass inspection. In this case, a 128-channel PA instrument is often required. To inspect components with a thickness of up to 25 mm, a DMA probe with a frequency of 4 MHz can be used. For deeper inspections or to inspect welds with larger grain sizes, ade- quate wave propagation can be challenging. In this case, a probe with a lower frequency (1.5 or 2.25 MHz) and a larger active aperture may be required. Calibration Calibration is an essential aspect of UT inspections, and a suitable calibration pro- cess must be determined once the proper combination of PA equipment has been selected. Choosing the Calibration Block The first important consideration is choosing the correct calibration block (Fig- ure 7). The two main options for a calibra- tion block are a reference block made of the parent material only (without any weld), and a reference block made of a production weld similar to the one to be inspected. Two parameters must be considered when selecting the blockā€”the sensitivity of the calibration and the repeatability of the inspection. Sensitivity refers to setting a reference level according to a well-known indication present in the calibration block

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