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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24 JUNE 2019 W W W.MATERIALSPERFORMANCE.COM FEATURE ARTICLE FIGURE 1 A pipe with dissimilar weld material. Photo courtesy of Olympus Scientific Solutions Americas Corp. FIGURE 2 The distribution of sound energy through a weld containing a similar weld material (left) vs. a dissimilar weld material (right). At the interface, skewing and attenuation of the UT beam occurs when using shear waves. Images courtesy of Olympus Scientific Solutions Americas Corp. FIGURE 3 A computer model of different wave types: (a) 2.25 MHz longitudinal waves, (b) 1 MHz shear waves, and (c) 2.25 MHz shear waves in a pulse-echo configuration. The material has a greater negative effect on the quality of the beam when using shear waves (as shown by the irregular shape of the red line). Images courtesy of Olympus Scientific Solutions Americas Corp. Scan Planning One of the main considerations for UT inspection is selecting the appropriate wave type. Propagation Mode— Longitudinal vs. Shear Waves When scanning challenging welds, longitudinal waves transmit the UT beam better than shear waves. Modeling the effect of the propagation mode on the UT beam (Figure 3) has shown that the beam undergoes greater skewing when shear waves are used. Therefore, despite potential interference from reflected shear waves, longitudinal waves are recommended for dissimilar weld inspection. Reducing Noise with the Transmit- Receive Longitudinal Technique Weld inspections can be carried out by using the same transducer for transmitting and receiving or using two separate trans- ducers for transmitting and receiving. Due to the high noise levels generated by aus- tenitic and dissimilar metal welds, a one- transducer approach (also known as the pulse-echo technique) is not considered to be optimal. The transmit-receive longitudi- nal (TRL) technique, with two transducers, is recommended. Since one transducer acts as a transmitter and the other as a receiver, the collected signals originate only from the area where the two beams cross each other. With a separate pulser and receiver, the size of the wedge—a plastic piece used in both shear-wave and longitudinal-wave UT applications that couples sound energy from the transducer to the component being tested—can be reduced. No dampen- ing material is required, and the probe can get closer to the weld, which provides higher sensitivity. When using this two-transducer approach with longitudinal waves, low interference is combined with better pene- tration, which results in receiving a signal with a lower noise level. This technique is particularly helpful when testing clad pipes or highly attenuating materials. The TRL technique can be accomplished with con- ventional UT transducers, or dual matrix array (DMA) probes (Figure 4) that consist of two probes—each with an array com- posed of several rows and columns of active elements—wired to the same connector. DMA probes offer certain advantages, such as sectorial scans that allow scan cov- erage without moving the probe back and forth; beam skewing that provides the abil- ity to steer the skew angle of the beam; and compensation for curvature when inspect- ing curved-surface parts (Figure 5). A limitation of the TRL technique is that the volume of signal energy is reduced. Figure 6 shows the region of energy gener- ated from the TRL technique at the beam intersection point as compared to the pulse-echo technique. For the TRL tech- nique, the energy is much lower in the region before the point where the beams intersect, as well as the area beyond the point of the beam intersection, which means that sensitivity in those areas is reduced. Due to a pseudo-focused beam, however, energy comes from the area of interest only (where the two beams cross each other), so the area covered by the scan is still sufficiently large. Inspecting the Surface and Ensuring Full Coverage of the Weld Ensuring full weld coverage is critical when using the TRL technique because of the coverage limitations when using longi- tudinal waves. One outcome when using longitudinal waves is that shear waves are simultaneously produced. Because the two types of waves have different velocities, shear waves interfere with the detection of the longitudinal waves reaching the weld (especially at the top of the weld) when longitudinal waves bounce off the bottom surface of the component. The ability of (a) (b) (c)

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