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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26 JUNE 2019 W W W.MATERIALSPERFORMANCE.COM FEATURE ARTICLE FIGURE 7 A calibration block for UT inspections. Photo courtesy of Olympus Scientific Solutions Americas Corp. FIGURE 8 A computer simulation showing how a material can cause a deviation of the UT beam. Image courtesy of Olympus Scientific Solutions Americas Corp. (usually a side-drilled hole) that can be compared to a real indication. Repeatability means that two inspectors using the same inspection procedure can produce the same report, and that the selected calibra- tion block can be duplicated consistently for different operators. ISO 22825 1 requires a block with a weld. Depending on the application (e.g., piping or pressure vessels), different reflectors can be used. Although it's more representative of an actual inspection, a reference block containing a real weld has several drawbacks, mainly in regard to repeatability. The inhomogeneous weld material can attenuate UT energy ; the detection sensitivity of indications may vary when drilled at slightly different depths; different beams generated at dif- ferent angles can have a different velocity ; and if two equal calibration blocks are required, the attenuation might differ from one block to another due to varia- tions in the manufacturing process. As a result of these issues, it's possible that repeatability cannot be ensured, which means the sensitivity level also cannot be guaranteed. When a reference block is made of the parent material only, it becomes easier to create two blocks with exactly the same properties, such as attenu- ation and velocity. It also ensures that the sound paths are the same and, therefore, have a similar attenuation. Wedge Delay Calibration When a wedge is used to generate angled beams, the entire setup must be calibrated. The cali- bration is similar to what is done for a stan- dard pulse-echo inspection using a refer- ence block that contains well-defined reflectors. Sensitivity Calibration In most cases, two sensitivity calibra- tions are carried out: one for the longitudi- nal waves covering the main volume and one for the surface waves covering the upper (near-surface) part of the weld. Once the repeatability of the calibration has been ensured by selecting a suitable calibration block, sensitivity calibrations can be performed. Detection Level After the sensitivity calibrations have been carried out, the gain level of the longi- tudinal and surface waves should be adjusted to provide a reference value. This is performed by measuring well-known indications, such as a real indication or a flat-bottomed hole in a real weld. The gain must be adjusted to minimize the noise level so that the different imaging modes— such as S-scan, B-scan, and C-scan views— provide the easiest possible detection. Scans with the best possible signal-to-noise ratio lead to clearer images and facilitate the analysis process. Sizing and Depth Tolerance of Indications Although flaw sizing and characteriza- tion may not be as accurate for austenitic/ dissimilar welds as they are when inspect- ing CS, ultrasonic PA inspection still pro- vides more information than radiography as long as proper care is taken. Determining the depth tolerance, or the maximum depth measurement that can be performed vs. the real depth, is particularly important when two different materials with different properties are present in the UT beam's path. In this situation, the energy is spread and can be deviated; there- fore, the beam angle value cannot be fully guaranteed (Figure 8). Consequently, the depth measurements might be affected, and the depth tolerance must be adjusted. It is often safer to oversize indications rather than undersize them; however, over- sizing can lead to an unnecessarily high rejection rate. A good way to validate sizing is to scan several samples with artificial reflectors that represent real indications to be detected, and then macro-section them to confirm the sizes. Coupling A final consideration for an inspection protocol is coupling. With the TRL tech- nique, the wedge holds two probes that generate two UT beams. This creates two exit points located at different areas on the wedge (vs. a single exit point for the pulse- echo technique), which makes the inspec- tion more sensitive to coupling issues between the wedge and the component. Careful attention must be taken to avoid air bubbles between the two probes and the component. This can be done using either a manual or an electric water pump. Ultrasound technology can be used for fast and detailed inspection of even the most challenging welds. However, many considerations regarding the techniques, equipment, and calibration need to be addressed to ensure a successful inspection procedure. As with any inspection method, it is important to understand the limita- tions of the technique and to adjust the procedure accordingly. In recent years, inspection professionals working in many different industries have demonstrated the clear benefits offered by off-the-shelf PA equipment. When the potential issues described here are ade- quately addressed, the TRL technique using DMA probes can bring precision and confi- dence to the inspection process. Source: Olympus Scientific Solutions Americas Corp., www.olympus-ims.com. Edited by Kathy Riggs Larsen. Reference 1 ISO 22825:2017, "Non-destructive testing of welds—Ultrasonic testing—Testing of welds in austenitic steels and nickel-based alloys" (Geneva, Switzerland: ISO, 2017).

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