QUALIFICATION OF A BUTT-WELD INSPECTION USING PHASED ARRAY IN LIEU OF RADIOGRAPHY

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1 More Info at Open Access Database QUALIFICATION OF A BUTT-WELD INSPECTION USING PHASED ARRAY IN LIEU OF RADIOGRAPHY 1 Context B. Dormoy, ALSTOM POWER, France J. Delemontez, T. Le Guevel, EDF, France The low profile scanner named Scan 13 and ultrasonic procedure allow for buttwelds to be inspected rapidly and without any shut down of production work as with radiographic process. This makes Alstom construction more competitive and safe. Since this scanner only requires 13mm of clearance, it opens up the door to do inspections that were previously not possible due to limited access. Since this scanner design adapts to various sized weld diameters, it can be used to inspect welds in virtually all business segments from turbine to HRSG, boiler, hydro, wind and transport Butt welds made during repairs on boiler screens or pendular tubes are particularly difficult to inspect. These welds are usually inspected radiographically. The consequences and impacts of the choice of this method during a unit outage are known and are restrictive. In order to speed up examinations and reduce analysis time, Alstom has developed a method of examination by ultrasonic phased array to inspect an entire weld quickly and efficiently. Figure 1 Buttweldscanner Design To facilitate monitoring and thereby promote effective management of the maintenance policy, the following are necessary: be able to characterize the indications located in the weld or HAZ (heat affected zone); perform the inspection, even if inspection conditions are not optimal (confined space, high temperature of the part being examined (up to 60 C); facilitate monitoring of machinery in the areas inspected by offering a reliable technique for recording the signals measured. Alstom proposed EDF to apply this new ndt technique for the the control of boiler tubes butt welds in lieu of radiography according criterias of french decree of March To meet the EDF requirements, a qualification dossier was constituted including: technical justifications and specifications, demonstrations of performance taking account of essential parameters, tests with representative mock-ups and representative CIVA simulation, specific ndt procedure, training program for the operators. 2 Specifications and objectives The purpose of the technique developed by Alstom is to be able to detect indications that may appear when butt-welding steel tubes (stainless steels are excluded from this study). 482

2 Their position and orientation may vary, but possible indications are expected at the welded joint and oriented along the axis of welding. Figure 2 illustrates the position of expected defects in different parts of a weld (lack of fusion or incomplete fusion, excess or lack of penetration, porosity, inclusions, cracks, etc.) From an operational perspective, it is necessary to develop a technique capable of taking into account the vagaries of site works (overlapping sites, confined space) so as not to penalize the rollout of the entire overhaul in progress during NDT. Moreover, it is also necessary to impact the inspection environment as little as possible so that NDT operators can be autonomous to carry out their work. Finally, the new inspection technique is semi-automated to ensure acquisition repeatability/ reproducibility and to record the acquisitions, which makes for proper monitoring. The semiautomated process will also save time because the analysis will be carried out almost simultaneously. Figure 2 Expected Defects 3 Justification for the choice of NDT technique The choice of ultrasounds for weld inspection is a natural one, since the requested inspection turns out to be a volume examination. So even though some indications may be surface-breaking, they are scarcely, if at all, accessible to conventional surface techniques (PT, MT, ET). It is therefore necessary to use a so-called volumetric technique. Only ultrasounds and radiography can be used to examine the volume of a component. But here, radiography, used up until now, has drawbacks when being used, as it is not possible to locate faults detected in the depth direction. Positioning the film can be tricky, depending on the weld being examined. In addition, radiographic testing has a major drawback due to the presence of a dead zone (limit of elliptic RTs) that mean two different shots have to be realized to ensure complete coverage of the weld. Moreover, the regulations on radiation protection imply major consequences in terms of site organization for the overhaul in progress. In addition, the use of radiography is becoming more and more subject to regulations due to changes in the Decree of 24 March The use of ultrasound will therefore be a definite advantage. The methodology is based on ultrasounds because they can also be used to examine a component in its volume and locate defects using the sound path (PS). The multitude of variables to be taken into account when making settings for the examination (groove type, thickness, angle, tube diameter, grade of material, etc.) leads to a large number of ultrasonic configurations (position of sensors, refraction angles, etc.). The use of phased array transducers will allow delay laws to be applied; these are chosen so as to generate ultrasound in several directions sequentially and build ultrasound images in real time. Automated - and especially encoded - movement can accurately locate the indications on the ultrasound image. To sum up, semi-automating acquisition allows for information repeatability (positioning relative to a physical reference), and reproducibility of the measurement (the various components of the UT channel can be interchanged without impacting performance). The impact of the human factor is strongly reduced. 4 Digital simulation of the inspection configuration To ensure the feasibility of the inspection and accurately define the components used, ultrasonic simulation resources available on the market are used. We used the ESBeamTool3 software to define the phased array configuration to be used. 483

3 The simulations were carried out in several stages: Ray tracing. This stage is to get the work started and determine the delay laws to be used depending on the position of the probe on the tube. It also provides an initial estimate of defect detectability; Definition and characterization of phased array transducers. In this stage the deflection capacity of the sensors is checked based on the intrinsic parameters of the transducers. The beam is then characterized ensuring that there are no interfering network lobes that may hinder detection; To perform simulations in a representative environment, the geometry of the weld + HAZ is drawn using ESBeamTool and this allows us to simulate a realistic configuration. - Groove angle: 37 - Tube thickness: 5mm - diameter 63.5mm - Gap between the two root faces: 2mm 4.1 Ray tracing Ray tracing or ray path allows us to define the examination positions for each of the transducers used together with the optimum refraction angle to be used to detect an indication. Ray tracing can also be used to define the angular range of the sectors to be used. These simulations allow us to choose the scan angles, the aperture and the depth of focus to be used for indications that may appear in this weld range. Several types of scanning can be chosen (linear or sector). All the simulations have shown that the optimal configuration for inspecting a weld comprises an initial acquisition of one side of the weld (upstream) and then a second on the other side (downstream). These two acquisitions provide a 100% examination of the weld. 4.2 Beam/defect interaction To digitally validate the inspection principles, new simulations have been conducted to define the nature of the interaction between the UT beam and the defect. Using these simulations in tandem with laboratory tests, it was possible to refine and validate the choice of scans, focusing and angles in order to obtain a configuration that is particularly sensitive to certain types of default. The linear scan turns out to be a perfect tool for detecting lack of fusion or incomplete fusion at the groove edge. This scanning is set so that the beam is incident perpendicularly on the groove. For example, for a groove at 37, the beam will be set at an angle of 53. Consequently, the ultrasonic response obtained is optimal. Conversely, sector scanning covering angles of 30 to 65 will allow indications such as porosity, inclusions, root crack, lack of penetration, etc. to be located. The phenomena involved in the simulations and tests are: - Corner echo (e.g. surface-breaking crack) - Specular reflection (e.g. incomplete fusion, porosity) Figure 3: Focal spot size at -6dB of the elementary beam 484

4 Representative CIVA simulations have allowed us to determine the length of the smallest detectable plane defect (i.e. one whose response is 50% of baseline level) with the SCAN13 inspection procedure. A plane defect, oriented along the groove axis and located in the chamfer, of size 1x1mm² is detected with a greater than 50 % response compared to a calibration on a side drilled hole (SDH) 1.5mm. A plane defect, oriented along the weld axis and perpendicular to the surface, of size 2x2mm² is detected with a greater than 50 % response compared to a calibration on a side drilled hole 1.5mm. From our experience feedback, it is unlikely to get a defect of this kind in the welding configurations discussed in this article but these results allow to define the limits of the sensibility performance of the UT phased array. A gas cavity greater than or equal to 3mm in diameter will be detected with a greater than 50 % response. CIVA simulations have shown that a gas cavity of diameter e detected with a response of -1.1 db. An elongated gas cavity of diameter 1.5mm and length 6mm is detected with a greater than 50 % response. CIVA simulations have shown that a gas cavity of diameter be detected with a response of +8dB. A root crack size 1x1 mm2 positioned at the boundary with HAZ on the inner diameter side and detected on the ½ skip will have a response in the order of +19 db (SDH at 16dB). When a crack is positioned at the boundary with the HAZ, on the outside diameter side and detected on the full skip, the response is in the order of +6 db (SDH at -3dB). 4.3 Identification of influential parameters and areas of change Groove angle According to the welding specifications, nominal groove angle is For inspection, we define the groove angle as The size of the incomplete fusion used for these simulations is 2x2mm². Simulations were performed on CIVA varying this parameter within the range of tolerance. Additional simulations were performed to measure the reflectivity difference between a 35 and a 37 groove with an ultrasound beam at 53 (linear sweep with 16 elements opened). The result obtained with these simulations indicates that there is a difference in reflectivity of 0.2dB. As long as we keep within the range of tolerances established by the DMOS, the difference in reflectivity does not affect the detection capability of the process Weld bead The width of the weld bead is an influencing parameter. During any inspection, the inspector will therefore measure a number of beads to corroborate his simulations with reality. In cases where the actual size of beads differs from the theory, a new simulation is performed in order to confirm the feasibility and the "offset index" parameter will be amended accordingly. From laboratory tests, a weld bead wider by 2mm at the most will not affect detection provided that this difference has been taken into account. In the case of a bead overlapping onto an area of the tube, the latter will be ground so as to have as regular as possible a weld bead HAZ: Heat affected zone Since no standard defines the size of the HAZ in tubes of thickness less than 8mm, we will define the size of the HAZ as being representative of a half thickness of the material considered on both sides of the weld bead. A specific test on mock-up is done to check the ultrasonic coverage of the volume to be examined Surface coupling Surface roughness or loss of water may affect the good coupling of the probe and cause damage regarding the sensibility during the scan. Surface is prepared before the examination and a specific 485

5 focal law (0 ) is generated during the scan in order to record the background echo. This allows the operator to check the good coupling and guarantee the sensibility of detection along the entire weld length. 5 Experimental measurements on artificial defects Many simulations were performed to determine the positions and angles to be used for each transducer. These are checked on calibration blocks made to be used in existing inspections. The following sections describe the blocks and transducers used for validation measurements on artificial defects. 5.1 Models used for testing The models used for the tests were made from a similar steel grade to that used in the majority of cases. The first model used represents a forest of pendular tubes containing butt welds. Some of these tubes have artificial representative defects such as incomplete fusion, porosity or inclusion. This model is necessary for testing implementation by simulating real-world conditions. Figure 4: Mock-up to test in real condition Comparison between conventional ultrasound and phased array ultrasound To make a comparison between conventional ultrasound and the phased array method, several specimens were manufactured. Each specimen includes a butt weld with real defects. There are several tubes by diameter range (1,5'', 2'', 2,5''). Each of these tubes has 2 different real defects. These reference tubes were manufactured by Sonaspection. The first tests were carried out by Sonaspection upon delivery of the manufacturing report. Their tests were performed using conventional ultrasound. An initial comparison could then be made, carrying out the inspection with the new process. The vast majority of faults are detected by both methods with the exception of lack of fusion between passes (not detected with conventional UT). However, it should be noted that the tests at Sonaspection required the use of several probes (45, 60 and 70 ) whereas using the phased array method, it is possible to significantly reduce the inspection time (10 minutes on average per weld). In addition, conventional ultrasound is feasible only when space permits. Using this method on the model, for example, is not feasible given the space between tubes Comparison between phased array ultrasound and radiography This section will be devoted to the comparison made between the use of phased array ultrasound and radiography. The 3 specimens cited above were X-rayed using an IR-192 source (36 curies). The exposure time was 2min and 36sec. 486

6 Figure 5 - Double-wall radiography of specimens After analysing the results, it appears that lack of fusion was not detected during X-ray examination. It appears that leaving this type of fault as it is on small thicknesses can be critical when using a boiler. Additional tests were performed on sleeves made of Z10CDNbV92 corresponding to a high-alloy steel. These comparative RT/UT tests have allowed us to identify all the defects in these sleeves. These tests required a calibration block of identical grade made from Z10CDNbV Sonaspection and EDF mock tests Additional tests on Sonaspection reference tubes were made on identical tubes (same diameter, thickness and grade to those encountered during the RHT (final reheater header) LE HAVRE 4 project in These tests involved tubes made of 10CrMo9.10 and X10CrMoVNb9.1 diam. 63.5mm th 4.5mm -> X10CrMoVNb9.1 diam. 63.5mm th 4mm -> 10CrMo9.10 diam. 44.6mm th: 6mm -> 10CrMo9.10 Tests were also carried out on EDF SC100 and SC101 mock-ups in order to demonstrate the good ultrasonic coverage of the SCAN 13 inspection process. These tubes had electro-eroded notches located throughout the weld (melt zone, groove and HAZ). The results mean that we can validate proper coverage of the process zone during the performance of a single scan per side. Figure 6: SC100 and SC101 EDF mock-ups check of the ultrasonic coverage Through these experimental tests, we were able to validate the fact of modifying the initial aperture of the linear scan by reducing. This allows us to cover a wider area of the groove zone without reducing detection sensitivity. Aditionnal CIVA simulations were added and confirmed the experimental results. 487

7 Figure 7: Planar defect perpendicular to the surface Figure 8: Planar defect in the chamfer In addition, we developed: - another group for monitoring coupling during the acquisition phase. This acquisition is performed with a sector scan, scanning from -6 to 6 with a criterion based on monitoring the background echo, - specific criteria based on amplitude and location of indications in order to characterized them and apply acceptance criterias according frenc decree of march Experience feedback This method is already used regularly on projects abroad. The use of SCAN 13 has been validated by Lloyds (UK) and ASNT (USA). We can assert a relatively large amount of feedback concerning this inspection technique. In the United States, 20 nuclear and conventional sites have already used this inspection method instead of radiography. Between 50 and 2200 tubes have been inspected per site. In Portugal, this system is already widely used in production on the Alstom Portugal site. In total, more than 5000 tubes have been inspected by this method. In France, we implemented this process during an outage that took place during the summer of 2012 on the site of Le Havre Tr4. During this overhaul the RHT was changed and we performed ultrasound inspections instead of radiography after EDF approbation of the qualification dossier. 608 welds were examined by the SCAN 13 method. EDF performed random X-ray shots on welds inspected using Scan 13 in order to confirm the results obtained by the ultrasonic method. No differences were observed in this project. Some indications were reported with the scan 13 (classified as lack of fusion,...) and repaired. 488

8 6 Conclusion Figure 9: Lack of fusion The technique developed by Alstom for the inspection of small diameter welds ( 2.75" and for a thickness of 8mm) was developed and validated according to a study in several stages. Work on documentation, the study of plans, forecasting and evaluation of the most appropriate techniques helped define the outline and the principle of phased array ultrasonic technique. Then development also took place in several stages: ultrasound simulation to verify the original ideas using ESBeamTool 3 and CIVA; Experimental validation on scale models including artificial defects; validation on the Seabank Power Company project in Bristol (UK). This last step was to verify that the new method performs much better than the usual technique because it allows us to characterize an indication by giving its length in a very short period of time. In addition, the new technique is encoded which ensures repeatability of the measurement. Finally, the inspection time can be reduced by using this device as compared to the use of radiographic examination. This technique allows complete inspection of a weld in less than 15 minutes (up to 60 welds inspected per day), whereas about several hours are required for a RT control. In addition, it is important to emphasize that these examinations do not interfere with other projects. In summary, semi-automation of the acquisition enables measurement repeatability and reproducibility, and so the impact of the human factor is strongly reduced. Furthermore, the use of phased array technology assists in the analysis through the use of an imaging system instead of a simple A-scan type view. Finally, being able to overlay the plan of the part further enhances the comfort of the data analysis. This technique is now implemented of the thermal power plant of EDF. To reach this, a complete qualification dossier with demonstration of performance, specific procedure and training program of the operators was approved from EDF. Finally the benefits for the utility are: if necessary, checks can be made immediately without affecting the continuity of the site; operating time is reduced; signals are recorded and can be consulted later on request; indication sizing is now possible; monitoring equipment is easier; reduces operating and maintenance costs radioactive source management work in parallel 489