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|NDT.net Issue - 2019-12 - NEWS ||NDT.net Issue: 2019-12|
Publication: e-Journal of Nondestructive Testing (NDT) ISSN 1435-4934 (NDT.net Journal)
Application Note: Ultrasonic Steel Weld TestingSONOTEC GmbH59, Halle, Germany
Ultrasonic testing is one of the most common methods for the non-destructive inspection of materials. One of the biggest advantages of this method is that it does not harm the human operator. Ultrasonic testing uses mechanical vibrations, in the form of ultrasonic waves, to search for internal discontinuities; the working principle is similar to the sonar technology, which was actually the predecessor to ultrasonic flaw detectors. Today, the technique is well established in the industry, having been used since the 1950s.
Weld testing is one of the main application fields of ultrasonic testing. Using ultrasonic flaw detectors, NDT technicians find and characterize discontinuities within the weld that might lead to material or component failure. Ultrasonic weld testing is part of weld quality control and is crucial for ensuring safety in a wide range of industries: for example, it can be instrumental in preventing plane crashes, reactor failures and oil platform explosions.
Capabilities, challenges and limitationsUltrasonic weld inspection is a complex process that requires a qualified inspectors. Many factors have to be considered before a test can be carried out, including:
However, it also has the following limitations:
StandardsWith the above requirements in mind, it is important that an NDT technician performing ultrasonic testing of welds or other materials has the necessary training and certification. The international standard (ISO 9712:2012 - Qualification and certification of NDT personnel) determines the criteria for the above.
Not only the operator must be qualified to perform reliable ultrasonic weld inspection; the inspection procedure, equipment, calibration test blocks and evaluation method are also regulated by an appropriate set of standards. A selection of relevant standards for weld testing is listed below:
Procedure1. Defining the ultrasonic testing task There is no doubt that ultrasonic weld inspection requires theoretical know-how and practical experience. To better understand this, let us take a closer look at the procedure itself:
2. Types of discontinuities and evaluationThere are different types of discontinuities
Defining reference, registration and Evaluation level
A digital ultrasonic flaw detector (e.g. SONOSCREEN ST10, SONO-FD 1 or SONOWALL 70 with flaw detector upgrade) is required. The gage should be capable of transmitting and receiving ultrasonic pulses and displaying them in full-rectified form (A-Scan). Angle beam probes supporting automatic trigonometric calculations and DAC, DGS or AWS software are the absolute minimum for weld inspection. It is crucial that the device is calibrated and manufactured in accordance to the EN-12668-1 standard. In most branches, the equipment used for formal weld inspection must be calibrated against this standard at least once per year.
Ultrasonic probes: dual element straight beam probes (e.g. TS and TL series), angle beam probes (e.g. WS, WM and WL series) - The probe selection depends strongly on the material being inspected; its geometry, weld type, thickness, the minimum defect size to be detected and many other factors must be considered. To simplify the probe selection process, a general rule of thumb is to use 4MHz probes for a thickness range from 8-50mm and 2MHz for 50mm and above. For proper inspection, two angles must be used. For thinner welds up to 20mm, angles of 70° and 60° are recommended. For thicker objects, 45° and 60° are preferred.
The next step is to draw the examination sketch. This should clearly display the probe movement zone, the part of the weld covered by the inspection, and the point where the ultrasonic beam is introduced to the part. On the tested object, the zero point of the measurement should be permanently set and marked on the sketch. It must be possible to exactly reproduce the test procedure based on the report made after test completion.
Usually this is a water-based gel, oil, grease or wallpaper glue. It is very important that the same couplant is used throughout the whole testing procedure (i.e. calibration, estimation of transfer losses, sensitivity adjustment and testing).
Performing the inspectionSurface preparation
First, a visual inspection of the weld and the surrounding material must be conducted, in order to determine if the surface is appropriate for ultrasonic testing. There may be weld spatters or other obstacles which could restrict probe movement; these should be removed prior to testing. Furthermore, the weld geometry should be inspected for possible root leakages or crown overlays as this will deliver geometry indications.
Before testing with angle beam probes, the beam index point and actual refracted angle has to be identified.
Performing the inspectionTesting the weld
To cover 100% of the weld volume, the angle beam probe has to be moved back and forth perpendicular to the weld axis, preferably from both sides.
To ensure the detection of transverse discontinuities, it is recommended to move the probe back and forth at a 90° and 45° angle to the weld axis.
Classification of discontinuities: indications should be evaluated according to their envelope. First, the maximum amplitude due to the indication must be found. If the amplitude drops steadily to zero in all directions when the probe is moved away from the indication, this means that the discontinuity is smaller than the ultrasonic beam from the probe. If the amplitude does not drop to zero and remains within a -6dB dynamic range, it means that the indication is bigger than the probe beam size.
False indications occur frequently: these are due to mode converted waves, arising from the object geometry, and must be correctly evaluated and ignored. This is the most difficult part in the whole process of ultrasonic testing.
Acceptance criteria: These criteria are defined in the standard for each quality class. To illustrate this, let’s consider the following example:
As seen in the above graph, the acceptance level is equal to the reference level (DAC curve level) as long as the length of the indication is less than the material thickness. For indications which are longer than the material thickness, the acceptance level is 6dB below the reference level (equal to the registration level). This is because long defects are a bigger threat to any construction than point defects and must therefore be evaluated with a lower acceptance level.
During inspection, those indications which are not accepted must be permanently marked.
Before finishing and leaving the inspection site, couplant and other test residues have to be removed.
Writing the test report
The final phase of the process is to write the test report. The whole testing procedure must be well documented and described. A typical report should contain:
*This report represents a suggestion how to perform an ultrasonic inspection. All operators should be qualified according to the required standard. All information provided is without legal engagement.
ABOUT USWith currently over 170 employees, SONOTEC GmbH is an international growing company. The company has established itself on the worldwide NDT market with UT products developed and made in Germany.
SONOTEC GmbH | Nauendorfer Straße 2 | 06112 Halle Saale
Certified according to ISO 9001 & EN13485
AUTHORSK. Rostkowski (application engineer) / A. Bodi (business unit director ndt) / M. Lucas (business unit director ndt) / M. Emmrich (marketing manager) - SONOTEC GmbH
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