INTRODUCTION
The welding quality control (https://www.delphincontrol.com/technical-pills-welding-quality-control/?lang=en) has been described in the previous article: indirect controls are envisaged, mainly relating to the issue and verification of documents relating to the qualification of welders, welding processes, materials, and direct controls, which involve the application of non-destructive tests (NDT) to verify the presence of defects during the manufacturing phase. The main surface-type NDTs have also been described, ie visual examination (VT), penetrating liquids (PT) and magnetic particles (MT).
The following article presents the main techniques of ultrasound control, a volumetric control capable of identifying defects present within the material.
This type of control involves the use of an ultrasonic wave beam (frequencies from 0.5 MHz to 5 MHz). The ultrasound beam propagating within the material to be inspected, in the vicinity of any discontinuity and / or defect, is subject to phenomena of reflection, but also of diffusion and diffraction. These are detected by the instrument providing indications to the operator according to specific representations, which he will proceed to interpret.
The techniques used are numerous. In recent years, thanks to the strong progress in electronics, new technologies are being developed that allow for increasingly intuitive representations, but above all to have the advantage of increasing the POD (Probability of Detection).
In the industrial field, the mostly used techniques are the following:
- Ultrasound traditional technique
- Phased Array
- TOFD
TRADITIONAL ULTRASOUND TECHNIQUE
It is historically the first technique to have been developed. The ultrasound beam is generated by a probe in which there is a piezoelectric crystal. A probe can also have two crystals, one transmitting and the other receiving. The ultrasound beam, propagating inside the piece, in correspondence with a defect and / or discontinuity, is reflected and, if the defect is properly oriented, the ultrasound beam goes back and is detected by the probe and instrument. The most used representation is the A-Scan, which allows you to represent a reflection through an echo.
In the field of welding control, the reference standards are ISO 17640 which indicates the techniques and levels of testing and evaluation, ISO 11666 which indicates the levels of acceptability and ISO 23279 which indicates how characterize a discontinuity.
In particular, the standards provide the control of the entire testing volume (welding and heat affected zone) according to different testing levels ranging from A to C, increasing the order of detection of defects. A more restrictive level of testing generally involves checks from more positions and with a greater number of angles of the probe, to increase the probability of detecting the defect. There is also a test level D, which is only used for special applications.
The acceptance or rejection criteria are based on the length of the defect and on the comparison between the amplitude of the echo of the defect and the amplitude of an artificial defect echo. For this reason, the sensitivity calibration involves the generation of a reference DAC (or DGS) curve. Levels of evaluation, registration and acceptance are then defined.
There is also a correlation between quality levels (ISO 5817), proof levels (ISO 17640) and acceptance levels (11666).
Nell’ambito del controllo delle saldature, le norme di riferimento sono la ISO 17640 nella quale vengono indicate le tecniche e i livelli di prova e di valutazione, la ISO 11666 in cui vengono indicati i livelli di accettabilità e la ISO 23279 in cui viene indicato come caratterizzare una discontinuità.
In particolare, le norme prevedono il controllo di tutto il volume di prova (saldatura e zona termicamente alterata) secondo differenti livelli di prova che vanno da A a C, secondo un ordine crescente di rilevazione dei difetti. Un livello di prova più restrittivo prevede generalmente controlli da più posizioni e con un numero maggiore di angoli della sonda, in modo da aumentare appunto la probabilità di rilevare il difetto. È presente anche un livello di prova D, utilizzato solamente per applicazioni speciali.
I criteri di accettazione o scarto si basano sulla lunghezza del difetto e sul confronto fra l’ampiezza dell’eco del difetto e l’ampiezza di un eco di un difetto artificiale. Per questo motivo la calibrazione della sensibilità prevede la generazione di una curva DAC (o anche DGS) di riferimento. Vengono quindi definiti dei livelli di valutazione, registrazione e accettazione.
È prevista inoltre una correlazione tra i livelli di qualità (ISO 5817), livelli di prova (ISO 17640) e i livelli di accettazione (11666).
| TESTING LEVELa
(ISO 17640) |
QUALITY LEVELS
(ISO 5817) |
ACCEPTANCE LEVELS
(ISO 11666) |
| B | B | 2 |
| A | C | 3 |
| A | D | 3b |
| Notes:
a if the characterization of the discontinuities is required, ISO 23279 must be applied b UTs are generally not used for this quality level. |
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Traditional ultrasound control is a consolidated method and still very widespread on an industrial level today. It is certainly also the cheapest when compared to the latest technologies. However, it turns out to be a control very dependent on the ability and interpretation of the operator, in many cases it involves the use of numerous probes and scans to perform a reliable control and generally there is no possibility to record the scans.
TECNICA PHASED ARRAY
This technique is based on the use of probes consisting of numerous piezoelectric crystals, each of which behaves like a point source of ultrasonic waves. Each crystal can be controlled independently. By applying appropriate delays in the excitation of one crystal compared to another, different focal laws can be defined that allow:
– Perform an electronic scan, or inspect more portions of material without moving the probe;
– Angle the beam according to different values;
– Focus the beam at different depths;
The typical representation of the Phased Array technology is the S-Scan. which can be used for a range of angles (for example from 40 ° to 70 °) and therefore you have a sectorial section view, coded according to a scale of colors whose information is obtained from each single angular step of the aperture cone of the ultrasound beam.
The main advantages are therefore the speed of execution thanks to electronic scanning, increase in POD, the possibility of improving the sizing of defects thanks to the dynamic focusing and the possibility of using more angles, ease of interpretation thanks to the numerous types of representations available. The disadvantages are higher costs, qualified personnel and in some cases, it is difficult to find regulatory references.
In the area of welding control, the reference standards are ISO 13588 regarding techniques and test levels and ISO 19285 about acceptance levels. These rules are actually very similar to those of traditional control. In this case there are 4 testing levels and 3 acceptance levels. In the following table the correlation between ISO 5817, ISO 13588 and ISO 19285.
| QUALITY LEVEL
ISO 5817 |
TESTING LEVEL
ISO 13588 |
ACCEPTANCE LEVEL
ISO 19285 |
| C, D | A | 3 |
| B | B | 2 |
| By agreements | C | 1 |
| Special applications | D | By agreements |
| NOTES
The acceptance criteria for level 1 are defined only on the basis of length and height. |
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The acceptance criteria also in this case are based on the length and width of the defect in reference to a sample defect, but there is also a criterion based only on the length and height of the defect. With the Phased Array technique, it is in fact possible to better exploit the diffraction phenomenon that occurs at the tip of a crack and to better size the height.
As for the execution of the control, if it is shown to be able to cover the entire test volume, it is possible to perform a single scan in the direction parallel to the weld at a fixed position, on both sides. If it is not possible to cover the entire test volume, it is necessary to perform multiple scans at different distances.
Hence the advantage to not carry out several inspections from numerous positions and with different angles of refraction, as necessary for traditional control, thus saving time.
TOFD TECNIQUE
This technique is different from the previous two (traditional and Phased Array). While the first two are based on the reflection of the ultrasound beam, with the TOFD (Time of Flight Diffraction) technique the detection takes place by exploiting the diffraction phenomenon at the apexes of the crack and / or along the cylindrical walls of a hole or a volumetric defect.
The operating principle is illustrated in the figure on the right. According to a pitch and catch configuration (one probe emits and the other receives), small, high frequency and longitudinal wave probes are used to generate high divergence waves. The signal received is characterized by a Lateral Wave (LW), two echoes produced by the crack apices (echo 1 and 2) and a background echo LL (echo 3). The signal is represented according to a B-Scan configuration in grayscale and radiofrequency (figure below).
The scan with the TOFD technique, like the Phased Array, occurs in a direction parallel to the weld. When there are high thicknesses (> 30-40 mm), it is not possible to cover the entire test volume with a single TOFD configuration. In this case, a configuration capable of inspecting different depths is used, using multiple probes with different angles.
This technique has the significant advantage of detecting defects regardless of their orientation and nature. For this reason, it is the ultrasound technique with the highest POD index. However, it is characterized by some limitations: the use of high frequencies limits the application to materials with low attenuation (carbon steel and low alloyed alloys), access is required from both sides of the weld, there are shaded areas near the surface (depth up to 4-8 mm) and near the bottom. For this reason, the TOFD scan is always accompanied by a Phased Array scan and is not applicable for thicknesses lower than 10 mm.
The reference standards for the TOFD control of welds are ISO 10863 which introduces the technique, ISO 15626 for acceptability levels, ISO 16828 method for detecting and dimensioning discontinuities.
The calibration of the TOFD technique does not provide for the generation of a DAC curve, typical of techniques based on reflection and echo amplitude. The TOFD calibration instead involves the use of sample blocks with artificial defects positioned at different depths, with the aim of evaluating the intensity of the gray tones of the image and defining the test volume covered by the scan set-up. Calibration is considered correct if there is sufficient contrast to make accurate measurement of the length and height parameters.
The acceptance criteria are based on the size of the defect, in particular the height and the length.
CONCLUSIONS
We have seen the main ultrasonic techniques used for the control of welds. The industrial world requires ultrasound checks that are fast, reliable, and cost-effective.
The traditional control appears to be consolidated and economical, but has numerous disadvantages compared to the most recent technologies: it is not possible to register scans, the standards require numerous scans with different angles to have a reliable control resulting in an increase in times, it remains a technique depending on the skill and interpretation of the operator.
The Phased Array technique overcomes the limits of traditional control by increasing the POD index thanks to the possibility of using electronic focusing and a range of angles. However, it involves higher costs, highly skilled personnel and the setup of the instrumentation can be complicated.
The TOFD technique, exploiting the phenomenon of diffraction, is the one that allows you to detect any type of defect regardless of orientation and nature. Traditional control and Phased Array struggle to find cross-cutting defects. However, it is characterized by gray areas and the interpretation of the signal is complicated.
To make the most of the potential offered by ultrasonic inspection, when it is possible to integrate the Phased Array and TOFD technique it is the best choice to maximize the probability of detecting defects.