Ultrasonic Weld Seam Testing

Welding Defects

There are different types of discontinuities and the most typical welding defects are:

• Point indications
• Large indications
• Longitudinal indications
• Transverse indications

Examination equipment

A digital ultrasonic flaw detector (e.g. SONOSCREEN ST10 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.

Examination sketch

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.

Couplant

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 Inspection

Surface 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.

Equipment preparation

Before testing with angle beam probes, the beam index point and actual refracted angle has to be identified.

• The Beam Index Point is the point where the center of the beam exits the wedge of the probe. This point is a zero point for all trigonometric distance and depth measurements. It can be estimated using calibration block K1\V1.
• The verification of a probe-refracted angle is important for the correct distance and depth measurements and can also be performed with a K1\V1 block.
• The next step is to perform distance calibration. This procedure depends on the probe selection and can be done with a K1 or K2 block. In order to perform a proper calibration, the thickness of the block should be greater than the probe width. Distance calibration is required to obtain the precise speed of sound inside the material and the probe delay for accurate distance measurements. The preferred calibration type is a two-point calibration, where both of these factors are calculated at the same time.
• At the end, a test sensitivity adjustment has to be performed. This involves selecting the appropriate evaluation method and creating the evaluation curve on the device display. The DAC method requires a specially prepared calibration block of the same material, usually with side drilled holes at different depths (to achieve high accuracy, the depth of the holes should cover the range of actual testing). DGS and AWS do not require additional blocks because the curve computation is done empirically.

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.