🔩 Ultrasonic testing (UT) of welded joints — angle-beam probes 2–5 MHz, four testing levels

Materials and NDT PN-EN ISO 17640

Overview

The standard specifies techniques for the manual ultrasonic testing of fusion-welded joints in metallic materials with a thickness of not less than 8 mm, exhibiting low ultrasonic attenuation, especially that due to scattering, at object temperatures from 0 °C to 60 °C. It is intended primarily for full-penetration joints in which both the weld metal and the parent material are ferritic. The material-dependent values given in the standard are based on steels with a longitudinal wave velocity of (5920 ± 50) m/s and a transverse wave velocity of (3255 ± 30) m/s.

The standard specifies four testing levels, each corresponding to a different probability of detection of imperfections; guidance on the selection of levels A, B and C is given in Annex A. Level D is intended for special applications and may be used only when it is defined in the specification; it covers the testing of metals other than ferritic steel, the testing of partial-penetration joints, testing with automated equipment and testing at object temperatures outside the range from 0 °C to 60 °C.

The assessment of discontinuities for acceptance purposes may be based on one of two techniques: on the length and echo amplitude of the discontinuity, or on the characterization and sizing of the discontinuity by probe movement techniques. The parameters specified in the standard, in particular the probe parameters, are consistent with the requirements of ISO 11666 (acceptance levels) and ISO 23279 (characterization of discontinuities).

Method principle

An ultrasonic wave, i.e. mechanical vibration with a frequency greater than 20 kHz, is introduced into the material and the reflected signals are recorded. Reflection takes place at the boundary of media with different acoustic impedance: at the surface of the object, at a discontinuity or at an inclusion of another material. In the pulse-echo method the height of the pulse depends on the size of the reflector, and its position on the time axis on the distance from the probe, which allows the location of the defect to be determined; at the beginning of the trace there is the transmission pulse forming a dead zone in which observation is not possible. Angle-beam probes have a built-in wedge causing mode conversion at the boundary of the media into transverse waves, introduced at an angle, usually 45°, 60° or 70°. Indications are assigned to longitudinal or transverse discontinuities depending on the direction of their largest dimension relative to the weld axis. The examination volume covers the weld, the parent material and the width of the heat-affected zone on each side of the weld, and where this width is not known — at least 10 mm.

Applications

• Acceptance testing of full-penetration butt joints in steel structures
• Testing of pipelines, tanks and machine subassemblies
• Detection of longitudinal and transverse cracks, lack of fusion, incomplete penetration, inclusions and undercuts
• Testing of joints where radiography is impossible because of radiation protection or access
• Testing of the parent material in the scanning zone with a straight-beam probe before or after welding

Key parameters

Standard

Standard number

PN-EN ISO 17640:2019-01

Title (PL)

Badania nieniszczące spoin — Badania ultradźwiękowe — Techniki, poziomy badania i ocena

Title (EN)

Non-destructive testing of welds — Ultrasonic testing — Techniques, testing levels, and assessment

Step-by-step procedure

1. Arrangements before testing

   Specify the method of setting the reference level, the method of assessing discontinuities, the acceptance levels, the testing level, the stage of manufacture or service, the personnel qualifications, the extent of testing for transverse discontinuities, any tandem testing according to ISO 16826, testing of the parent material before or after welding, and whether a written test procedure is required.

2. Information for the operator

   The operator must have access to the type of parent material and product form (casting, forging, rolled product), the stage of manufacture and heat treatment, the joint preparation and dimensions, the requirements for surface condition, the welding procedure, the reporting requirements, the acceptance levels, the extent of testing, the testing level, the required qualification level and the action to be taken when unacceptable discontinuities are detected.

3. Preparation of the scanning surfaces

   Scanning surfaces are to be wide enough to cover the entire examination volume, even and free of rust, loose scale, spatter, notches and grooves that hinder coupling. Surface waviness must not give a gap under the probe greater than 0.5 mm; machine the surface if necessary. Surfaces from which the beam is reflected must permit undisturbed reflection.

4. Selection of probes

   Select a frequency of 2–5 MHz and a transducer size appropriate to the sound path. Ensure an angle between the beam and the normal to the opposite surface within 35–70°, and one of the angles such that the fusion faces of the weld are examined as close to perpendicular as possible. On curved surfaces check the gap from the formula g = a²/4D; when it exceeds 0.5 mm, contour the probe to the surface and set the sensitivity and range anew.

5. Testing of the parent material

   Test the parent material in the scanning zone with a straight-beam probe before or after welding, unless it can be shown that imperfections or high attenuation do not affect the angle-beam testing. If imperfections are detected, assess their influence and change the technique if necessary; where coverage is seriously restricted, consider another method, for example radiography.

6. Range and sensitivity setting

   Carry out the setting before each test in accordance with ISO 16811, taking into account the influence of temperature; temperature difference between setting and testing within ±15 °C. Set the reference level using one of the techniques: DAC curve for 3 mm side-drilled holes, the DGS system, a 1 mm × 1 mm notch for thicknesses from 8 to below 15 mm and angles of at least 70°, or a 6 mm flat-bottomed reflector for the tandem technique at an angle of 45° and a thickness from 40 mm.

7. Scanning of the examination volume

   Scanning is to cover the entire examination volume: the weld, the parent material and the heat-affected zone on both sides, and where its width is not known — at least 10 mm. If part of the volume cannot be covered in at least one scanning direction or the angles of incidence do not meet the requirements, agree alternative or supplementary techniques: dual-element angle-beam probes, creeping-wave probes, other ultrasonic techniques or another method, for example penetrant, magnetic particle or radiographic testing; sometimes removal of the weld reinforcement is necessary.

8. Checks during testing and assessment

   Check the settings at least every 4 h and at the end of the test, and after any change of a system parameter. With a sensitivity deviation above 2 dB correct the setting before further testing, with a decrease above 4 dB repeat the tests of the preceding period, with an increase above 4 dB re-assess the recorded indications; for the range the limits are 1 % and 2 % of the range. Assess the indications against the acceptance levels of ISO 11666 or characterize them according to ISO 23279 and give them in the report together with their position in the x, y, z coordinate system.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Oil-based couplants — risk of slipping, skin irritation and surface contamination; gloves and removal of residues after testing
• Testing at height, in confined spaces and on operating installations — safety harness, ventilation, work permit
• Objects at temperatures up to 60 °C — risk of burns to the hands, heat-resistant gloves
• Cables and power supply of the flaw detector in field conditions — check of the insulation condition, supply with residual current protection
• Sharp edges and spatter on scanning surfaces — gloves, prior surface preparation

Frequently asked questions