Overview
ISO 4761:2022 defines acceptance levels for phased array ultrasonic testing (UT-PA) of full‑penetration welds in thin‑walled low‑alloy and fine‑grained steels. It applies to wall thicknesses from 3.2 mm to 8 mm and aligns UT‑PA acceptance criteria with the quality levels of ISO 5817. Indications are evaluated when detected according to ISO 20601.
Keywords: ISO 4761:2022, phased array ultrasonic testing, UT‑PA, thin‑walled steel, acceptance levels, weld inspection, NDT.
Key topics and technical requirements
- Scope and materials: Full‑penetration welds in low‑alloy/fine‑grained steels, t = 3.2–8 mm.
- Reference and sensitivity: Sensitivity is set using a 1.0 mm side‑drilled hole per ISO 20601. Three level types are used - reference level, acceptance level, and evaluation level (evaluation level = reference level −12 dB).
- Indication measurement: Length is measured along the weld by the 6 dB drop method; a length correction for unfocused lateral beams is specified.
- Acceptance levels mapping:
- Acceptance level 1 (stringent) - corresponds to ISO 5817 quality level B.
- Acceptance level 2 (intermediate) - corresponds to ISO 5817 quality level C.
- Acceptance level 3 (moderate) - corresponds to ISO 5817 quality level D.
- Amplitude/length criteria (examples):
- Two‑sided testing: short longitudinal indications (≤4 mm) may be accepted up to +6 dB (relative to reference); larger lengths have progressively lower allowable amplitudes with defined breakpoints (e.g., level‑specific limits at 4 mm, 6–8 mm, and >7–8 mm).
- Single‑sided testing: no criteria for acceptance level 1; level 2 and level 3 have defined length/amplitude thresholds (e.g., level 2 permits ≤4 mm at +3.5 dB).
- Cumulative length limits: Over a weld section of length 12·t, the summed lengths of individually acceptable indications shall not exceed:
- ≤ 3.5·t (level 1),
- ≤ 4.0·t (level 2),
- ≤ 4.5·t (level 3).
(Single‑sided testing uses ≤4.0·t and ≤4.5·t for levels 2 and 3.)
Applications and users
ISO 4761:2022 is intended for:
- NDT engineers and ultrasonic inspection teams applying UT‑PA on thin‑walled steel components.
- Welding inspectors, fabricators and quality managers in industries such as automotive, pressure equipment, piping, rail, and general steel fabrication where thin‑walled welds are common.
- Procurement, conformity assessment bodies and project engineers specifying acceptance criteria consistent with ISO 5817.
Practical benefits include consistent, reproducible acceptance decisions, reduced ambiguity in UT‑PA reporting, and clearer linkage between ultrasonic results and weld quality classes.
Related standards
- ISO 20601 - Use of automated phased array technology for thin‑walled steel components (detection & procedure)
- ISO 5817 - Quality levels for welding imperfections
- ISO 5577, ISO 23243 - NDT and ultrasonic vocabularies
- ISO 17635, ISO 17640, ISO 9712 - General NDT rules, UT techniques, and personnel qualification
For inspection programs and procedure qualification, use ISO 4761:2022 together with ISO 20601 and ISO 5817 to ensure consistent UT‑PA weld acceptance for thin‑walled steel components.