ISO 15626:2018

Overview

ISO 15626:2018 - Non‑destructive testing of welds - Time‑of‑flight diffraction technique (TOFD) - Acceptance levels - defines acceptance criteria for TOFD inspections of full‑penetration welds in ferritic steels of 6 mm to 300 mm thickness. The standard links TOFD acceptance levels to the quality levels of ISO 5817 and requires indications to be classified in accordance with ISO 10863. It specifies how to determine the geometric size of discontinuities (length and height) from TOFD data and sets numerical acceptance limits and total‑length limits for different acceptance levels.

Key topics and technical requirements

• Scope & applicability
  • Applies to TOFD of full‑penetration welds in ferritic steels (6–300 mm).
  • Acceptance levels correspond to ISO 5817 quality levels (mapping to Acceptance levels 1, 2, 3).
• Indication classification
  • Indications must be classified per ISO 10863 before acceptance evaluation.
• Size definitions
  • Length (l): difference in x‑coordinates of an indication.
  • Height (h): maximum difference in z‑coordinates at a given x.
• Length sizing techniques
  • Elongated straight indications: fit hyperbolic (arc‑shaped) cursors at ends.
  • Elongated curved indications: place hyperbolic cursor at ends at 1/3 penetration delay and measure distance between positions.
• Height measurement (A‑scan based)
  • Recommended four methods: transit time between leading edges, first peaks, maximum amplitudes, or first zero crossings (Methods 1–4). Phase reversals must be considered.
• Surface‑breaking vs embedded discontinuities
  • Height determined by comparing lateral wave or back‑wall reflections with tip diffraction signals as appropriate.
• Acceptance levels & limits
  • Three levels with specific maximum acceptable lengths/heights by thickness range.
  • Total sum of indication lengths over 12·t has level‑dependent limits (e.g., 3.5·t, max 150 mm for Acceptance level 1).
• Special cases
  • For welds joining different thicknesses, acceptance is based on the thinner section.
  • More stringent criteria or additional NDT recommended for dynamically loaded or crack‑sensitive welds.
  • If surface‑breaking depth cannot be resolved by TOFD, alternative techniques must be used; otherwise such indications are unacceptable.

Applications and users

• Practical use cases:
  • Weld quality acceptance in fabrication, pressure vessels, pipelines, shipbuilding, power plants, and heavy structural steel.
  • Routine quality control and acceptance testing where TOFD is used as a primary ultrasonic method.
• Typical users:
  • NDT engineers and technicians, inspection bodies, welding inspectors, QA/QC managers, fabricators, and regulatory authorities.
• Benefits:
  • Provides repeatable, standardized acceptance criteria for TOFD results, improving consistency and defensibility of weld inspections.

Related standards

• ISO 5817 - Welding - Quality levels for imperfections (related quality mapping)
• ISO 10863 - Examination levels and classification of indications
• ISO 5577 - NDT vocabulary (terms used)
• Prepared by ISO/TC 44 (Welding and allied processes), SC 5 (Testing and inspection of welds)

Keywords: ISO 15626:2018, TOFD, time‑of‑flight diffraction, non‑destructive testing, weld inspection, acceptance levels, ferritic steels, ISO 5817, ISO 10863.