This International Standard specifies the application of the time-of-flight diffraction (TOFD) technique to the semi- or fully automated ultrasonic testing of fusion-welded joints in metallic materials of minimum thickness 6 mm. It applies to full penetration welded joints of simple geometry in plates, pipes, and vessels, where both the weld and parent material are low-alloyed carbon steel. Where specified and appropriate, TOFD can also be used on other types of materials that exhibit low ultrasonic attenuation (especially that due to scatter).Where material-dependent ultrasonic parameters are specified in this International Standard, they are based on steels having a sound velocity of (5 920 ? 50) m/s for longitudinal waves, and (3 255 ? 30) m/s for transverse waves. It is necessary to take this fact into account when examining materials with a different velocity. This International Standard makes reference to the basic standard EN 583-6 and provides guidance on the specific capabilities and limitations of TOFD for the detection, location, sizing and characterization of discontinuities in fusion-welded joints. TOFD can be used as a stand-alone method or in combination with other non-destructive testing (NDT) methods or techniques, for manufacturing inspection, and for in-service inspection. This International Standard specifies four testing levels (A, B, C, D) in accordance with ISO 17635 and corresponding to an increasing level of inspection reliability. Guidance on the selection of testing levels is provided. This International Standard permits assessment of TOFD indications for acceptance purposes. This assessment is based on the evaluation of transmitted, reflected and diffracted ultrasonic signals within a generated TOFD image. This International Standard does not include acceptance levels for discontinuities.