ISO 17640:2017 specifies techniques for the manual ultrasonic testing of fusion-welded joints in metallic materials of thickness ≥8 mm which exhibit low ultrasonic attenuation (especially that due to scatter) at object temperatures from 0 °C to 60 °C. It is primarily intended for use on full penetration welded joints where both the welded and parent material are ferritic.Where material-dependent ultrasonic values are specified in ISO 17640:2017, they are based on steels having an ultrasonic sound velocity of (5 920 ± 50) m/s for longitudinal waves and (3 255 ± 30) m/s for transverse waves.ISO 17640:2017 specifies four testing levels, each corresponding to a different probability of detection of imperfections. Guidance on the selection of testing levels A, B, and C is given in Annex A.ISO 17640:2017 specifies that the requirements of testing level D, which is intended for special applications, be in accordance with general requirements. Testing level D can only be used when defined by specification. This includes tests of metals other than ferritic steel, tests on partial penetration welds, tests with automated equipment, and tests at object temperatures outside the range 0 °C to 60 °C.ISO 17640:2017 can be used for the assessment of discontinuities, for acceptance purposes, by either of the following techniques:a) evaluation based primarily on length and echo amplitude of the discontinuity;b) evaluation based on characterization and sizing of the discontinuity by probe movement techniques.