NDT for HACC
What NDT method should I use to detect Hydrogen Assisted Cold Cracking (HACC)?
HACC can occur in the heat affected zone (HAZ) known as HAZHACC or in the weld metal (WM) known as WHHACC. In the welding of modern high-strength pipelines WMHACC is the much more likely form of HACC.
Part 2 allows the use of radiographic testing and ultrasonic testing (both volumetric methods). Both can be used to detect HACC, but ultrasonic methods are generally preferred as detailed in Section B.5. Where the presence of HACC is credible, careful consideration should be given to the volumetric test method that is selected.
Radiographic testing relies on density differences in order to provide radiographic contrast and interpretation of the defect. HACC may result in very fine cracking which is not detectable even using the most optimal method, as the volume of the cracking is very small and possibly not oriented in the same plane as the radiographic source. Ultrasonic testing is much better at detecting HACC as the crack is a good reflector of the ultrasonic signal. The preferred methods of ultrasonic testing are Automated Ultrasonic Testing (AUT) and Phased Array Ultrasonic Testing (PAUT) due to the capability of providing a permanent record (refer Section 22.1), and manual ultrasonic testing should only be performed in the circumstances described in Section 22.3. If transverse cracking is possible (WMHACC) appropriate transverse scanning patterns or ToFD shall be used (Section 22.1).