Probability of detection of discontinuities in 9% Ni steel pipe girth welded joints: comparison between phased array ultrasonic and radiographic testing

Phased array ultrasonic testing (PAUT) in ferritic metals offers advantages over radiographic testing (RT), leading to its increasing adoption and potential as a substitute for RT. However, PAUT presents challenges in austenitic metals due to sonic attenuation, particularly in weld metal, which has a coarse and anisotropic microstructure. The present study compares the reliability of PAUT and RT using a probability of detection (PoD) analysis. To achieve this, both techniques were applied to inspect discontinuities artificially introduced in 9% Ni steel thick-section pipes girth welded using the Ni-based superalloy 625 as filler metal. These discontinuities are introduced at the root and hot pass region of the welded joint and include typical weld joint discontinuities (lack of root pass fusion, incomplete root penetration, hot cracking in the root pass, cluster porosity, lack of fusion on the bevel side, and lack of fusion at the interpass). The PoD curves demonstrated that PAUT was more sensitive in detecting discontinuities than RT. PAUT and RT showed a a 90 (size of the discontinuity that has a PoD of 90%) of 6.87 mm and 12.72 mm, respectively. PAUT is more efficient in detecting planar defects (e.g., lack of fusion); otherwise, RT showed better results for volumetric defects (incomplete root penetration). However, the probability of false positives of the PAUT (13.59%) was higher than that for RT (0.58%), i.e., PAUT exhibits greater uncertainty. Therefore, replacing RT with PAUT for inspecting thick-section welded joints in critical applications remains a challenge. • Thick-walled pipe joints were inspected using phased array ultrasonic (PAUT) and radiographic testing (RT). • Artificial welding discontinuities were introduced during the root and hot pass welding. • Probability of detection (POD) curves were experimentally computed for PAUT and RT. • PAUT can detect more minor discontinuities than RT. • Replacing RT with PAUT for inspecting thick-section welded joints remains a challenge.