Analysis of the failure causes in the flattening test of 2205 duplex stainless steel seamless tubes

•Employed industrial CT scanning to investigate crack defects in 2205 duplex stainless steel seamless pipes.•Identified large-sized CaO-Al2O3-MgO inclusions as the root cause of crack initiation and propagation.•Demonstrated the novel application of industrial CT for non-destructive evaluation.•Proposed actionable measures to prevent similar failures in the future. This study aims to analyze the fundamental causes of crack defects in 2205 duplex stainless steel seamless pipes during flattening tests. The failed steel pipe samples were analyzed using various experimental methods, including macroscopic observation, outer diameter and wall thickness measurement, ultra-depth-of-field microscopy, scanning electron microscopy, and industrial CT scanning. The results indicate that the crack extended by 5.79 mm with a trapezoidal cross-section. Large-sized CaO-Al2O3-MgO type inclusions were observed at the bottom and root of the crack, indicating deformation mismatch with the steel matrix. During the flattening test, cracks were initiated at the locations of inclusions. Voids formed and subsequently expanded. Ultimately, tensile stress on the outer arc surface of the steel pipe reached its peak at the end of the flattening test. The voids caused by inclusions penetrated the wall thickness. This resulted in macroscopic crack defects and the failure of the steel pipe during the flattening test.