Through-Thickness Properties of TMCP Steels

Two failure modes were observed in tensile test specimens machined from TMCP steel pipe. Longitudinal centerline cracks were formed after necking followed by the conventional cup and cone transverse failure. The formation of the longitudinal cracks was attributed to sulfur segregation and grain growth at the centerline. This gave rise to concerns that TMCP steels might exhibit low through-thickness ductility that could lead to lamellar tear failures in highly constrained joints or joints with through-thickness loadings such as in the case of tubular joints and lifting lugs. It also gave rise to a concern regarding the potential of hydrogen-induced cracking in TMCP steel vessels, pipelines, and cathodically protected tubular joints. The results of through-thickness tensile tests showed that this TMCP steel has good through-thickness ductility in spite of the presence of high sulfur segregation at the centerline. However, the results of slow strain tests on a hydrogen charged through-thickness tensile specimen showed some deterioration in ductility. In addition, the fracture of the specimen was characterized by the presence of multiple circumferential cracks that are characteristic of hydrogen embrittled materials.