Microstructure and Thickness Effects on Impact Behavior and Separation Formation in X70 Pipeline Steel

In an effort to optimize the transportation of oil and gas, the pipeline industry is developing large-diameter, thick-walled pipelines that can withstand low temperatures and high pressures. In this study, three X70 steel plates of similar chemistry, ranging in thickness from 13.5 mm to 22 mm, were subjected to drop-weight tear and Charpy V-notch tests to determine the effects of plate thickness and microstructure on the formation of separations and impact behavior. Constraint induced by specimen thickness appears to dictate the location of separations, the three microstructures exhibited different separation behaviors, and microstructural banding was not found to promote separation formation. Separations were most frequent when the primary fracture plane was parallel to the rolling direction. This study also found that standardized empirical relationships between Charpy V-notch and drop-weight tear tests do not estimate to the advanced high-strength and -toughness steels investigated.