PR-276-184501-R01 Toughness Specification to Avoid Brittle Fracture in New Linepipe Steels

Abstract

Historically, Charpy test results have been used for assessing linepipe steels for fitness-for-service or new pipeline design, and the linepipe steels should be chosen such that the brittle-to-ductile transition temperature of the steel is below the minimum design service temperature. The transition temperature is typically much different for crack initiation versus crack propagation. This study focused on the transition temperature for fracture initiation which is important for liquid as well as high-energy lines. However, in recent studies, different behavior between vintage and newer steels has been noticed for the fracture initiation transition temperature relative to Charpy data. Even the fracture surface appearance in the Charpy test is much different for vintage steels than for newer steels. To investigate the above issues, the objectives of the current project were set to: a) determine if the fracture initiation transition temperature for new linepipe steels is similar to that in older steels, and how to predict it better if Charpy data are insufficient; and b) understand if the Charpy energy requirement for API 5L PSL-2 linepipe is always adequate to ensure ductile fracture initiation at the Charpy test temperature. In this project, the transition temperature behaviors of three linepipe steels from the Charpy test data, results from drop-weight tests to determine the nil-ductility temperature or DWT-NDT, and findings from more sophisticated fracture tests such as single-edge-notched tension [SEN(T)] that is representative of surface-cracked pipe behavior, and compact tension [C(T)] specimen tests that are representative of through-wall-cracked pipe behavior were determined. The report summarizes the key findings of the project for newer steels and attempts to contrast fracture initiation behavior in vintage versus newer linepipe steels.