L51450 Origins of High-Fracture-Initiation Energy in High-Strength Pipe

For the study, duplicate pairs of pressed-notch DWTT specimens were selected that had exhibited large differences in fracture resistance in the tear tests. Metallurgical evaluations were conducted in an effort to identify correlations between metallurgical characteristics of the specimens and the large differences in fracture resistance observed for a given pipe. The chemical composition, microstructure, inclusion content, extent of midwall segregation, hardness profiles through the pipe-wall thickness, and the appearance of the fracture surfaces were documented, and comparisons were made between the duplicate specimens. The various pairs of DWTT specimens that were evaluated exhibited widely varying metallurgical characteristics among the pipes, particularly the content of secondary (or minor) microstructural constituents. However, significant differences between duplicate DWTT specimens were not observed consistently among the various pairs. Thus, metallurgical factors apparently were not responsible for the differences in fracture resistance measured during testing of the specimens.