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PRCI Report 140
- Origin of High Fracture Initiation Energy in High-Strength Pipe
- Report / Survey by Pipeline Research Council International, 03/30/1984
- Publisher: PRCI
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L51450e
Battelle Memorial Institute
Need: Large discrepancies in the absorbed energies of adjacent dropweight-tear-test (DWTT) specimens with pressed notches from the same pipe have occasionally been observed in high-strength controlled-rolled steels in the NG-18 Line Pipe Research Program. Therefore, a research investigation was undertaken with the objective of determining the cause of those large energy variations.
Result: 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.
Benefit: Metallographic sections through notches in untested specimens revealed considerable notch-root deformation in some specimens, to the extent that cracks along flow lines sometimes were observed. The extent of deformation varied from notch to notch despite efforts to press the notches to a uniform depth using a standardized procedure, Microhardness traverses near the notch root in metallographic sections through fractured specimens indicated that in many cases the higher energy specimen had more preexisting' deformation due to the notch-pressing operation than did the corresponding lower energy specimen. Although all pairs of specimens did not show a direct correlation between notching deformation and DWTT energy, it was concluded that these differences in notch-root deformation probably were responsible for the observed differences in fracture resistance.
Battelle Memorial Institute
Need: Large discrepancies in the absorbed energies of adjacent dropweight-tear-test (DWTT) specimens with pressed notches from the same pipe have occasionally been observed in high-strength controlled-rolled steels in the NG-18 Line Pipe Research Program. Therefore, a research investigation was undertaken with the objective of determining the cause of those large energy variations.
Result: 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.
Benefit: Metallographic sections through notches in untested specimens revealed considerable notch-root deformation in some specimens, to the extent that cracks along flow lines sometimes were observed. The extent of deformation varied from notch to notch despite efforts to press the notches to a uniform depth using a standardized procedure, Microhardness traverses near the notch root in metallographic sections through fractured specimens indicated that in many cases the higher energy specimen had more preexisting' deformation due to the notch-pressing operation than did the corresponding lower energy specimen. Although all pairs of specimens did not show a direct correlation between notching deformation and DWTT energy, it was concluded that these differences in notch-root deformation probably were responsible for the observed differences in fracture resistance.
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