Fracture Toughness of Contemporary EW Seam Welds for EW Specification

摘要

Fracture toughness tests using standard single-edge notched bend, SE(B), specimens were performed for seam welds of ten contemporary high-frequency (HF) electric welded (EW) pipes to develop a database for engineering critical assessment (ECA) and steel pipe specification requirements. The test temperatures were selected to provide a large number of tests at temperatures where the Charpy absorbed energies (CVN) were in the range 27-40 J. Limited fractography confirmed that low-energy fracture in SE(B) specimens was generally associated with ERW weld penetrators as was the case for Charpy specimens. The effect of initial crack length (for a/W of 0.2 or 0.5) on fracture initiation toughness was found to be very small for two tested pipes. The K-CVN equation in BS 7910-2013 Annex J gives a good fit to the lower-bound estimates of toughness for these EW pipe welds with CVN taken as the average of three specimens and lower-bound toughness taken as the average values minus one standard deviation. To specify minimum toughness values for safe operation of pipelines from first principles, the failure assessment diagram (FAD) in the CSA pipe standard combined with the BS 7910 K-CVN correlation (average values) were used to determine CVN values to prevent initiation. A hypothetical semi-elliptical surface flaw with depth (a)=2 mm and length (2c)=30 mm was assessed for 16 typical cases. The required CVN values were in the range 10-50 J, and were generally higher (more demanding) than the weld fusion-line requirements (18 J) in the current (2018) edition of the CSA pipe standard.