The HFW process involves uncoiling and levelling a hot rolled coil, forming the strip into a cylinder, electric heating the edges and forging them together to form a longitudinal weld. The resultant autogenous weld has a very narrow bond line, which can present difficulties in testing, particularly for the Charpy impact test (CVN). Incorrect bond line CVN testing can result in unrepresentatively high toughness being reported. It is particularly critical to accurately locate the notch relative to the position of the bond line. It is, therefore, essential that a suitable procedure is followed. Current product standards give only limited guidance on how to test the bond line.
EPRG has developed a procedure for the Charpy impact testing of the HFW bond line. This defines the positioning of the notch and requirements for macrographic confirmation of correct positioning. The procedure has been validated by an extensive interlaboratory round robin study.
10 test laboratories (from 5 countries) agreed to participate in the round robin study.
A single pipe (457.2x9.8mm API 5L X60) was sectioned and test coupons were supplied to participating laboratories.
Each laboratory was required to machine, notch and test nine Charpy specimens according to the EPRG procedure.
Photomicrographs of the notched test pieces prior to testing were used to determine specimen validity according to the procedure.
A statistical analysis of the results was performed according to ASTM E691-14 Standard Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method.
Both between-laboratory and within-laboratory consistency parameters were found to be below the critical values.
The test programme has demonstrated that the EPRG procedure is practical, applicable, repeatable and reproducible.