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Stainless steel flange toughness testing

Connect, no. 145, November - December 2006, p.1

Fracture faces of the CTOD specimen
Fracture faces of the CTOD specimen

'We need to know if they're fit for can you do an Engineering Critical Assessment... preferably, right now', asked one of the world's leading oil and gas operators when it found that several of its crucial stainless steel pipe flanges had failed to meet their Charpy impact test requirements.

The requests made to TWI were unambiguous. Carry out fracture mechanics tests on the flange parent materials and calculate the tolerable flaw sizes to see if they are fit for service with reduced Charpy impact properties. They were meant to satisfy a 60J average and a 45J minimum impact energy at -70 Celsius.

Upon delivery of the specimens in question to TWI, fracture mechanics testing to BS 7448-1:1991 was started on the neck and swivel flange material supplied. Three single edged notch bends fracture toughness specimens were machined from each flange body and notched appropriately.

An ECA was carried out on the weld neck flange and swivel flange using a material specific Failure Assessment Diagram to Level 2B procedures in the British Standard BS 7910. This is incorporated in TWI's CRACKWISE 4 software.

Both the weld neck flange and the swivel flange showed very low minimum fracture toughness values of less than 0.05mm CTOD at the test temperature of -70 Celsius. The ECA determined sizes of tolerable flaws in the two flanges with below specification Charpy impact toughnesses. The flaw sizes varied depending upon the type of flaw assessed, the applied membrane stress, and whether the imperfections were adjacent to or remote from bolt holes.

Armed with the results of this ECA the client was then able to evaluate whether or not such flaws could subsequently be found reliably using an appropriate non-destructive testing method.

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