weld with backing ring & radiography test 1

[liby]

Hi
We are producing a CR vessel, 2m diameter, 4m long, cylinder 8mm, heads 10mm. No manhole, so the second head had to be welded from one side only. We used a backing strip & 100% radiography. There were a few rejects which were repaired. Second test showed three rejects, two porosity & a tranversal 2cm crack. The manufactor says that welds with backing strip should not be filmed.
My first question: Does anyone know of a spec or work order which specify that a backing strip weld should not be tested ?
Second question: What is the accuracy difference, or which is better. Radiography or Magnetic particle test.
Thank you
Liby

 

[Ron]

Welds with backing rings can be radiographed. There is a chance of obscuring a defect, but not much. As long as the backing ring thickness is included in the time calculations and the appropriate density is obtained in the film, there should not be in issue.

Radiography and magnetic particle testing serve two distinctly different functions. Magnetic particle testing is for detecting anomalies that are at or very near the surface of the weld. This method is not used for subsurface (i.e., root) indications. Further, there is no record of MP testing after the test is done, thus very dependent upon operator evaluation at the time.

Radiography is a true "picture" of the weld and detects anomalies that are perpendicular to the plane of the film. Anomalies or defects that are parallel to the film plane are not as detectable, such as laminations, lamellar tears, etc. While radiography is also susceptible to interpretive evaluation, at least there is a record of the test from which others may form an opinion as well.

Noting your comment about transverse cracking, it is likely that your fabricator is not controlling temperatures very well. This type of defect is often caused by inappropriate pre-heat or post-heat temperatures.