Clearence btw backing and base metal

[waqasmalik]

Hi all, i am qualifying welder for groove weld in flat position with backing per AWS D1.2. I have read somewhere that if backing is not to be removed then it must be 3 inch wide atlesat to keep the tack welds away from the radiography area.Also, i have heard that there must be close proximity btw backing and base metal to avoid any non relevant indication like lack of fusion and penetration.
Having said that i have two questions.

1) What problems could tack welds create if i use smaller width backing lets say 10 mm ?

2) Is there any value for acceptable max clearence btw base metal and backing to avoid any non relevant indication? what would be the tolerance on that?

Thanx all

 

[metengr]

From what you stated, why use tack welds along the edges to secure backing on base material? The backing should be tack welded at the root to secure it before depositing the root to fuse with the backing material.

 

[waqasmalik]

What about clearance?

 

[gtaw]

Since you are working to the requirements of AWS D1.2, why not look in the "book" to see what requirements apply?

The figures in the code are pretty self explanatory. By the way, the prequalified joint details and the included tolerances do not apply to performance qualification or procedure qualification.

Smaller, i.e., narrower backing simply does not meet the code requirements. The code isn't a guide, it delineates the minimum requirements that must be met. In regards to welder performance qualification, the figures included in the AWS code are the tests that are required. Once the requisite tests have been passed, the contractor can invoke any additional tests deemed necessary to ensure the welder is capable of producing acceptable production welds. An example would include the contractor's desire to assess the welder's ability to properly back gouge a groove weld. Should that be the case, the contractor can require the welder to demonstrate the ability to weld a grooved plate, back gouge the root side, and back weld the back side. That test is in addition to the requisite performance test that includes the use of backing. There is no standard AWS test that includes a back gouge operation.


Best regards - Al

 

[waqasmalik]

Which figure or paragraph in AWS D1.2 talks about the intimate proximity BTW permanent backing and base metal if the joint is to be radiographed?

Lack of penetration defect is reported BTW base metal and permanent backing in radiograph film. Its not in the weld area. It's both right and left of the weld root.
When backing removed then the defect disappeared.Test plates and backing strip were not very flat.

I did some R & D. I machine both test plates and backing strip and after fit up the max clearance was 0.2 max. I welded the same way as before then got them radiographed.No lack of penetration reported.

Does this relates to sensitivity of radiograph? How would I know the max clearance value BTW the two? Any range? I m sorry I could not find in the code.

 

[gtaw]

Can't help you there bud. I'm on the road this week and I don't have access to my copy of the code.

My recommendation is to bevel your plates to a knife edge, that is, no root face. Make sure everything fits tight. I fabricated a fixture plate out of 1 inch thick aluminum plate to hold every thing in alignment and to clamp everything tight. Fit up is important. Gaps between the backing and the plates are not your friend.

Best regards - Al