Removable Backings

By Lawrence (*****)

Date 10-22-2007 12:00

You may get other answers here, but to me it seems like the whole idea of working to a code is to gain control of a process or procedure.

If your root openings are uncontrolled you have a process problem that needs to be solved. Inconsistant within a range is one thing.. Uncontrolled is another.

Rather than investing time a and materials on backing rings it might be wiser to invest in process control to bring your root openings into reasonable dimensions.

By Kaveh Date 10-23-2007 05:21

Thanks for the advice. But actually the structure we are working on has already been erected by another contractor which has been disqualified due to problems like this. We are here to finish a messed up project and therefore we can only focus on solutions.

By Root Pass (***)

Date 10-23-2007 20:18

Can you post some pictures? Might help with some suggestions.

By swsweld (****)

Date 10-24-2007 00:13

Do the backing bars have to be removed? Could be UT/RTd with them in place. Unless required to remove them I would leave them in place.

By 803056

Date 10-24-2007 04:07 Edited 10-24-2007 04:10

The excessive root openings can be reduced by building up the root face and groove face with weld metal and then reshaping them if you can disassemble the components. This is permitted by paragraph 5.22.4.3. There are some restrictions that are applicable, so check the details. I'm looking at D1.1-2002 because that's the one that's handy, later editions say the same thing.

If that isn't feasable, you can use backing, but I would not recommend copper or any copper alloy. If the copper is melted (high probability), you will have issues with cracking. This isn't a small possibility, you will have cracks in the weld.

I would use permenant backing with the same chemistry as the members you are welding. I have included a sketch to help in the explanation of the procedure. I show a V-groove, because you didn't tell us your groove configuation. The backing should be tack welded to only one member and not attached to the second member. Deposit weld metal against the member to which the backing is tack welded. Build up the groove face until the root opening is reduced to about 1/4 inch. Allow the joint to cool. Reapply any required preheat and complete the joint using the minimum number of passes, i.e., use weave beads, but limit their width to 1/2 inch if you are using FCAW or SMAW. This procedure should be reviewed and approved by the EOR or someone that has the authority to approve welding procedures. This is not typicall within the job discription of a CWI.

Good luck.

Al

Attachment: RootReduction-Buidup23-Oct-07.dwg (25k)

Attachment: RootReduction-Buidup23-Oct-07.wmf (10k)

By Kaveh Date 10-24-2007 15:08

Thanks Al, this is very nice of you.
I just have a question: why should the backing strip be tack welded only to one member? Is it mandatory? Correct me if I'm wrong but the distortion caused by the build-up process and the tack welding might form a gap between the backing and the second member which would result a slag line or LOF in the RT. In your sketch too (which is exactly like my case) the first pass of the main weld is placed on this unwanted gap and therefore in the middle of the RT film. By the way, my groove is a 60 degree V and the Electrode is E7018.
Thanks again.

By 803056

Date 10-24-2007 16:10

The reason for tacking the backing to one member is that after building up the groove face to reduce the root opening, I allow the weld to cool and contract. If the backing was tack welded to both members, the tack welds securing the backing to the opposite member would crack. Likewise, I do not weld the groove in layers as one would expect because the residual stresses introduced as the weld cools could induce lamellar tears in non-butting member in the case of a corner joint or crack the base metal in the HAZ or the weld (whichever is weaker) in a butt joint. By tack welding the backing to only one member (the one which is being built up) and allowing the weld and surrounding base metal to cool, the member (and weld) is free to contract without introducing any stress or causing any damage to the other component(s).

Only after everything is cooled and fully contracted do you add the minimum amount of preheat to both members and complete the joint.

The backing bar extends well beyond the root of both members and beyond the toe of the weld face so that there will be no issues with interpreting the radiograph. As you build up the groove face with weld, the residual stresses will tend to pull the backing up tighter to the second member in the case of the V-groove. You are correct if the joint is a corner or T-joint, there may be a separation between the end of the backing bar and the opposite member, i.e., the non-butting member. Hopefully it isn't so severe that the welder can't close the "gap" with his root pass. The resulting gap between the backing and the non-butting member shouldn't be that large when all is said and done. In any event, even if the backing is tight up agaainst the non-butting member (in a corner or T-joint) there with be a line in the radiograph between the end of the backing and the non-butting member. There will be no reason for a slag line in a butt joint where the residual forces are pulling the backing bar tighter against the underside of the joint.

If the joint is long and the tack welds at the ends are not sufficient to secure the backing in place, place the tack welds in the joint, but be sure to grind and feather them before depositing your first weld bead in the groove. Tack welds on the back edge of the backing can cause the backing to rotate downward away from the underside of the second member if the joint is long and several tack welds are needed to secure it in place. I prefer to see the tacks in the groove, but that is not what the welders prefer.

Once again, pass this by the individual that has the authority and responsibility to approve the repair before it is implemented.

Best regards - Al