Welding Backing Rings 2

[Metcorr]

In one of the steam service pipeline, we are evaluating using welding backing rings to ensure full weld penetration. Are there backing rings available in the market which are fully consumable? Can any one kindly guide us to the available literature on type of welding backing rings and their properties. Should the backing ring be of same material, or anodic to the pipeline or cathodic to the pipeline? Assistance from colleague will be greatly appreciate.

 

[metengr]

Metcorr;
I would strongly advise against using any backing rings (unless it is absolutely necessary) to assure a full penetration weld in a steam line. I have seen welds that were made 50 years ago with backing rings that have failed during some point in service, resulting in FOD downstream in the steam line. Backing rings can be a source for chemical hideout or possible cracking from thermal or mechanical fatigue because of the natural crevice that is formed by the ring on the ID surface of the pipe.

If you absolutely need to use a backing ring, you can counter bore the pipe ID surface at the weld root region and use a rounded ring to avoid failure in service. However, you still have a crevice and that is unavoidable.

I would suggest an open root butt weld using the GTAW process to assure 100% fusion. This is the best way to go in my opinion.

 

[Metcorr]

Thanks Meteng,

I came to know from a colleague that there are backing rings which are fully consumable that is they 'go away' before or during service and do not cause any contamination of the process. Just wondering if you have come across any such thing. Also, should the backing ring be cathodic or anodic to the pipe. I thing, they should not be cathodic although anodic can be acceptable.

 

[Metcorr]

Dear Colleagues,

I have been searching net for the possible answers to my question and found the following which seems to have answered my own querry.

1. Consumable inserts are intended to be fully consumed by a welding process without the use of additional filler metal. They are are actually a root pass in themselves. They are the filler metal.And hence, called ' consumable'.

2. The backing ring should neither be anodic or cathodic to the base metal. Its composition should be closely matched with the base metal.

I just thought to put in this thread for information. Thanks.

 

[metengr]

Metcorr;
Yes, I would agree to using a consumable insert but this is NOT the same as a backing ring. Normally, a backing ring is not designed as part of the weld joint and offers no strength. The consumable insert is part of the weld itself and should match the chemcial composition of either of the two base metal being joined along with the weld metal.

 

[Metcorr]

Metengr,

Thanks. Are there consumable backing rings? Will appreciate your feedback.

 

[metengr]

Metcorr;
No, not consumable backing rings. You normally purchase consumable rings to provide for the root.

 

[unclesyd]

Adding to the sage advice from metengrconcerning backing rings.
In my failure museum at work I had two samples of steam line failures that originated at backing rings. Both failures were from material, NaOH, being trapped by the ring and concentrated and initiating cracking. One was purely caustic embrittlement and the other was corrosion fatigue due to the cyclic nature of this line.

A consumable insert does not a welder make.
It takes a welder with the skills to make an acceptable weld, meaning full penetration and smooth root, to properly handle consumable inserts. We have no trouble routinely achieving the quality weld that you require without the use of backing rings or consumable inserts.

 

[MJCronin]

metcoor...

I would like to add to the wise advice given by "metengr" and "unclesyd".

In almost all industrial projects, you do not want to have welded piping joints that include backing rings AND consumable inserts. It has been my experience that careful pipefitting (yes, including tedius trial-and error field fit-up)coupled with a quality TIGed root pass and SMAW balance of weld is the preferred method of joint design.

In the 1980s and 1990s, I have worked at a few nuclear power stations where this became the preferred method for a number of reasons.

Some of these are:

- Ease of inspection, backing rings complicate well x-rays and sometines ultrasonic joint inspection

- Quality of the joint, By forcing the contractor to carefully fit-up his joint, this will ensure that any gap or "non-consumption of weld material will not occur

- Elimination of gaps for corrosion - covered above

- Ease of future rework, should there be any rework in the future, the contractor can more easiliy rework the pipe without chipping away at internal "chill rings"

In summary, I have found that the use of consumable inserts and backing rings is a 1950s technology that has been surpassed by a TIGed root pass and a careful welder and crew.

Yes, in mid-job you will probably get a contractor to ask to use these ancient devices, but IMHO its only to cover up poor workmanship and enable him to speed up his job because the fit-up and TIGed root pass is time consuming.....

Again, my opinion only

-MJC

 

[mshimko]

In addition to the comments above, you need to consider if you have a clearly defined requirement for full penetration welding, and what are the QA/QC/inspection requirements to ensure the stated requirements are met.

Often the welding procedure is the primary "QA tool" to ensure full penetration is achieved, as the alternative is making an inspection hole on opposite sides of the piping (on long sections, at least).